The Cessna 170

Service Manual

Published by

The International Cessna 170 Association

ELECTRICAL SYSTEM

©2004 The International Cessna 170 Association and George Horn

All Rights Reserved
It was in the lobby of the hotel during the 2001 Texas Christmas Party held in
Nacogdoches that TIC170A Historian Cleo Bickford approached me. If I hadn’t been so
overwhelmed by the warm welcome and atmosphere of all the fellow Cessna 170 owners
present I might have been more alert to the twinkle in his eye. He was on a mission and I
didn’t realize it.

Cleo, charter member number C42, being an accomplished aviator, experienced

aeronautical engineer, Cessna 170 owner, and devoted promoter of TIC170A, had long
observed the painful lack of a service manual specifically for the 170 and made it his goal
to address the issue. A manual had been contemplated for many years but had never
been brought to fruition and Cleo apparently decided there would be no further harm in
soliciting the services of yet another procrastinator. The difference this time was, …Cleo
had his “don’t even THINK about telling me no” charm turned fully ON! I was caught
completely unawares. In all fairness, he did allow me to think it over before committing to
the project. Those who know Cleo well can appreciate what that truly means. His quiet,
well-respected manner plays upon the conscience until the decision was made: I WILL
complete the project, no matter how long it takes!

This is the first attempt at a service manual for me, so I ask forbearance if it appears less
than complete. It is very much a “work in progress”. This first section is dedicated to
the electrical system, and subsequent sections will be presented at upcoming convention
seminars until the entire manual is complete. As new material (and authorship technique)
is developed, this section will be revised and amended. The complete manual
encompassing all systems is expected to be a five-year project. A person attending all
five convention seminars will have obtained a complete manual, and helped influence the
final products outcome.

Wherever possible part numbers have been included in the text for convenience’s sake.
Be aware that part numbers may be superceded at some point in time and be certain to
confirm any part requirements. An attempt has been made to help the owner with
unsophisticated repair and troubleshooting techniques. Some of those activities may not
meet the FAA’s definition of “preventive maintenance” and therefore undertaking such
actions should be considered carefully. By all means, do not violate the law, or good
operating procedures.

Several sources have been consulted in preparing this manual including but not limited to
manufacturer service manuals, overhaul manuals, and FAA Advisory Circulars (especially
AC 43.13-1A “Methods and Materials”.) Much of the information herein is the result of
years of experience gathered from many individuals, most of whom will go unfairly
unrecognized. To my friends and fellow aviators I apologize if I’ve “borrowed” your ideas
too readily. I hope you take consolation in seeing them put to good use. If as the reader
you find something to correct or improve the manual, please contact me and let me know
for inclusion in future revisions. None of the information in this manual is intended to
replace manufacturer or FAA approved methods and practices. This manual is intended
only for the purpose of furthering the understanding of maintenance issues as regards
this airplane. Neither The International Cessna 170 Association nor the author makes any
representation as to the fitness of purpose or the suitability of the information contained
herein. Use it at your own risk.

Having given to disclaimer, now please join me in the pleasure of dedicating this manual
to the person who I credit as the real motivating force in its production:
Cleo Bickford, C42, Historian, TIC170A, without whom it would never have been started,
and doubtless with whom it will never be finished. ;Þ

George Horn
Parts/Mx coordinator, TIC170A
Cessna 170 Service Manual
Electrical System

ELECTRICAL
Cessna 170 Service Manual
Electrical System

TABLE OF CONTENTS

ELECTRICAL SYSTEM……………………………...Page 1
BATTERY AND EXTERNAL POWER SYSTEM
Battery
Battery Box
Battery Solenoid.
Master Switch.
Ground Service Receptacle.
Troubleshooting the Battery System……………..Page 2
Removal and Replacement
Cleaning, Testing, and Charging…………………..Page 3
Battery Box Removal
Battery Box Maintenance……………………..…….Page 4
Battery Solenoid Replacement

GENERATOR POWER SYSTEM.

Generator
Radio Noise Capacitor
Generator Warning Light…………………………...Page 5
Ammeter
Removal and Replacement of Generator
Polarizing the Generator/Regulator
Voltage Regulator…………………………………...Page 6
Removal and Replacement
Troubleshooting the Generator/Regulator ……..Page 7
Generator Tests, Repairs, and Adjustments
Simple Generator Test……………………………...Page 8
Generator Short Test
Voltage Regulator Repair and Adjustment
High RPM Gen OnLine Speed
Adjustment of Voltage
Adjustment of Current
Isolating Too Low/High Charge……………….….Page 9
Inspecting Regulator
Regulator Point Cleaning

Engine Starter…………………………………….…Page 10
Starter Electrical Circuit
Starter Lever/Cable Adjustment

AIRCRAFT LIGHTING SYSTEM. ,

Landing and Taxi Lights
Navigation Lights…………………………………..Page 11
Navigation Lights Flasher.
Rotating Beacon.
Interior Lights Map Light and Dome Light,
Removal and Replacement of Dome and Instrument Lights
Compass Light
Removal and Replacement of Navigation Lights

PlTOT AND STALL WARNING HEATER………..Page 12

CIRCUITS
System description
Removal and Replacement of Pitot Heater

STALL WARNING
System description

TROUBLESHOOTING THE ELECTRICAL SYSTEM...Page 13

EXAMPLE: Stall Warning troubleshooting
Cessna 170 Service Manual
Electrical System

Table of Contents cont’d

Electric Wire…………………………………………Page 14
General
Aircraft Electrical Wire

Wire Gauges and Ampacity Chart..………...……Page 15

Battery Cables
Splices
Open Wiring…………………………….……………Page 16
Heat Precautions
Protections Against Chafing
Stripping
Terminals

Attachment of Terminals to Studs……………….Page 17

Bonding Jumper Installations
Corrosion Prevention
Ground Return Connections
Current Limiters

Wiring Schematics

Electrical Schematics
SNs 20267 THRU 26372
SNs 25373 THRU 26995
SNs 26996 and ON
SNs 26996 and On – Inst Pnl

Electrical Wiring Diagram Identification

(Wire ID, Length, Sizes)

Appendix EL (Illustrations)

Ground Service Plug and Early Battery Solenoid (Relay)….EL 1

Late Style Battery Solenoid (Relay)

Battery Solenoid (Relay) and Master Switch Circuit………...EL-2

Landing Light Adjustment

MultiMeter…………………………………………………………...EL-3

Test Leads

Generator Motor Test Schematic……………………………….EL-4

Test for Generator Short

Voltage Regulator………………………………………………….EL-5

Test 1 to Confirm Regulator Ground

Test 2 to Isolate Regulator

Test 3 Cut Out Relay (Reverse Current Relay)……………..EL-7

Starter Cable/Lever Adjustment

Cessna 170 Service Manual
Electrical System

Twelve-volt electrical systems are used on all models. An engine-driven generator supplies the normal
source of power during flight and maintains a battery charge controlled by a voltage regulator. An external
power source receptacle is offered as optional equipment on all models to supplement the battery-generator
system for ground operation. See Appdx. EL, Fig. EL-01.

BATTERY AND EXTERNAL POWER SYSTEM.

The battery and external power system consists of a battery, a battery solenoid, a master switch and an
external power receptacle (optional equipment). The battery and solenoid are mounted on the fire- wall. No
electrical power is supplied to the aircraft bus until the master switch is turned on. The master switch closes
the battery solenoid, connecting the battery to the bus.

BATTERY. Multi-cell, wet-type batteries, which have non-spill type filler caps, are used on all models. The
battery capacity is 25 ampere-hours (originally 24 ampere-hours). All batteries are 12-volt and are housed in
a battery box attached to the firewall. The battery on the 170 is accessible by removing the cowl, or on
certain models thru an access door on the cowl.

BATTERY BOX. The battery is contained in an aluminum box, which is riveted to the aircraft firewall
structure. The battery is vented by a tube which attaches to the bottom of the battery box and extends
downward thru the cowling exhaust opening. A battery box lid completely encloses the battery to reduce
any spillage of electrolyte or accumulation of battery gases inside the cowl. Internal metal parts of the
battery boxes are best coated with an acid resistant paint.

BATTERY SOLENOID. The battery solenoid is bolted to the forward side of the battery box. It is a plunger
type contactor, which is actuated by turning the master switch on. When the master switch is off, the battery
is disconnected from the electrical system. A silicon diode has been added to some aircraft to eliminate
spiking of transistorized radio equipment. The large terminal of the diode connects to the battery terminal of
the battery solenoid and the small terminal of the diode connects to the minus terminal of the solenoid coil.
The minus terminal of the solenoid coil is the small terminal that the master switch wire connects to. Several
models of battery solenoid with differing appearance have been utilized over the life of the airplane, but their
operation is essentially identical. The original solenoid was contained within a metal box, while subsequent
models resemble a metal cylinder with two large threaded terminals for main battery cable connections (one
from positive battery terminal, the second connected to the rest of the aircraft electrical system at the starter
switch mounted upon the starter motor) and either one or two smaller terminals. One of these smaller
terminals is utilized to connect the solenoid to the cockpit master switch (where it provides a pathway to
ground so the normally open solenoid will close, thereby completing the connection from battery to the
previously described aircraft electrical system. The other smaller terminal, if present, completes an external
solenoid circuit function providing battery power to activate the solenoid. On single terminal solenoid models
that circuit is internally provided for and therefore hidden/not required.) Most modern replacement solenoids
are of the single, small-terminal type. See Appdx. EL, Fig EL-02.

MASTER SWITCH. Operation of the battery and generator power system for all models is controlled by a
switch located on the instrument panel. Master switches are double pole, single throw. (In other words,
there are two circuits actuated by the Master switch. When the master switch is turned on, one circuit
provides a ground connection for the battery solenoid (small terminal) and the normally open solenoid
closes, connecting the battery to the electrical system. The switch also closes a second circuit, completing
the generator field connection to the voltage regulator. This allows the voltage regulator to regulate the
generator when it comes online after engine start. See Appdx. EL, Fig EL-03.

