C210

0.8 PPL 0.8 PPL/WHT

RH REAR
MARKER
LIGHT

Figure 11–1 The center wire is a solid color wire, meaning

0.8 BLK
that the wire has no other identifying tracer or stripe color.
The two end wires could be labeled “BLU/WhT,” indicating a Figure 11–2 Typical section of a wiring diagram. Notice
blue wire with a white tracer or stripe. that the wire color changes at connection C210. The “0.8”
represents the metric wire size in square millimeters.

Wiring SChemaTiCS abbreviation

BRN
color
Brown
and SymbolS BLK Black
GRN Green
WHT White
Terminology The service manuals of automotive
PPL Purple
manufacturers include wiring schematics of every electrical
PNK Pink
circuit in a vehicle. A wiring schematic, sometimes called a
TAN Tan
diagram, shows electrical components and wiring using symbols
BLU Blue
and lines to represent components and wires. A typical wiring
YEL Yellow
schematic may include all of the circuits combined on several
ORN Orange
large foldout sheets, or they may be broken down to show
DK BLU Dark blue
individual circuits. All circuit schematics or diagrams include:
LT BLU Light blue
■■ Power-side wiring of the circuit DK GRN Dark green
■■ All splices LT GRN Light green
■■ Connectors RED Red
GRY Gray
■■ Wire size
VIO Violet
■■ Wire color
■■ Trace color (if any) Chart 11–1
■■ Circuit number Typical abbreviations used on schematics to show wire color.
Some vehicle manufacturers use two letters to represent a wire
■■ Electrical components color. Check service information for the color abbreviations used.
■■ Ground return paths
■■ Fuses and switches Wire Size Wire size is shown on all schematics.
● Figure 11–2 illustrates a rear side-marker bulb circuit
diagram where “0.8” indicates the metric wire gauge size in
CirCuiT informaTion Many wiring schematics include
square millimeters (mm2) and “PPL” indicates a solid purple wire.
numbers and letters near components and wires that may
The wire diagram also shows that the color of the wire
confuse readers of the schematic. Most letters used near or on
changes at number C210. This stands for “connector #210” and
a wire identify the color or colors of the wire.
is used for reference purposes. The symbol for the connection
■■ The first color or color abbreviation is the color of the
can vary depending on the manufacturer. The color change from
wire insulation.
purple (PPL) to purple with a white tracer (PPL/WhT) is not impor-
■■ The second color (if mentioned) is the color of the stripe tant except for knowing where the wire changes color in the cir-
or tracer on the base color. ● see Figure 11–1. cuit. The wire gauge has remained the same on both sides of the
Wires with different color tracers are indicated by a slash connection (0.8 mm2 or 18 gauge). The ground circuit is the “0.8
(/) between them. For example, GRN/WhT means a green wire BLK” wire. ● Figure 11–3 shows many of the electrical and
with a white stripe or tracer. ● see charT 11–1. electronic symbols that are used in wiring and circuit diagrams.

144 Ch AP TER 11
 POSITIVE DIODE

NEGATIVE ZENER DIODE

BATTERY LIGHT-EMITTING
DIODE (LED)

OR OR CAPACITOR
GROUND

FUSE MOTOR

CIRCUIT BREAKER
CASE GROUNDED

RESISTOR

SOLID BOX
VARIABLE RESISTOR REPRESENTS
ENTIRE
COMPONENT
VARIABLE RESISTOR
(POTENTIOMETER) DASHED LINE
REPRESENTS PORTION
(PART) OF A
COMPONENT
BULB (LAMP)

NORMALLY OPEN
(N.O.) RELAY
DUAL-FILAMENT
BULB

NORMALLY CLOSED
MALE TERMINAL (N.C.) RELAY

FEMALE TERMINAL

CONNECTOR

DELTA ( ) WINDINGS
SPLICE

WIRES NOT
ELECTRONICALLY
CONNECTED

WYE (Y) WINDINGS

COIL WINDING

COIL WITH STEEL

LAMINATIONS
Figure 11–3 Typical electrical and electronic symbols used in automotive wiring and circuit diagrams.