GROUND SERVICE RECEPTACLE. The AN-2552-2 ground service receptacle is optional equipment and
may be located on the lower firewall in the engine cowl. A 12-volt battery cart or ground service generator
may be plugged into the unit for starting and operation of the electrical and radio equipment on the ground.
If the operator has specially constructed battery “jumper” cables, the ground service receptacle may be used
to provide electrical power to the aircraft from a ground vehicle or other source. Caution: No reverse polarity
protection exists for the aircraft. One must be careful to connect jumper cables in the correct polarity. The
center contactor pin of the AN-2552-2 is connected to the positive terminal of the battery solenoid at the
same location as the battery positive terminal connection. Therefore, any connection to the AN-2552-2
receptacle is, in effect, a direct connection to the battery. The end contactor pin of the AN-2552-2 is
directly grounded to the airframe.

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TROUBLE SHOOTING THE BATTERY SYSTEM.

BATTERY DOES NOT SUPPLY POWER TO BUS WHEN MASTER SWITCH IS ON.

Dead battery: Check specific gravity of electrolyte. Gravity reading should be at least 1.256, which indicates
a 75% charge at normal temperature. Replace or charge battery. Check charging rate of generator.

Defective master switch, battery solenoid or wiring: (IF you are CERTAIN the battery is good and well
charged) Short the battery solenoid terminal that is wired to the master switch to ground. This is the small
terminal of the solenoid. If the solenoid does not operate, check the jumper wire connecting the solenoid
coil to the "hot" solenoid terminal (two small terminal models only). If the solenoid does not activate, repair
defective wiring or replace solenoid. If the previous test activated the solenoid then check master switch for
continuity across circuit terminals. (Caution: Note the correct wiring schematic for the cockpit master switch,
re: two circuits. See above MASTER SWITCH commentary notes.) Check for good ground continuity
between Master switch grounded circuit and battery ground terminal.

Faulty battery cable: Inspect the battery cables for good connection. Clean and/or Replace cable,
reconnect.

Battery Box: Check battery box drain line to be open and clear of insect nests, or rubbish. Clean battery
box with battery removed using water and baking-soda solution. Rinse battery box thoroughly with clear
water. Inspect interior of box for condition of acid -resistant paint (if equipped.) Renew as necessary.
Reinstall battery.

BATTERY SUPPLIES POWER TO BUS BUT WILL NOT CRANK ENGINE

Low Battery. Check specific gravity. Charge battery

Faulty battery cables, Inspect for corrosion and poor connection. Repair wiring. Clean and reconnect

Battery cell shorting under load. (This cause is very difficult to detect. Even a good battery connected to
a shorted battery will behave similarly. Only when a shorted battery is completely removed from the system
will operations return to normal.) Test battery with a load tester. Replace defective battery.

Defective starter contactor or solenoid. On aircraft with starter switch check operation of switch and
solenoid. Replace switch. Replace solenoid.

BATTERY USES EXCESSIVE AMOUNT OF WATER. Charging rate too high. Test voltage regulator or try
a new unit. Adjust or replace regulator,
NOTE: Voltage regulators are adjustable, however adjustment should not be attempted unless proper
equipment is available. Refer to Delco-Remy service bulletins and see Voltage Regulator section for
instructions.

REMOVAL AND REPLACEMENT OF BATTERY

a. Remove the battery box cover and open cover. b. Disconnect the ground cable from the negative
battery terminal.

Always remove the ground cable first and replace it last to prevent accidental short circuits.

c. Disconnect the cable from the positive terminal of the battery.

d. Lift the battery out of the battery box.
e. To replace the battery, reverse this procedure.

CAUTION: Storage batteries are filled with corrosive electrolyte, usually sulphuric acid. Wear protective
goggles and clothing. Avoid contact with electrolyte. Prior to working on batteries, provide a ready source
of clear water to flood accidental spills. Electrolyte may be neutralized with a solution of baking soda and
water. Do NOT contaminate the battery’s electrolyte with baking soda solution or it will be damaged.

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CLEANING THE BATTERY. For maximum efficiency, the battery and connections should be kept clean at
all times.
a. Remove the battery in accordance with the preceding paragraph.
b. Tighten battery cell filler caps to prevent the cleaning solution from entering the cells.
c. Wipe battery cable ends, battery terminal and the entire surface of the battery with a clean cloth
moistened with a solution of baking soda and water.
d. Rinse with clear water, wipe off excess water and allow batteries to dry.
e. Brighten up cable ends and battery terminals with emery cloth or a wire brush.
f. Coat the battery terminals and the cable ends with petroleum jelly or battery terminal grease.
g. Install the batteries according to the preceding paragraph.

ADDING ELECTROLYTE OR WATER TO THE BATTERY. A battery being charged and discharged with
use will decompose the water from the electrolyte. When the water is decomposed hydrogen and oxygen
gases are formed which escape into the atmosphere through the battery vent system. The acid in the
solution chemically combines with the plates of the battery during discharge or is suspended in the
electrolyte solution during charge. Unless the electrolyte has been spilled from a battery, acid should not be
added to the solution. The water however will decompose into gases and should be replaced regularly. Add
distilled water as necessary to maintain the electrolyte level with the horizontal baffle plate or the split ring on
the filler neck inside the battery. When "dry charged" batteries are put into service fill as directed with
electrolyte. When the electrolyte level falls below normal with use, add only distilled water to maintain the
proper level. The battery electrolyte contains approximately 25% sulphuric acid by volume. Any change in
this volume will hamper the proper operation of the battery.

Do not add any type of "battery rejuvenator" to the electrolyte. When acid has been spilled from a battery,
the acid balance may be adjusted by following instructions published by the Association of American Battery
Manufacturers

TESTING THE BATTERY .The specific gravity, of the battery may be measured with a hydrometer to
determine the state of battery charge. If the hydro- meter reading is low, slow-charge the battery and
retest. Hydrometer readings of the electrolyte must be compensated for the temperature of the electrolyte.
Some hydrometers have a built-in thermometer and conversion chart. The following chart shows the battery
condition for various hydrometer readings with an electrolyte temperature of 80 degrees Fahrenheit. For
higher temperatures the readings will be slightly lower. For cooler temperatures the readings will be slightly
higher.

BATTERY HYDROMETER READINGS

1.280 Specific Gravity 100% Charged

1.250 Specific Gravity 75%Charged
1.220 Specific Gravity 50% Charged
1.190 Specific Gravity 25% Charged
1.160 Specific Gravity Practically Dead

CHARGING THE BATTERY. When the battery is to be charged, the level of electrolyte should be checked
and adjusted by adding distilled water to cover the tops of the internal battery plates. The battery cables and
connections should be clean. If the aircraft is equipped with the optional ground service receptacle the
charge may be applied to the battery by plugging a ground service generator into the ground service
receptacle. Turn off all electrical switches including the master switch when charging.

Note: When a battery is charging, hydrogen and oxygen gases are generated. Accumulation of these gases
can create a hazardous explosive condition. Always keep sparks and open flame away from the battery.
Allow unrestricted ventilation of the battery area during charging. The main points of consideration during a
battery charge are excessive battery temperature and violent gassing. Test the battery with a hydrometer to
determine the amount of charge. Decrease or stop the battery charging temporarily if the battery
temperature exceeds 125 degrees F.

REMOVAL AND REPLACEMENT OF BATTERY BOX. The battery box is riveted to the firewall. It is
necessary to drill out the rivets to remove the box. When an aluminum box is installed and riveted into place,
any rivets or scratches inside of the battery box should be coated with black acid-proof lacquer such as
Enmar Type TT-L-54 or Randolph Acid-Proof #345 (also meets Mil Spec TT-L-54.)

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MAINTENANCE OF BATTERY BOX. The battery box should be inspected and cleaned periodically. The
box and cover should be cleaned with a strong solution of baking soda and water. Hard deposits may be
removed from aluminum boxes with a wire brush. After all corrosive deposits have been removed from the
box flush it thoroughly with clean water.

Do not allow acid deposit to come in contact with skin or clothing. Serious acid burns may result unless the
affected area is washed immediately with baking soda/water solution or soap and water. Clothing will be
ruined upon contact with battery acid.

Inspect the cleaned box and cover for physical damage and for areas lacking proper acid proofing.
A badly damaged or corroded box should be replaced, If the box or lid require acid proofing paint the area
with acid-proof black lacquer such as Enmar Type TT-L-54 or Randolph Acid-Proof #345 (also meets Mil
Spec TT-L-54.)

REMOVAL AND REPLACEMENT OF BATTERY SOLENOID.

a. Open battery box and disconnect ground cable from negative battery terminal. Pull cable clear of battery
box.
b. Remove the nut, lock washer and the two plain washers securing the battery cables to the battery
solenoid.
c. Remove the nut, lock washer and the two plain washers securing the wire, which is routed to the master
switch.
d. Remove the bolt, washer and nut securing each side of the battery solenoid to the battery case. The
solenoid will now be free for removal.
e. To replace a battery solenoid, reverse this procedure.

REMOVAL AND REPLACEMENT OF GROUND SERVICE RECEPTACLE.

a. Open battery box and disconnect the ground cable from the negative terminal of the battery and pull the
cable from the battery box.
b. Remove the nuts and washers from the studs of the receptacle and remove the battery cable.
c. Remove the screws and nuts holding the receptacle and the ground strap will then be free from the
bracket.
d. To install a ground service receptacle, reverse this procedure. Be sure to place the ground strap on the
negative stud.

GENERATOR POWER SYSTEM.

The generator power system consists of the generator, voltage regulator and master switch. The generator
output is controlled by the voltage regulator to compensate for the amount of electrical power being
consumed and the condition of the battery. The master switch allows the pilot to shut the generator off
completely. An alternative method to disable the generator is to pull the generator fuse or circuit breaker.
A 12-ampere generator system is standard equipment. A 20, 25, or a 35-ampere generator is optional on
these models. If additional power is needed on the 170, an alternator conversion is possible resulting in
various ratings up to 60 amperes.

GENERATOR. Generators used on Cessna aircraft are two brush-shunt wound types with negative ground.
The generator output is controlled by the current passing thru the field winding of the generator. The field
winding is connected to the armature circuit of the generator internally and must be grounded externally (by
the regulator after passing through a master switch circuit) in order for the generator to operate. The
generator is driven by a gear train in the engine accessory case. The output is approximately 14 volts at 12,
20, 25, or 35 amperes, depending upon the particular unit. 35 ampere generators require a cooling air blast
tube supplying fresh air directly to the brush/commutator shroud and that the engine be equipped with a
dampened crankshaft.
Three electrical connections are required for the generator. Ground is provided thru the generator case and
mounting brackets. The field terminal is connected thru the master switch to the voltage regulator FieLD
terminal and the armature terminal connects directly to the voltage regulator ARMature terminal. On some
aircraft a capacitor is attached to the ARMature terminal of the generator or the regulator. The capacitor
suppresses any radio interference that might be created by the generator/regulator.

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CAUTION: If the generator system has a filter capacitor connected for suppression of radio noise, be
certain that it is connected ONLY to the armature terminal of the generator or regulator. If it is connected to
the field terminal it will cause burning of regulator points and possible damage to the battery and/or fire due
to overcharging.

GENERATOR WARNING LIGHT. Cessna 170’s were not factory equipped with generator warning lights.
However a few owners desired this feature and adapted it from other Cessna models. The following
description of this system is provided for those aircraft so modified. The generator warning light is designed
to indicate when the generator is not charging the battery. The light is electrically connected across the
cutout relay contacts of the voltage regulator. Whenever the cutout contacts are open the light will measure
the potential difference between the battery and the generator armature. When the battery voltage is in
excess of the generator output (such as an idling condition) the light will come on. As the generator speed
(output) is increased, the potential difference across the light will diminish and the light will grow dim. When
the cutout relay connects the generator output to the battery the light circuit is bypassed through the voltage
regulator and the light will go out.

AMMETER. The ammeter is connected between the battery and the aircraft bus. The meter indicates the
amount of current flowing either to or from the battery. With a low battery and the engine operating at cruise
speed the ammeter will show the full generator output when all electrical equipment is turned off. When the
battery is fully charged and cruise RPM is maintained with all electrical equipment off, the ammeter will show
a minimum charging rate. Important: Any time the ammeter is replaced, a gauge of sufficient generator
capacity must be installed that also includes an external or internal shunt amperage-rated at LEAST as great
as the generator.

REMOVAL AND REPLACEMENT OF GENERATOR.

a. Remove the cowl from the aircraft and drain the oil.
b. Loosen the clamp securing the blast tube and pull the tube clear of the generator.
c. Disconnect the generator wiring and pull it clear of the generator area.
d. Remove the three mounting nuts and washers attaching the generator to the engine accessory
case.
e. To replace the generator, reverse this procedure.

It is recommended that anytime the generator be replaced that a new oil seal (PN 352068) be installed on
the front of the generator. Remember that the oil seal open end faces TOWARD the engine to keep oil
inside the engine. Examine the generator drive shaft to detect interference between the woodruff key and
the oil seal. Any interference will damage the oil seal with resultant loss of engine oil. Generators are
usually received less drive gear and hub coupling. After installing the oil seal, drive the hub coupling to
where it bottoms on the generator shaft. Make certain the woodruff key remains in place. Assemble the
rubber disc with its groove side up. Assemble the drive gear on shaft, fitting the lug on the gear with the
rubber groove. Insert the special 5/16” washer over the generator shaft, and screw on the shear nut and
secure with a cotter pin.

A light coating of Permatex No. 2 Aviation gasket sealant may be used on both sides of the generator mount
gasket.

CAUTION: It is recommended that anytime the generator be removed that a completely new generator-
mounting gasket (PN 652072) be utilized upon reinstallation. It is VERY IMPORTANT that the gasket NOT
be cut or modified in any way. This requires that the tach-drive housing also be removed and reinstalled, as
the generator mount gasket includes provision for that tach-drive housing. Cutting the generator mount
gasket at this area as a “short-cut” method is highly discouraged as the potential for a high-volume oil leak to
develop in-flight can occur without warning to the pilot with the potential of complete oil pressure loss.
Whenever the tach-drive housing is removed/replaced, it is also highly recommended that the tach-drive
housing oil seal (PN 642714) be replaced to prevent oil from being carried up the tachometer cable into the
instrument and into the cockpit. Do not violate this procedure regardless of how innocent cutting that gasket
may appear.

POLARIZING THE GENERATOR/REGULATOR. A generator of the type used on aircraft utilizes a residual
magnetism in the field pole shoes in order to produce a charge. Whenever any work is performed on the
basic electrical system the generator should be polarized to make sure a charge will be produced. A
common defect after installing a battery, replacing a regulator, or replacing a generator is a NO CHARGE
condition due to lack of proper polarization. To polarize a generator/regulator system, connect a jumper
momentarily between the ARMature and BATtery terminals of the regulator BEFORE starting the engine, but
with the battery connected and the master switch ON. A momentary surge through the generator is enough
to correctly polarize both it and the regulator.

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If a generator is not correctly polarized the regulator and generator may be damaged.

VOLTAGE REGULATOR. The voltage regulator is a vibrating contact type containing three relays. The
voltage and current limiting relays control the output of the generator according to the demand of the battery.
This control of the generator is accomplished by changing the amount of field current flowing through the
generator. The maximum current output of the generator is controlled by the current limiting relay. If the
current output of the generator exceeds a preset amount the current limiting relay interrupts the generator
field circuit reducing the output by inserting a resistance into the field circuit. Similar conditions apply for
generator output voltage control. When the generator is not producing, such as in an idle condition it is
necessary to disconnect the battery from the generator. If the generator is not disconnected during no-output
conditions the armature appears as a path to ground for the battery voltage. To prevent this loss of battery
potential the cutout relay portion of the regulator disconnects the battery from the armature circuit when the
charging voltage drops below a specified level and current begins to flow in a reverse direction. The
operation of the voltage regulator controls the output of the generator with respect to certain preset
maximum levels, however the regulator must be informed of the batteries condition to taper the charge and
prevent overcharging. This information is supplied to the regulator by the battery counter electromotive force
on the regulator. The rate of charge is determined by the voltage-limiting relay. The higher the battery's
charge, the higher the charging voltage must be and the more effect the voltage limiter will have on the
charging rate. This reduces the charging rate as the battery comes up to charge.

PHYSICAL CHARACTERISTICS. A voltage regulator is designed for one purpose only and that is to
regulate or control the generator-battery circuit. When a battery charge condition is low the regulator will
automatically increase the charging rate until the battery becomes fully charged. As soon as the battery is
charged the regulator will automatically reduce the charging rate.
The regulator consists of three units, a Cut-Out relay (also known as a circuit breaker or Reverse Current
Relay), a Voltage Regulator Relay, and a Current regulator Relay. All three relays are nothing more than
magnetic switches that operate at high speed. See Appdx EL, Fig. 09.
The Cut-Out Relay removes the generator from the battery circuit when the generator is rotating so slowly
(low engine RPM) that it cannot produce useful electricity. This prevents the Battery from discharging into
the generator circuits. The Cut-Out Relay may be identified as it is directly connected to the system via the
BATtery terminal on the regulator.
The Current Regulator Relay controls the amperage output of the generator by connecting directly to the
generator armature thru the regulators ARMature terminal.
The Voltage Regulator Relay controls the actual voltage of the generator-battery circuit by either
weakening or strengthening the current within generator field windings, thereby altering the magnetic field
surrounding the armature windings.
Both Current and Voltage regulator relays accomplish their work by means of resistances that automatically
are cut into or out of the generator circuits. When the battery is fully charged resistance is introduced to
reduce the charging rate of the generator so that the battery voltage is held within design limits.

REMOVAL AND REPLACEMENT OF VOLTAGE REGULATOR.

a. Disconnect the wiring from the voltage regulator terminals. CAUTION: When removing the battery
lead from the voltage regulator, tape the terminal on the end of the wire to prevent accidental short
circuits. GOOD OPERATING PRACTICE dictates that the battery ground strap be disconnected
PRIOR to any work involving removal of wiring.
b. Remove the three bolts securing the regulator to the firewall and remove the regulator.
c. To replace the regulator, reverse this procedure and polarize the generator field when completed.
d. Be certain the voltage regulator base-plate is well grounded to the firewall. Usually this is
accomplished with a short ground strap across the shock mounts or directly to the case.

When replacing the generator or regulator, it is necessary to re-polarize the generator to establish proper
polarity. Connect a jumper momentarily between the ARMature and BATtery terminals of the regulator while
the master switch is on but before starting the engine. A momentary surge through the generator is enough
to correctly polarize it. Failure to properly polarize the generator/regulator may cause a failure to generate
electrical power condition.

IMPORTANT : Generators and Regulators must be matched in order to produce the rated current and to
avoid damage. While a high amperage generator may be operated with a low amperage regulator, such a
mismatch will result in the generator not producing maximum rated current. (The system will only produce
the maximum rated regulator amperage.) However, the reverse condition…that of a high amperage rated
regulator connected to a low amperage rated generator will result in a damaged/burned generator due to an
overheat condition as the regulator attempts to demand more and more current from the lower rated

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generator. Therefore, it is important that both generator and regulator be of the same capacity to obtain best
performance without damage.