WI RI N G SCh E MAT I C S AN D C I R C U IT T E S T IN G 145

 TO ELECTRICAL
TO BATTERY
COMPONENT  
Figure 11–4 In this typical connector, note that the positive Figure 11–5 The symbol for a battery. The positive plate
terminal is usually a female connector. of a battery is represented by the longer line and the negative
plate by the shorter line. The voltage of the battery is usually
Tech Tip stated next to the symbol.

Read the Arrows OR

Wiring diagrams indicate connections by symbols
that look like arrows. ● see Figure 11–4. Figure 11–6 The ground symbol on the left represents an
Do not read these “arrows” as pointers showing earth ground. The ground symbol on the right represents a
chassis ground.
the direction of current flow. Also observe that the
power side (positive side) of the circuit is usually the
HOT AT ALL TIMES
female end of the connector. If a connector becomes
IGNITION
disconnected, it will be difficult for the circuit to START
SWITCH
become shorted to ground or to another circuit
because the wire is recessed inside the connector. B C2

0.5 YEL 5

SChemaTiC SymbolS B3 C202

0.5 YEL 5
In a schematic drawing, photos or line drawings of actual com-
ponents are replaced with a symbol that represents the actual
component. The following discussion centers on these sym- B6 C101
bols and their meanings.

baTTery The plates of a battery are represented by long

and short lines. ● see Figure 11–5. Figure 11–7 Starting at the top, the wire from the ignition
The longer line represents the positive plate of a battery switch is attached to terminal B of connector C2, the wire is
and the shorter line represents the negative plate. Therefore, 0.5 mm2 (20 gauge AWG), and is yellow. The circuit number is 5.
The wire enters connector C202 at terminal B3.
each pair of short and long lines represents one cell of a bat-
tery. Because each cell of a typical automotive lead–acid bat-
■■ circuit numbers. Each wire in part of a circuit is labeled
tery has 2.1 volts, a battery symbol showing a 12 volt battery
with the circuit number to help the service technician
should have six pairs of lines. however, most battery symbols
trace the wiring and to provide an explanation of how the
simply use two or three pairs of long and short lines and then
circuit should work.
list the voltage of the battery next to the symbol. As a result, the
battery symbols are shorter and yet clear, because the voltage
■■ Wire color. Most schematics also indicate an abbrevia-
is stated. The positive terminal of the battery is often indicated tion for the color of the wire and place it next to the wire.
with a plus sign (+), representing the positive post of the bat- Many wires have two colors: a solid color and a stripe
tery, and is placed next to the long line of the end cell. The color. In this case, the solid color is listed, followed by
negative terminal of the battery is represented by a negative a dark slash (/) and the color of the stripe. For example,
sign (-) and is placed next to the shorter cell line. The negative Red/Wht would indicate a red wire with a white tracer.
battery terminal is connected to ground. ● see Figure 11–6. ● see Figure 11–7.
■■ Terminals. The metal part attached at the end of a wire
Wiring Electrical wiring is shown as straight lines and with is called a terminal. A symbol for a terminal is shown in
a few numbers and/or letters to indicate the following: ● Figure 11–8.
■■ Wire size. This can be either AWG, such as 18 gauge, or ■■ splices. When two wires are electrically connected, the
2
in square millimeters, such as 0.8 mm . junction is shown with a black dot. The identification

146 Ch AP TER 11
 B
300–399
INSIDE PASSENGER
200–299 COMPARTMENT 400–499
100–199 TRUNK
UNDER
UNDER HOOD
A DASH

Figure 11–11 Connectors (C), grounds (G), and splices (S)

Figure 11–8 The electrical terminals are usually labeled are followed by a number, generally indicating the location in
with a letter or number. the vehicle. For example, G209 is a ground connection located
under the dash.