TROUBLESHOOTING THE GENERATOR/REGULATOR SYSTEM

NO CHARGE INDICATED ON AMMETER. If the charging circuit does not indicate a charge or if the
ammeter indicates the generator is not producing current at normal cruising RPM, the trouble can be
isolated without special tools or instrumentation. First determine that no obvious difficulty exists with the
system wiring, i.e., that the generator ARMature terminal (large one) and the generator FieLD terminal ends
are secure with no broken wires. Also check at the voltage regulator BATtery, ARMature, and FieLD
terminals that the wiring is secure, terminals are tight and that no wires are broken. Confirm that the voltage
regulator base is grounded to the firewall and that the rubber shock mounts do not insulate it from achieving
such a ground (ground strap is secure.) Confirm the generator fuse or circuit breaker is good and/or reset.
If none of these items are defective, then using a small jumper wire, MOMENTARILY connect the voltage
regulator ARMature terminal to it’s BATtery terminal while the Master switch is ON, but the engine NOT
RUNNING. This will insure that the generator and voltage regulator is polarized. Start the engine and run it
at approx. 2,000 RPM and observe if the generator is now charging.
If the generator still does not charge, then shut down the engine and with a small jumper wire connect the
generator FieLD terminal (small one) directly to ground and then start the engine and run it at 2,000 RPM.
If the generator still does not produce a current then the generator is defective and should be removed and
repaired, overhauled, or exchanged with a serviceable one.
If the generator DID produce a charge then the fault lies with the voltage regulator or the aircraft wiring
system. Next, remove the small test jumper wire from the generator FieLD terminal, and use it to connect
the Voltage REGULATOR CASE to ground as in Appdx. EL-Fig.EL-09, TEST 1 (Confirm Regulator Ground.)
Again run the engine at approx. 2,000 RPM and observe if the generator is now charging. Is so, then the
problem is a faulty regulator ground. Repair the regulator ground connection between the reg. case and
aircraft ground. If there was STILL NO CHARGE, then connect the small test jumper from the regulator
FieLD terminal to ground as in Appdx. EL, Fig. EL-09, TEST 2 (Isolate Regulator.) Start the engine and run
it at 2,000 RPM and if the generator now produces a charge then the fault lies with the voltage regulator and
it must be repaired, overhauled, or replaced. However, it there is STILL NO CHARGE then connect a small
test jumper wire between the ARMature and BATtery terminals of the voltage regulator as illustrated in
Appdx. EL, Fig EL-09, TEST 3 (Cut Out Relay.) This shorts out the Reverse Current Relay and the Current
Regulating Relay and connects the regulator directly to the battery. Run the engine again at approx. 2,000
RPM and if a charge now occurs the voltage regulator is still at fault and must be replaced, overhauled, or
repaired.
If the generator STILL did not produce a charge, then the fault lies with the aircraft wiring and/or master
switch. Use a jumper wire to connect the generator FieLD terminal directly to the voltage regulator FieLD
terminal. Again, start the engine and run it at 2,000 RPM, and if the generator now produces a charge then
the fault lies in the generator-to-master switch-to-Voltage regulator FieLD circuit. This may be due to a
faulty Master switch.
Check the wiring at the cockpit master switch terminals for continuity when the switch is actuated.
Remember that there are four terminals at the master switch. Two are used to connect the battery solenoid
(relay) to ground, and the other two (the ones we’re interested in here) connect the generator FieLD terminal
to the voltage regulator FieLD terminal. Confirm this circuit makes continuity across those two terminals
when the switch is activated. If found defective, repair or replace that switch and its associated wiring.

GENERATOR TESTS, REPAIRS AND ADJUSTMENTS. There is very little to go wrong with the Delco-
Remy generators if they are polarized and have not suffered mechanical failure such as damage from a
leaking oil seal, or failed brushes or bearings resulting in contact between the armature and the field pole
shoes, or burned windings from failed or incorrect regulator. Inspecting the generator in-situ is a simple
process and may be done with the engine cowling removed. Remove the blast-tube/brush shield-cover to
examine the brushes. When new the brushes are approximately three-quarters inch long. When the
brushes are more than half worn, they should be replaced. It is best to replace them on the workbench.
Remove the generator in accordance with the previous instructions. Examine for evidence of oil
contamination. Using a screwdriver, pry the brush holders away from the commutator and prop them in that
position with a wooden stick or tongue depressors. Be careful not to damage the brushes if they are not to
be replaced. Examine for rough or worn bearings by spinning the armature by hand and listening and/or
feeling for roughness, grinding, or points of increased friction. Remove the brush end-plate by removing the
long generator thru-bolts. Replace the brushes at this time if desired. Be certain that brush leads do not
ground against the case or springs. This is also the time to remove the armature and inspect, and/or
replace the bearings and oil seal.

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A SIMPLE GENERATOR TEST involves recognition that a generator is merely the mirror image of an
electric motor. A generator may be easily tested on the workbench by securely clamping it in a vice or
placing it upon the floor between two aircraft chocks to prevent its rolling, and placing one’s foot firmly upon
it. Using your hand, spin the armature to confirm that no interference exists between the armature windings
and the field windings/shoes.
Using a heavy gauge wire or jumper lead connect the generator FIELD terminal (small terminal) to the
generator case. Using a 12-volt storage battery (car or aircraft battery will do) and starting jumper cables,
connect one end of the red/positive cable to the POSitive terminal of the battery and the other end of the
red/positive jumper cable to the generator ARMature (large terminal). Next connect the black/negative
jumper cable to the generator case and…while firmly holding the generator from violently jumping away from
it’s location…connect the other end of the black/negative jumper cable to the NEGative terminal of the
battery. (Alternatively, a 10 Amp or larger battery charger may be used for this test.)
A healthy generator will instantly and vigorously spin its armature shaft as it becomes motorized. Bearings
may be listened to for roughness or chattering. (Limit this test to only short bursts of 5 seconds or less as
armature RPM (speed) can be exceeded and damage can subsequently occur.) See Appdx EL, Fig 07 for
diagram of this test.

TEST THE GENERATOR FOR SHORTS. A simple test for a shorted generator is to remove the two leads
from the generator FieLD and ARMature terminals. Raise the grounded brush and insert a piece of
cardboard beneath it to insulate it from the commutator. Using a Volt-Ohm meter or battery powered test
lamp, test for a short between the ARMature terminal and the generator case. If current flows with this
setup, the generator is internally shorted either in its field coils, its armature or its brush holders. Send the
generator for repair, overhaul, or exchange. See Appdx EL, Fig. 08 for a diagram of this test.

VOLTAGE REGULATOR REPAIR AND ADJUSTMENT Most owners feel ill equipped to undertake voltage
regulator repair and/or adjustment. However certain minor adjustments may be made with unsophisticated
tools.

HIGH RPM GENERATOR ON-LINE SPEED. Ordinarily the generator should come online at 1200 engine
RPM or slightly above. If this does not occur, then adjustment may be made to cause a reduction of the
Cut-Out relay spring tension. To adjust the ON-Line speed the battery must be less than fully charged. A
voltage meter reading of approximately 11.5 volts is ideal. Use a short test jumper lead as in Appdx. EL,
Fig. EL-09, TEST 3 (Cut Out Relay) and from engine idle, slowly increase engine speed until the first
indication of a charge occurs. If this speed is appreciably below the previously observed ON LINE speed,
then adjustment of the Cut Out Relay spring tension to a lower tension value may be warranted. . Most
regulators will have either a tension-type coil-spring attached to bend-able steel hooks (use needle nose
pliers), or a screw may adjust the spring tension with a screwdriver. In either case a reduction of spring
tension will lower the required engine RPM (generator voltage actually) to bring the generator on line.
Carefully note that when the Cut-out Relay brings the generator online that indicated voltage does not
decrease but should increase with any additional engine RPM. After the battery has charged above 12.6
volts, the engine should be slowly brought back to low idle and it should be determined that the Cut-out relay
opened before 800 RPM and/or before battery voltage drops below 12.6 volts. Note: On regulators that
have dual springs (two springs on each relay) then only one spring should be adjusted to increase or
decrease total tension.

ADJUSTMENT OF VOLTAGE: Examine the voltage regulator with the cover removed. Determine if the
relays are operated by direct spring tension through “hangers” or hooks, or if they are adjusted by screws.
(Caution: Some models utilize screws with left-hand threads.) Voltage is adjusted by adjusting the spring
tension of the Voltage Regulator Relay (connected to the FieLD terminal). Operating voltage is Increased by
increasing spring tension and decreased by decreasing spring tension. It is recommended that a DIGITAL
voltmeter be used if adjusting voltage for accuracy. Maximum voltage should be 13.8 volts.

ADJUSTMENT OF CURRENT (Amperage): To check this setting the voltage regulator relay must be
disabled. To do this connect a jumper across the voltage regulator contacts. Turn on landing/taxi lights and
non-avionic accessories to prevent high voltage during this test. With the engine running at 1800-2000 RPM
note the current reading on the ammeter. If the current reading is not within specifications, adjust the spring
tensions of the Current Relay (attached to the ARMature terminal) in the same manner as that outlined for
the voltage regulator relay. Increase of spring tension will INCREASE current output. NOTE: It is important
that operating accessories demand the maximum possible current in order to determine the output of the
generator;/regulator for this method to be valid. Consult your electrical loads to confirm this.

ISOLATING “Too Low or Too High CHARGE” TROUBLE. There are three common symptoms of high or
low charging difficulties.

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Battery Fully Charged, Charging Rate Low: This is an indication of a generator and regulator that are
operating satisfactorily.
Battery Fully Charged, Charging Rate High: This is an indication that the charging rate is not being
reduced when it should be. This condition will cause armature trouble, overheating of the generator,
regulator, and shorten life of avionics and light bulbs, and also cause excessive “boiling” of the battery
thereby shortening it’s life. Extreme cases will result in battery “boil-over” (sometimes accompanied by a
strong “sulfur” smell) and will cause corrosion due to spilled electrolyte. Run the engine at approx. 2,000
RPM and remove the wire from the regulator FieLD terminal. If the charging rate drops to zero the regulator
is at fault. If the high charging rate continues then either the generator is faulty or there is a grounded wire
within the generator-to-master-switch-to-regulator field circuit or the master switch is improperly wired or
defective.
Battery Low, Charging Rate Low: In order to isolate this trouble the battery and its cables must first be
confirmed to be in good condition with clean connections. Run the engine at 1,500 RPM and temporarily
ground the regulator FieLD terminal (Appdx EL, Fig EL-09, TEST 2 (Isolate Regulator)). Now increase
engine RPM to approx. 2,000 and if the charge rate now increases the trouble is with the voltage regulator.
If the charging rate remains low the trouble is either in the generator or in the wiring circuit. Generators with
rough/worn commutator or worn brushes or weak brush springs are a distinct possibility in this case. (The
Cessna 170 has a gear-driven generator, but other aircraft with belt-driven generators may also show this
symptom with loose or worn drive belts.)

The above tests are conclusive in determining whether the regulator is at fault or whether the fault lies
elsewhere. Assuming the regulator is at fault, remove it for inspection.

INSPECTING THE REGULATOR: The regulator may be inspected in such a manner that will uncover any
trouble as you progress so that by the time the preliminary tests are made you will know whether or not to
replace or repair it and not waste valuable time making final adjustments. No equipment other than a
battery and test lamp is required for these tests. From a practical standpoint, although component parts of
regulators are available for rebuilding, the labor plus parts costs associated with rebuilding is usually not
warranted. But sometimes regulators can be returned to service with minor repairs covered under these
inspection procedures.