SPLICE

Figure 11–9 Two wires that cross at the dot indicate that 32 BLK 1
the two are electrically connected. BATTERY

WIRES NOT
ELECTRONICALLY
GROUND
CONNECTED DISTRIBUTION BLK
SCHEMATICS
Figure 11–10 Wires that cross, but do not electrically IN WIRING 50
SYSTEMS
contact each other, are shown with one wire bridging
over the other.
32 BLK 50

of the splice is an “S” followed by three numbers, such

as S103. ● see Figure 11–9. When two wires cross in
a schematic that are not electrically connected, one of
the wires is shown as going over the other wire and does
G305
not connect. ● see Figure 11–10.
Figure 11–12 The ground for the battery is labeled
■■ connectors. An electrical connector is a plastic part
G305 indicating the ground connector is located in the
that contains one or more terminals. Although the ter-
passenger compartment of the vehicle. The ground wire is
minals provide the electrical connection in a circuit, it is black (BLK), the circuit number is 50, and the wire is 32 mm2
the plastic connector that keeps the terminals together (2 gauge AWG).
mechanically.
■■ Location. Connections are usually labeled “C” followed
by three numbers. The three numbers indicate the general
Even-numbered connectors are on the right (passenger
location of the connector. Normally, the connector num-
side) of the vehicle and odd-numbered connectors are on
ber represents the general area of the vehicle, such as:
the left (driver’s side) of the vehicle. For example, C102
is a connector located under the hood (between 100 and
100–199 Under the hood
199) on the right side of the vehicle (even number 102).
200–299 Under the dash
● see Figure 11–11.
300–399 Passenger compartment
■■ grounds and splices. These are also labeled using
400–499 Rear package or trunk area
the same general format as connectors. Therefore, a
500–599 Left-front door
ground located in the passenger compartment could
600–699 Right-front door
be labeled G305 (G means “ground” and “305” means
700–799 Left-rear door
that it is located in the passenger compartment).
800–899 Right-rear door ● see Figure 11–12.

WI RI N G SCh E MAT I C S AN D C I R C U IT T E S T IN G 147

 RESISTOR
BULB (LAMP)

VARIABLE RESISTOR
DUAL-FILAMENT
BULB (LAMP)
VARIABLE RESISTOR
Figure 11–13 The symbol for lightbulbs shows the (POTENTIOMETER)
filament inside a circle, which represents the glass ampoule Figure 11–15 Resistor symbols vary depending on the type
of the bulb. of resistor.

B Figure 11–16 A rheostat uses only two wires—one is

connected to a voltage source and the other is attached to the
movable arm.
M

A
OR

Figure 11–14 An electric motor symbol shows a circle Figure 11–17 Symbols used to represent capacitors. If one
with the letter M in the center and two black sections that of the lines is curved, this indicates that the capacitor being
represent the brushes of the motor. This symbol is used even used has a polarity, while the one without a curved line can be
though the motor is a brushless design. installed in the circuit without concern about polarity.

eleCTriCal ComponenTS Most electrical components

have their own unique symbol that shows the basic function LIGHTER
or parts.
■■ bulbs. Lightbulbs often use a filament, which heats and
then gives off light when electrical current flows. The Figure 11–18 The grid-like symbol represents an
symbol used for a lightbulb is a circle with a filament in- electrically heated element.
side. A dual-filament bulb, such as is used for taillights
and brake light/turn signals, is shown with two filaments. A two-wire rheostat is usually shown as part of another
● see Figure 11–13. unit, such as a fuel level sensing unit. ● see Figure 11–16.

eleCTriC moTorS An electric motor symbol shows CapaCiTorS Capacitors are usually part of an electronic
a circle with the letter M in the center and two electrical component, but not a replaceable component unless the
connections, one at the top and the other at the bottom. ● see vehicle is an older model. Many older vehicles used capacitors
Figure 11–14 for an example of a cooling fan motor. to reduce radio interference and were installed inside alternators
or were attached to wiring connectors. ● see Figure 11–17.
reSiSTorS Although resistors are usually part of another
component, the symbol appears on many schematics and eleCTriC heaTed uniT Electric grid-type rear window
wiring diagrams. A resistor symbol is a jagged line representing defoggers and cigarette lighters are shown with a square box–
resistance to current flow. If the resistor is variable, such as a type symbol. ● see Figure 11–18.
thermistor, an arrow is shown running through the symbol of a
fixed resistor. A potentiometer is a three-wire variable resistor, boxed ComponenTS If a component is shown in a box
shown with an arrow pointing toward the resistance part of a using a solid line, the box is the entire component. If a box uses
fixed resistor. ● see Figure 11–15. dashed lines, it represents part of a component. A commonly