After removing the regulator from the aircraft, remove the regulator cover and inspect for any obvious
indication such as burned components, burned paint or smell, corroded contacts or broken solder joints.
Examine the underneath side of the regulator for obviously broken or burned resistors and damaged potting.
(Aircraft regulator resistors are usually, though not always, covered with a high temp RTV or “potting”
substance to delay the onset of corrosion. Do not remove this potting material, but if it or the resistors are
observed to be damaged then good cause exists for rejecting the regulator.

If the above steps show the regulator appears to be serviceable then the next step is to make a continuity
test of the windings and points. To check the continuity you will need a battery-powered test lamp or a volt-
ohm meter with continuity test ability (Ohmmeter or continuity mode) with test leads.

Connect one lead to the ARMature terminal of the regulator and the other to the BATtery terminal. There
Should be no continuity. Close the Reverse Current relay (the one directly connected to the BATtery
terminal) and the lamp or meter should indicate continuity. If not, then replace the regulator.

Move the test lead from the BATtery terminal and connect it directly to the regulator case. Continuity should
exist. Using the aircraft battery in series with a test lamp this connection should make the voltage regulator
contacts move. If not, replace the regulator. While the connection is still made, lightly touch the Reverse
Current Relay with your finger and the contacts should close. If not, replace the regulator.

Connect one lead of the meter or battery powered test lamp to the FieLD terminal and the other to the
regulator case. Continuity should exist, but when the Voltage Regulator Relay contacts (the ones next to the
FieLD terminal) are opened the lamp or continuity should indicate an open circuit. If not, the insulators at
the relay attachment are shorted and should be replaced. Otherwise, replace the regulator.

REGULATOR Points/Contacts CLEANING: Regulators operate by rapidly opening and closing their
contact points as demanded by their relay coils. The points/contacts may become pitted and burned and
may be cleaned/adjusted with ordinary ignition point files or other small files/smoothing boards such as
those used by manicurists. Never use emery on generators or regulators. Using a magnifying glass, lift the
contact arms and examine the points for evidence of pitting/burning and remove it by gently filing and

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cleaning. Be certain to clean the points when finished with electronics cleaner or alcohol (NOT rubbing
alcohol which may contain glycerin.)

REMEMBER: The single most overlooked solution to generator/regulator problems is lack of proper
polarization, followed by improper grounds. Disconnect your battery before working on electrical circuits and
re-polarize the system after re-connection EVERY TIME.

ENGINE STARTER. The electric starter is mounted to the upper rear of the engine accessory case and is
mechanically engaged to the crankshaft gear through a starter adaptor. The starter adaptor consists of a
reduction gear (resembling a large and a small gear fixed to each other) mounted on a sliding gear-shaft.
The reduction gear incorporates a clutch mechanism that provides one-way impetus when turned by the
starter motor. This helps prevent a running engine from damaging the starter by turning it to an overspeed
condition.
When the cockpit starter handle is pulled the associated Bowden cable pulls a starter adaptor lever which in
turn forces the starter adaptor gear-shaft forward into the engine gear case. When the starter adaptor
reduction gear has meshed with the crankshaft gear, the lever comes against the starters integral mounted
electric switch. Further action of pulling the cockpit starter handle forces the lever to activate the starter
switch thereby placing battery current at the disposal of the starter motor. The starter motor rotation drives
rollers within the clutch to lock the reduction gears to each other, and with the reduction gear meshed with
the crankshaft gear the engine crankshaft is thereby turned over for starting.

STARTER ELECTRICAL CIRCUIT. The starter receives its electrical power at its integrally mounted
switch. This starter switch receives this power directly from the battery solenoid only after the solenoid is
activated by the cockpit master switch, however the starter motor itself is not energized until that integral
switch is depressed/activated via the cockpit starter handle cable and the starter adaptor lever. (Note: Two
wires are connected to the starter integral switch terminal. One is the large (#4 gauge minimum) power
supply cable from the battery solenoid. The other is a 10 or 8 gauge wire that brings electrical power directly
to the cockpit main electrical buss. It is merely using the starter switch terminal as a source of electrical
power, since that terminal is energized whenever the battery solenoid is activated by the cockpit master
switch. )

STARTER LEVER AND BOWDEN CABLE ADJUSTMENT . The starting lever should have a minimum
amount of free-play before it’s lower end contacts the sliding pinion shaft of the starter adaptor. The lever
should move the pinion shaft a minimum of 7/16” into the engine accessory case before the adjusting stud at
the levers upper end contacts the starter switch button. See Appdx. EL, FIG 10.
When the cockpit starter pull-knob is released, it should decisively return to the forward position thereby
releasing the starter lever from contact with the pinion shaft. If this does not occur, then the cable should be
repaired/replaced and/or the lever return spring at the top of the lever replaced/adjusted. Note: Some
aircraft are not equipped with this optional return spring. It may be added to all aircraft with pull starters.

AIRCRAFT LIGHTING SYSTEM.

Lighting equipment consists of landing and taxi lights, navigation lights, interior and instrument panel lights,
courtesy lights and on some aircraft, a rotating beacon and/or anti-collision light system.

LANDING AND TAXI LIGHTS. The landing and taxi lights are mounted in the leading edge of the left wing.
They are PAR-36, 100-Watt GE-4509 bulbs. A clear plastic cover provides weather protection for the lamps
and is shaped to maintain the leading edge curvature of the wing. The landing lamp is mounted on the
inboard side and adjusted to throw its beam further forward than the taxi light. A single switch controls both
lights. The lamps point of aim may be adjusted by movement of four (each) adjusting screws. Preliminary
adjustment specifications are found in Appdx..-EL.

REMOVAL AND REPLACEMENT OF TAXI/LANDING LIGHT.

a. Remove the 18 screws securing the landing light cover and remove the cover and its retainer strip.
b. Two channels hold lamps to the wing bracket. One must be removed by removing at least
two screws. (Note the number of turns each screw requires for its removal. This is
important in order not to shift the point of aim of each light)
c. Remove the two brass screws attaching the wires to the lamp. (Polarity is not important.)
d. Attach the wiring to the new lamps using their supplied brass screws/toothed washers.

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e. Re-install the lamps within their channels and install the adjustment screws by counting the same
number of turns as was required during removal. (See Appdx EL, Fig. EL-04 for preliminary
adjustments.)

NAVIGATION LIGHTS. The navigation lights are located on each wing tip and on the trailing edge of the
rudder. Some early aircraft are equipped with a flasher, which blinks the lights at a regular rate. A single
switch controls the navigation lights. If flasher-equipped, they are controlled by a single three position pull
type switch.

NAVIGATION LIGHTS FLASHER. The navigation lights flasher is mounted behind the panel on the glove
box. Early aircraft are equipped with a Narco thermal operating type flasher. The Narco flasher requires a
noise filter when used in aircraft with radio equipment. The Narco flasher is no longer available and when
replacement is required a transistor type should be installed. Some aircraft are equipped with a Van Duesen
flasher, which has a replaceable thermal type timing cartridge. The cartridge actuates a relay, which controls
the lights. If the timing cartridge fails the lights remain on, providing fail-safe operation.
The latest flasher is a transistor type, which does not use mechanical switching. The conducting properties
of the transistors are used to turn the navigation lights on and off. Two transistor circuits are used to provide
two-circuit navigation light switching. Each of the circuits triggers the other into operation. This flasher also
provides fail-safe operation. Navigation light flasher systems have been largely superceded by more
modern anti-collision light systems.

ROTATING BEACON. Rotating beacons (anti-collision lighting) was not required but was an option during
certification of the 170, but most owners have subsequently retrofitted these aircraft with such systems. The
rotating beacon is mounted to the fuselage, either/or top or bottom. Most units are either Grimes or Whelen
and mount similarly with three machine screws through the case into a mounting ring. The lens may be
removed by loosening the clamp and the bulb(s) may then be accessed for replacement. The usual difficulty
with Grimes-type units is failure of the drive motor that rotates the bulbs point of focus thereby creating the
flashing effect. When these motors fail it is usually more economical to simply replace the entire unit with a
later model. Most owners find the Whelen direct-replacement strobe system the best solution.

INSTRUMENT LIGHTS, MAP LIGHT AND DOME LIGHT. Several types of instrument light have been
utilized in Cessna 170 aircraft. . Early instrument panels incorporated individual indirect lighting within the
floating instrument panel. Some later models were equipped with dual, ultra-violet, fluorescent lamps,
however most models utilized plain incandescent lamps within an overhead console either mounted within
dedicated sockets and directed through a red lens at the instrument panel, or mounted within Grimes
“torpedo” lamp assemblies and directed at the panel. This latter system is the most common. Instrument
lighting is controlled through a rheostat mounted on the lower instrument panel. This rheostat is a
resistance-wire-wound ceramic variable resistor. By increasing/decreasing the resistance experienced by
the supply current, the rheostat varies the brilliance of the lighting.
The Map light is a “torpedo” Grimes lamp assembly mounted on the left doorpost and operated by a slide
switch. The assembly may be focused by adjusting the small knob on its upper end, and the entire unit may
be directed about the cockpit by moving it on it’s mounting pedestal. The friction of the pedestal is adjusted
by a small clamp-and-screw with a small screwdriver.
The lamps in both the Grimes Map light and the Grimes overhead Instrument lights may be replaced by
simply pulling the “torpedo-tube” from it’s base/pedestal. Only GE-67 bulbs should be used for replacement.
These bulbs are rated at 3 candlepower.

COMPASS LIGHT . The compass is internally lit and it’s light is controlled by the same rheostat as the main
instrument lighting. It consists of a T 1-1/4 size GE-330 bulb. This lamp may be replaced by removing the
false instrument panel, rotating the flat lamp-cover away from its closed position, and removing the lamp.
Reversing the procedure completes the installation.

REMOVAL AND REPLACEMENT OF NAVIGATION LIGHTS. Remove the navigation light cover being
careful not to allow the colored lens to fall and break. Using a protective cloth, grasp the navigation lamp
and push it in, rotating it 1/3 counterclockwise and pulling it from it’s socket. Examine the socket for
corrosion and dirt, removing any found with a pencil eraser. Lubricate the socket bottom VERY LIGHTLY
with Dow Corning DC-4 on a cotton swab. Do NOT overdo this. Install the new lamp being careful NOT to
touch the glass with your fingers, as any contamination will shorten the life of the lamp. If oil, or
contamination should get on the lamp, remove it with alcohol. Clean the interior of the colored lens with
glass cleaner and reinstall with its retainer/cover. Note: Early aircraft utilized the No. 1512 lamp in the
navigation light position. This is a 12/14-volt, 21-watt lamp and was the original lamp specification for the

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170. Subsequent regulations increased candlepower requirements of aircraft lighting systems and aircraft
designed to meet those regulations specify the No. 7512-12 lamp which is a 12/14 volt, 26 watt lamp of
increased candlepower. Either may be used in the 170, however the 7512-12 lamps will provide brighter
light with only minimal increase (.66A) in load of the existing system.

The rudder navigation lamp was originally Cessna PN 0433006-3, which has been superceded to a common
GE-93. This is a 12/14-volt, 15 candlepower lamp.

PITOT AND STALL WARNING HEATER CIRCUITS. All models 170 are capable of being provided a pitot
heater and stall warning heater system. The system originally designed for the aircraft is a Kollsman heated
pitot unit in accordance with Cessna Dwg. 0511051. This is a straight pitot similar to that used on C-
190/195 series aircraft. Some aircraft have been modified with “L”-type units of the AN-5812 types found on
later series Cessna aircraft. The electrical circuits for both systems are identical. Power is supplied via a
current limiter and a switch usually located on the instrument panel, to the heater elements contained within
the pitot tube and within the lift detector assembly. Each heater element draws approximately 5 amperes.
Both heated pitot types and lift detector assembly units heaters are field replaceable ceramic units
containing resistance wire. The only indication of failure of the heating element is loss of heat at the unit
itself coupled with a slightly lower amperage indication on the ammeter. This drop in electrical requirement
is only slightly observable. In rare instances fracturing of the heating units ceramic body may cause current
limiter activation (blown fuse/activated circuit breaker.) Pre-flight testing of the systems is imperative when
freezing precipitation is anticipated in flight. The system is tested during pre-flight by turning the Master
switch ON, turning the pitot/static system heat switch ON for 30 seconds, and feeling the units by hand for
generated heat. This test period must be kept to a short duration to avoid overheating the units on the
ground. Caution: The pitot/static heaters are capable of extremely high temperatures. Caution must be
taken to avoid serious burns and overheating of the units which will cause damage. Avoid static testing of
the units for periods exceeding 2 minutes.

REMOVAL AND REPLACEMENT OF PITOT/STATIC HEATERS. The pitot tube has a nose-cone that is
removable by unscrewing it from the main body, exposing the ceramic heating unit surrounding the inner
pitot tube. Remove the local inspection cover and disconnect the wiring terminals from the harness. Slide
the heater unit from the pitot tube. Installation of the replacement unit is the reverse procedure.
The stall warning lift detector heating unit is removed by first removing the lift detector from the leading edge
of the wing. First note the relative position of the lift detector on the leading edge of the wing so as to avoid
the necessity of flight-testing the aircraft to correct an altered stall warning speed. A grease pencil may be
handy to mark the position of the lift detector prior to removal. Remove the local inspection cover and the
four PK screws mounting the lift detector to the wing. Locate the tubular, ceramic heater element held by a
small clamp inside the lift detector assembly, and remove it by loosening the clamp. This will require only a
screwdriver and a 5/16” wrench or socket. Remove the two heater wire leads from the harness terminals.
There is no polarity preference of this harness. Installation of the replacement heater element unit is the
reverse procedure.
Caution: Some pitot systems utilize poly tubing to carry air pressure to the cockpit instrumentation. Be
certain not to allow any resistance wiring to come into contact with the poly tubing or the tubing may melt
giving erroneous airspeed indications to the pilot. Use only the correct wiring terminal ends and isolate any
poly tubing from wiring.

STALL WARNING SYSTEM. All models 170 were capable of having a stall warning system installed in
accordance with Cessna Dwg. 0511062. It is required equipment on the 170 B-model. The system consists
of a current limiter (shared with the turn-and-bank indicator) a warning horn/lamp unit located on the
instrument panel, a lift detector located on the left wing leading edge, and associated wiring. Electrical
power (12/14 volts DC) is supplied to the warning horn/light via the current limiter (fuse/circuit breaker)
whenever the Master Switch is ON. After the voltage passes through the warning unit it finds it’s ground out
on the wing, by passing first through the Lift Detector switch (operated by relative wind against the lift
detector vane on the leading edge) then to a nearby ground through a short wire terminating at a screw in
the airframe, thereby completing the circuit. When the lift detector completes the circuit, the warning horn
sounds and the indicator lamp illuminates. The lamp bulb is a GE-1816, 12/14volt .33 amp,
T-3 ¼ miniature bayonet.
The lift detector position on the leading edge of the wing is adjustable and should be adjusted to give
approximately 5 M.P.H. warning above a stall in level flight.

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TROUBLESHOOTING THE ELECTRICAL SYSTEM There are two simple and inexpensive tools with which
your electrical system troubleshooting may be performed. An inexpensive Volt/Ohm meter or MultiMeter is
the single, most useful tool you can have to troubleshoot your electrical system. Along with the MulitMeter,
an inexpensive set of heavy-duty jumper wires or “test leads” with “alligator” style clips can make any
troubleshooting an easy if not downright pleasant task, instead of a puzzling one. These items may be
purchased at local hardware and electronic shops at a total cost of less than ½ hour technician shop rates.
See Appdx EL, Fig 05 and Fig 06.

The aircraft electrical system is predominately a one-wire system, meaning that the airframe structure
provides the ground path for most circuits. As a result, the single most common fault will lie with that ground
return path. Whichever lighting circuit is causing difficulty may be diagnosed using these tools and the
following procedure.
Anytime a lighting (or other) circuit fails to operate properly the first two items to check are 1) proper, clean,
and secure ground connection, and 2) proper, clean, and secure positive connection and circuit. Item
number one will eliminate many failures of various electrical system failures. Item number two will usually
correct any outstanding defect. The most expeditious method to test these items is with the MultiMeter and
test leads.
Since one of the most frustrating systems to diagnose can be the stall warning horn/light in later aircraft, lets
use that system as an example.

Example Problem: The horn/light does not operate when the Master switch is ON and the Lift Detector is
raised.
System Description: Electrical power (12/14 volts DC) is supplied to the warning horn/light via a fuse/circuit
breaker whenever the Master Switch is ON. After the voltage passes through the warning unit it finds it’s
ground out on the wing, by passing first through the Lift Detector switch (operated by relative wind against
the lift detector vane on the leading edge) then to a nearby ground through a short wire terminating at a
screw in the airframe, thereby completing the circuit.
Procedure: Whenever troubleshooting any circuit FIRST re-read the system description to better understand
its operation. Second, always check the current limiter (fuse or circuit breaker.) Do not accept a visual
check. (Fuses may look fine but still fail a continuity check just as circuit breakers may look fine but also
have internal failures.) With the Master switch OFF: Use the continuity mode/function of your MultiMeter
across the input side and exit side of the fuse holder or C.B. to confirm continuity across the unit. If no
continuity then repair/replace the current limiter. Caution: Do not accidentally allow voltage to flow through
the MultiMeter in the continuity mode or the meter will be damaged. This is why we checked it with the
Master switch OFF.
Third, Gain access to the Lift Detector thru the left wing nearby inspection hole. Turn the Master switch OFF
and using the MultiMeter in it’s OHMmeter function (or it’s continuity test mode) check that the lift detector
switch-to-ground-wire has continuity to ground by placing one meter test-lead against the switch terminal and
the opposite lead against the airframe at a convenient location. No continuity? Problem solved. Repair the
lift detectors short lead to ground by replacement or cleaning of the terminal ends.
Continuity OK? Turn the Master switch ON and using the MultiMeter DC voltage measurement mode (15
volt range) place the positive tester lead against the lift detectors terminal leading from the cockpit, and
the negative lead to the other terminal and measure the available voltage.
Voltage near 12 volts? Use an alligator test-lead to ground the lift detectors terminal leading from the
cockpit to a convenient location on the airframe. Stall Warning Horn/Light operate satisfactory? Problem
lies with lift detector switch. Horn/Light still does not operate? Problem lies with Warning unit.
Voltage nonexistent at lift detector cockpit terminal? Problem lies with warning horn/light unit or wiring circuit
from main electrical buss. Use Multimeter at the warning unit and test for voltage at both terminals on rear
of warning unit between each terminal and airframe ground.
Voltage OK (approx 12 volts) at BOTH terminals? Connect alligator lead to warning unit terminal leading to
wing and a nearby airframe ground. Warning unit operate satisfactory? Problem lies with wiring from
warning unit to lift detector. Warning unit still not operate? Problem lies with warning unit. Remove it for
bench test and repair or replacement.
Voltage OK at one warning unit terminal but not both? Problem lies with warning unit.
Voltage nonexistent at either terminal? Problem lies with circuit from main buss. Check for power at fuse or
circuit breaker. Repair/replace faulty fuse/circuit breaker or defective wiring from the main buss.

This example demonstrates how a MultiMeter and set of test leads/alligator leads can solve most electrical
system troubleshooting problems. This procedure will work similarly for most systems.

1. Re-read the system description.

2. Test the current limiter (fuse/circuit breaker) for power and continuity.

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3. Test the circuits final ground at the end of the circuit.

4. Test the primary item/accessory both for power and with a temporary ground.
5. Test/eliminate each connection/item between the final ground and the current limiter.
6. Murphy’s 6 th rule of electricity: Never assume a part (not even a new one) is faultless purely by
visual inspection. TEST IT!

ELECTRIC WIRE

GENERAL. Aircraft service imposes severe environmental conditions on electric wire. To assure
satisfactory service, inspect the wire at regular intervals for abrasions, defective insulation, condition of
terminal posts, and buildup of corrosion under or around swaged terminals.

AIRCRAFT ELECTRICAL WIRE. Use aircraft quality wire. Correct wire selection is dependent upon
knowledge of current requirements, operating temperatures, and environmental conditions involved in
the particular installation. (Authors Note: The Cessna 170 was originally designed and built utilizing
MIL-W-5086A wire or previous standards. Some of these wires were insulated with PVC (poly-vinyl-
chloride) that emits poisonous gases when ignited. While adequate for continued use in circuits of good
condition, these standards are no longer acceptable for replacement, repair, and/or new installations.
The most common aircraft electrical wire in modern applications is MIL-W-22759/16 that is constructed
of a copper conductor that has been tinned and insulated with ETFE (ethylene/tetraflourethelene.) This
is commonly referred to by its commercial moniker “Tefzel”. )

a. Conductors. Copper conductors are coated to prevent oxidation and to facilitate soldering. Tinned
copper or aluminum wire is generally used in installations where operating temperatures do not
exceed 221°F (105°C). Aluminum wire shall be restricted to size 6 and larger. Aluminum wire shall
neither be directly attached to engine mounted accessories nor installed in other areas of severe
vibration. It shall not be installed where frequent connections and disconnections are required. All
installations of aluminum wire shall be relatively permanent. Aluminum wire shall not be used
where the length of run is less than 3 feet, in areas where corrosive fumes exist. It is not
recommended for use in communication or navigation systems. Note: Aluminum and Silver
coated wire was never used in Cessna 170s originally, but this information is being supplied in the
event any is found subsequently installed.

SILVER COATED WIRE is sometimes found and used where temperatures do not exceed
392°F (200°C). CAUTION: An inflammability hazard exists when silver or silver plated conductors
impressed with direct current potential are saturated with water/glycol solutions (anti -freeze/de-icing fluids.)
The positive (cathodic) may be of any conductive material. If the anode and cathode are in sufficient
proximity to permit current (in the milliampere range) to flow through a glycol solution that has contaminated
the space between the two conductors, oxidation is rapid and an intensely hot flame appears. This
phenomenon is not known to occur when the anode is other than silver or when the impressed voltage is
alternating current.

Nickel coated copper wire is sometimes used for temperatures up to 500°F (260°C). Nickel coated wire is
more difficult to solder than tinned or silver coated wire, but with proper techniques, satisfactory connections
can be made.

b. Insulation. Silicone rubber is rated at 392°F (200°C), is highly flexible, and self -extinguishing
except in vertical runs. Polytetrafluoroethylene (TFE Fluorocarbon) is widely used as high
temperature insulation. It will not burn, but will vaporize when exposed to flame. It is resistant to
most fluids. Fluorinated ethylene propylene (FEP Fluorocarbon) is rated at 392° F (200°C), but will
melt at higher temperatures. Other properties of FEP are similar to TFE.
c. Thermal and Abrasion Resistant Materials. Glass braid has good thermal and abrasion qualities
but moisture absorption is high. Asbestos and other minerals provide high temperature and flame
resistance, but are highly absorbent. Moisture absorption is reduced by use of silicone rubber, TFE,
or other saturants. Nylon is widely used in low temperature wires for abrasion and fluid resistance.
Polyimide, a new material, has excellent thermal and abrasion resistant characteristics.
d. Wire Selection. When selecting wire, refer to structural and environmental characteristics. Wire
normally used for chassis wiring, in enclosed areas, or in compact wire harnesses protected by
molded or braided coverings, usually has low abrasion resistance. Wire used to interconnect units,

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or in long, open runs in the airframe, is designed to withstand normal aircraft environment without
sleaving, jacketing, or other protection. Care must be taken in making all installations because no
wire insulation or jacketing will withstand continuous scuffing or abrasion.

Authors Note: The following wire sizes information is based upon wire meeting MIL -W-22759/16 (tin-
plated copper with ETFE insulation) in lengths of 25 feet or less for non-intermittent circuits at 14
volts.

AMPS WIRE GAUGE (SIZE)

2 Amps or less 20 Gauge

3 Amps 18 Gauge
4 Amps 16 Gauge
5 Amps 14 Gauge
7 Amps 14 Gauge
10 Amps 10 Gauge
15 Amps 10 Gauge
20 Amps 10 Gauge
30 Amps 8 Gauge
50 Amps 6 Gauge
70 Amps 4 Gauge

Battery cables are used for starting only intermittently and therefore may be somewhat smaller gauge than
the 300 Amp and greater currents sometimes generated during start. Therefore, to save weight and costs
Cessna originally provided battery cables of 4 gauge. It is recommended that larger gauges be considered
for replacement purposes. Sizes 1 or 1/O is recommended for the short runs from battery to solenoid to
starter terminal lug, and from battery negative terminal to aircraft ground as well as from the Ground
receptacle to the battery solenoid.

SPLICES IN ELECTRIC WIRE. Splicing of electric wire should be kept to a minimum and avoided entirely in
locations subject to extreme vibrations. Individual wires in a group or bundle may be spliced provided the
completed splice is located so it can be periodically inspected. Stagger splices so the bundle does not
become excessively enlarged. Many types of aircraft splice connectors are available for use when splicing
individual wires. Use of the self-insulated splice connector is preferred; however, a noninsulated splice
connector may be used provided the splice is covered with plastic sleaving (or heat –shrink) which is
secured at both ends. Solder splices may be used; however, they are particularly brittle and not
recommended.
a. There should be not more than one splice in any one wire segment between any two
connectors or other disconnect points, except as allowed by c and g below.
b. Splices in bundles should be staggered and shall not increase the size of the bundle so as
to prevent the bundle from fitting in its designated space or cause congestion which will
adversely affect maintenance.
c. Splices shall not be used to salvage scrap lengths of wire.
d. Splices shall not be used within 12 inches of a termination device, except for e below.
e. Splices may be used within 12 inches of a termination device when attaching to the pigtail
spare lead of a potted termination device, or to splice multiple wires to a single wire, or to
adjust the wire sizes so that they are compatible with the contact crimp barrel sizes.
f. The application of splices shall be under design control and shall be authorized by engineering
drawings.
g. Splices may be used to repair manufactured harnesses or installed wiring when approved by
engineering.

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OPEN WIRING. Electric wiring is often installed in aircraft without special enclosing means. This practice is
known as open wiring and offers the advantages of ease of maintenance and reduced weight.
a. Wire Bundles. To simplify maintenance and to minimize the damage that may result from a single fault,
limit the number of wires in the run. Shielded wire, ignition wire, and wires which are not protected by a
circuit breaker or fuse are usually routed separately. Avoid bending radii less than 10 times the outer
diameter of the bundle to prevent excessive stresses on the wire insulation.

b. Insulating Tubing. Soft insulating tubing (spaghetti) cannot be considered as mechanical protection
against external abrasion of wire, since at best it provides only a delaying action. Use conduit or ducting
when mechanical protection is needed.
c. Clamping of Wire Bundles. Use clamps lined with nonmetallic material to support the wire
bundle along the run. Tying may be used between clamps, but should not be considered as
a substitute for adequate clamping. Adhesive tapes are subject to age deterioration and, therefore are not
acceptable as a clamping means.

Authors Note: Do not use automotive electrical tapes, wires, or terminals in aircraft due to lack of proper
tinning and use of PVC insulation materials.

d. Separation from Flammable Fluid Lines. An arcing fault between an electric wire and a metallic
flammable fluid line may puncture the line and result in a serious fire. Consequently, make every effort to
avoid this hazard by physical separation of the wire from lines or equipment containing oil, fuel, hydraulic
fluid, or alcohol. When separation is impractical, locate the electric wire above the flammable fluid line and
securely clamp to the structure. In no case, should a wire be supported by a flammable fluid line.

HEAT PRECAUTIONS. Separate wires from high temperature equipment, such as resistors, exhaust
stacks, heating ducts, etc., to prevent insulation breakdown. Insulate wires that must run through hot areas
with a high temperature insulation material such as fire sleeve meeting TSO C53a or C75, fiberglass, or
Teflon. Avoid high temperature areas when using cables having soft plastic insulation such as polyethylene
because these materials are subject to deterioration and deformation at elevated temperatures. Many
coaxial cables have this type of insulation.

PROTECTION AGAINST CHAFING. Protect wire and wire groups against chafing or abrasion as damaged
insulation may result in short circuits, malfunctions, or inoperative equipment. Support wire bundles using
MS-21919 cable clamps. When clamped in position, if there is less than ¼ inch clearance between a
bulkhead cutout and the wire bundle, install a suitable grommet. The grommet may be cut at 45-degree
angle to facilitate installation provided it is cemented in place and the slot is located at the top of the cutout.

STRIPPING INSULATION. Attachment of wire to connectors or terminals requires the removal of insulation
to expose the conductors. This practice is commonly known as stripping. When performing the stripping
operation, remove no more insulation than is necessary. Stripping may be accomplished in many ways;
however, the following basic principles should be practiced: Make sure all cutting tools used for stripping
are sharp. When using special wire stripping tools, adjust the tool to avoid nicking, cutting, or otherwise
damaging the strands.

TERMINALS. Terminals are attached to the ends of electric wires to facilitate connection of the wires to
terminal strips or items of equipment. The tensile strength of the wire to terminal joint should be at least
equivalent to the tensile strength of the wire itself, and its resistance negligible relative to the normal
resistance of the wire. Terminals specifically designed for use with the standard sizes of aircraft wire are
available through normal supply channels. Haphazard choice of commercial terminals may lead to
overheated joints, vibration failures, and corrosion difficulties.

a. Solder Terminals. For most applications, soldered terminals have been replaced by solderless
terminals. The solder process has disadvantages that have been overcome by use of the solderless
terminals. A few of these disadvantages are listed as follows:

(1) A more skilled operator is required.

(2) A corrosive flux may be used causing the joint to deteriorate.
(3) Maintenance is extremely difficult.
(4) The wire strands are stiffened by the solder and become more susceptible to breakage due
to vibration.
(5) The wire insulation may be charred during the soldering process.

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b. Solderless Terminals. The terminal manufacturer will normally provide a special crimping or swaging
tool for joining the solderless terminal to the electric wire. Aluminum wire presents special difficulty in that
each individual strand is insulated by an oxide coating. This oxide coating must be broken down in the
crimping process and some method employed to prevent its reforming. In all cases, follow the terminal
manufacturer’s instructions when installing solderless terminals.

c. Terminal Strips. Wires are usually joined at terminal strips. Use a terminal strip fitted with barriers to
prevent the terminals on adjacent studs from contacting each other. Studs must be anchored against
rotation. When more than four terminals are to be connected together, use two or more adjacent studs and
mount a small metal bus across the studs. In all cases, the current is to be carried by the terminal contact
surfaces and not by the stud itself. Replace defective studs with studs of the same size and material as
terminal strip studs of the smaller sizes may shear due to over tightening the nut. Assure that the
replacement stud is securely mounted in the terminal strip and that the terminal securing nut is tight. Mount
terminal strips in such a manner that loose metallic objects cannot fall across the terminals or studs. It is
good practice to provide at least one spare stud for future circuit expansion, or in case a stud is broken.
Inspect terminal strips which provide connection of radio and electronic systems to the aircraft electrical
system for loose connections, metallic objects which may have fallen across the terminal strip, dirt and
grease accumulation, etc. Such condition can cause arcing that may result in a fire.

d. Terminal lugs. Wire terminal lugs shall be used to connect wiring to terminal block studs or equipment
terminal studs. No more than four terminal lugs or three terminal lugs and a bus shall be connected to any
one stud (total number of terminal lugs per stud includes a common bus bar joining adjacent studs. Four
terminal lugs plus a common bus bar thus are not permitted on one stud). When the terminal lugs attached
to a stud vary in diameter, the greatest diameter shall be placed o n the bottom and smallest diameter on top.
Terminal lugs shall be selected with a stud hole diameter that matches the diameter of the stud. Tightening
terminal connections shall not deform the terminal lugs of the studs. Terminal lugs shall be so positioned
that bending of the terminal lug is not required to remove the fastening screw or nut, and movement of the
terminal lugs will tend to tighten the connection.

e. Copper terminal lugs. Solderless crimp style copper wire terminal lugs shall be used. Terminal lugs
shall conform to MIL-T-7928. Spacers or washers are not permitted between the
tongues of terminal lugs.

f. Aluminum terminal lugs. Aluminum terminal lugs conforming to MIL-T-7099 (MS-25435, MS-25436,
MS-25437 and MS-25438) shall be crimped to aluminum wire only. The tongue of the aluminum
terminal lugs or the total number of tongues of aluminum terminal lugs when stacked, shall be
sandwiched between two MS-25440 flat washers when terminated on terminal studs. Spacers or
washers are not permitted between the tongues of terminal lugs. Special attention shall be given to
aluminum wire and cable installation to guard against conditions that would result in excessive
voltage drop and high resistance at junctions that may ultimately lead to failure of the junction.

Examples of such conditions are improper installation of terminals and washers, improper torsion "torquing"
of nuts, and inadequate terminal contact areas.

g. Class 2 terminal lugs. Class 2 terminal lugs conforming to MIL-T-7928 may be used for installation by
contractors, provided that in such installations Class 1 terminal lugs are adequate for replacement without
rework of installation or terminal lugs. Class 2 terminal lugs shall be the insulated type unless the conductor
temperature exceeds 105 degrees C in which case un-insulated terminal lugs shall be used. Parts lists shall
indicate the appropriate Class 1 terminal lugs to be used for service replacement of any Class 2 terminal
lugs installed.

ATTACHMENT OF TERMINALS TO STUDS. Electrical equipment malfunction has frequently been

traced to poor terminal connections at terminal boards. Loose, dirty, or corroded contact surfaces
will produce localized heating which may ignite nearby combustible materials or overheat adjacent
wire insulation to the smoking point.

BONDING JUMPER INSTALLATIONS. Make bonding jumpers as short as practicable, and install in such
a manner that the resistance of each connection does not exceed 0.003 ohm. The jumper must not interfere
with the operation of movable aircraft elements, such as surface controls, nor should normal movement of
these elements result in damage to the bonding jumper.

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a. Bonding Connections. To assure a low resistance connection, remove nonconducting finishes; such as
paint and anodizing films, from the attachment surface to be contacted by the bonding terminal. Do not
ground electric wiring directly to magnesium parts.

b. Corrosion Prevention. Electrolytic action may rapidly corrode a bonding connection if suitable
precautions are not taken. Aluminum alloy jumpers are recommended for most cases; however, use copper
jumpers to bond together parts made of stainless steel, cadmium plated steel, copper, brass, or bronze.
Where contact between dissimilar metals cannot be avoided, the choice of jumper and hardware should be
such that corrosion is minimized, and the part likely to corrode would be the jumper or associated hardware.
At locations where finishes are removed, apply a protective finish to the completed connection to prevent
subsequent corrosion.

c. Bonding Jumper Attachment. Avoid the use of solder to attach bonding jumpers. Bond tubular
members by means of clamps to which the jumper is attached. Proper choice of clamp material will
minimize the probability of corrosion.

d. Ground Return Connection. IMPORTANT: When bonding jumpers carry substantial ground return
current, determine that the current rating of the jumper is adequate, and that a negligible voltage drop is
produced. This condition is frequently found at engine-to-airframe or firewall jumpers where substantial
return current is found during starting.

Current Limiters The Cessna 170 was originally equipped with fuses which protect each electrical circuit.
These fuses are usually located in the lower instrument fuse-panel and are size AGS glass-tube fuses
measuring 9/32” X 1 1/4”. AGS is a size no longer used by manufacturers and sometimes difficult to find
therefore many owners substitute AGC fuses in similar amperages. The AGC series is not an exact fit (due
to ¼” diameter) and their performance is sometimes intermittent due to reduced contact area within the
holder.
The original Cessna part number (0411023) is the AGS series fuse with a following “dash” number indicating
the amperage. If this PN is searched for results may be frustrating. The modern equivalent Cessna part
number is: S1091 followed by the appropriate dash number.
(Example S1091-2 is a 10 Amp fuse. S1091–3 is a 15A fuse, S1091-4 is 25A, S1091-6 is 20A and so on as
indicated in the Illustrated Parts Catalog) Your Cessna distributor should have them in stock. Some large
fuse supply companies also carry the AGS fuses.
Many aircraft have been modified to utilize circuit breakers of the “pull” type. Examples are the 7277 and
7271 series of “Klixon” brand devices or the W23 series of Potter and Brumfield brand. Circuit breakers
have the advantage of being quickly re-settable, an especially useful feature at night. Caution: Resetting
circuit breakers in flight should be done with care. Most guidelines either suggest or require that a circuit
breaker be re-set only ONCE while inflight. Similar procedures should be used for fuses. Never substitute a
larger capacity fuse for a failed fuse.
Note: Current limiters are not intended to protect devices such as radios, lighting, etc. Current limiters are
intended to protect the wiring from damage or fire due to excessive load demand such as may occur during
shorts or failed equipment. A current limiters size is determined by the wire gauge and its intended purpose.
A 10% over-size is usually specified for current limiters in order to accommodate power surge requirements.
Replace current limiters only with approved, aircraft quality units of similar capacity.

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WIRING CIRCUIT DIAGRAMS AND IDENTIFICATION

The following pages contain diagrams and wiring identification information as the
B-model Cessna 170 aircraft was originally designed. They are offered for the
purpose of providing original documentation for the noted aircraft and as a
resource guide for previous models. Subsequent alterations and modifications
may have significantly changed these. Many aircraft have specific electrical
blueprints appropriate to that serial number, and any new electrical work
performed should include the production of appropriate electrical schematics
which should be kept with the aircraft maintenance records.

Be especially cautious when replacing equipment and wiring in the aircraft.

Original wiring gauges, switches, and current limiters may not be adequate for
subsequently added equipment. Example: An aircraft originally equipped with a
12 ampere generator was equipped with a 10 gauge wire in the main electrical
supply buss circuits and in the generator-to-battery circuit. If the aircraft was
subsequently modified with the addition of a 35 ampere generator and regulator
that wiring may be insufficient and the associated current limiting device will also
be insufficient. Alternator conversions will exacerbate this problem.

Therefore, confirm that your aircraft is adequately equipped with regard to wire
sizes and current limiting devices (fuses and/or circuit breakers.) Consult the
Electric Wire section of this manual and/or FAR 43.13-1A, Acceptable Methods,
Materials, and Techniques when utilizing these diagrams.
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ELECTRICAL SCHEMATIC SNs 20267 THRU 25372

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Electrical System

ELECTRICAL SCHEMATIC SNs 25373 THRU 26995

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ELECTRICAL SCHEMATIC SNs 26996 AND ON

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INSTRUMENT PANEL - ELECTRICAL SCHEMATIC SNs 26996 AND ON

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Appdx. EL 1

Fig. EL-01 Ground Service Plug and Early Battery Solenoid (Relay)

Fig. EL-02 Late Style Battery Solenoid (Relay)

Cessna 170 Service Manual
Electrical System
Appdx. EL-2

Fig. EL 03, Battery Solenoid (Relay) and Master Switch Circuit

Fig. EL-04, Landing Light Adjustment

Land Lamp Taxi Lamp

Position 1 = .56” Position 2 = .62” Position 5 = .56” Position 6 = .90”

Position 3 = .62” Position 4 = .68” Position 7 = .50” Position 8 = .84”

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Appdx. EL-3

Fig. EL-05, MultiMeter

Fig. EL-06, Test Leads

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Appdx EL-4

Fig. EL-07. GENERATOR MOTOR TEST SCHEMATIC

NEGATIVE BATTERY CABLE TO CASE POSITIVE BATTERY CABLE TO ARM TERMINAL
FIELD TERMINAL TO CASE

FIG. EL-08, TEST FOR GENERATOR SHORT

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Electrical System
Appdx. EL 5

FIG. EL-09, VOLTAGE REGULATOR

CIRCUIT BREAKER
OR REVERSE CURRENT RELAY
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Electrical System
Appdx. EL 6

TEST 1 (CONFIRM REGULATOR GROUND)

TEST 2 (ISOLATE REGULATOR)

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Appdx EL 7

Fig. EL-09 cont’d, VOLTAGE REGULATOR

TEST 3 (CUT OUT RELAY)

Fig. EL-10. STARTER CABLE/LEVER ADJUSTMENT