148 Ch AP TER 11
Figure 11–19 A dashed outline represents a portion (part)
of a component.

SPST SPDT

Figure 11–20 A solid box represents an entire component.

(a) (b)

Figure 11–21 This symbol represents a component that is

case grounded.

used dashed-line box is a fuse panel. Often, just one or two

fuses are shown in a dashed-line box. This means that a fuse
panel has more fuses than shown. ● see Figures 11–19 DPST DPDT
anD 11–20.

SeparaTe replaCeable parT Often components (c) (d)

Figure 11–22 (a) A symbol for a single-pole, single-throw
are shown on a schematic that cannot be replaced, but are
(SPST) switch. This type of switch is normally open (N.O.)
part of a complete assembly. When looking at a schematic of
because nothing is connected to the terminal that the switch
General Motors vehicles, the following is shown. is contacting in its normal position. (b) A single-pole, double-
■■ If a part name is underlined, it is a replaceable part. throw (SPDT) switch has three terminals. (c) A double-pole,
single-throw (DPST) switch has two positions (off and on) and
■■ If a part is not underlined, it is not available as a replace-
can control two separate circuits. (d) A double-pole, double-
able part, but is included with other components shown throw (DPDT) switch has six terminals—three for each pole.
and sold as an assembly. Note: Both (c) and (d) also show a dotted line between the two
■■ If the case itself is grounded, the ground symbol is arms indicating that they are mechanically connected, called a
“ganged switch.”
attached to the component as shown in ● Figure 11–21.

A single-pole, single-throw (spsT) switch has only two posi-

SWiTCheS Electrical switches are drawn on a wiring
tions, on or off. A single-pole, double-throw (spDT) switch
diagram in their normal position. This can be one of two
has three terminals, one wire in and two wires out. A headlight
possible positions.
dimmer switch is an example of a typical SPDT switch. In one
■■ normally open. The switch is not connected to its inter-
position, the current flows to the low-filament headlight; in the
nal contacts and no current will flow. This type of switch
other, the current flows to the high-filament headlight.
is labeled n.O.
■■ normally closed. The switch is electrically connected Note: A SPDt switch is not an on or off type of switch
to its internal contacts and current will flow through the but instead directs power from the source to either the
switch. This type of switch is labeled n.c. high-beam lamps or the low-beam lamps.

Other switches can use more than two contacts. There are also double-pole, single-throw (DpsT)
The poles refer to the number of circuits completed by the switches and double-pole, double-throw (DpDT) switches.
switch and the throws refer to the number of output circuits. ● see Figure 11–22.

WI RI N G SCh E MAT I C S AN D C I R C U IT T E S T IN G 149

 Figure 11–23 (a) A symbol for a normally open (N.O.) momentary
(a) (b) switch. (b) A symbol for a normally closed (N.C.) momentary switch.

Hot at All Times

Fuse
INT LP Block
Fuse Rear
10 A
INT
Lamp
Relay

E12 D12 E11 C2

BLU/WHT 149 DK BLU/WHT 149

B A
SP306 SP310 TAN 2183

B5 C2
Rear
Inadvertant
Power Relay
Integration Figure 11–24 Color the parts of the circuit that
Module
Control
(RIM) have 12 volts, then take to the vehicle to see if
power is available at each location marked.

Note: All switches are shown on schematics in their

Tech Tip
normal position. this means that the headlight switch
will be shown normally off, as are most other switches
Color-Coding Is Key to Understanding
and controls.
Whenever diagnosing an electrical problem, it is
common practice to print out the schematic of the
momenTary SWiTCh A momentary switch is a switch circuit and then take it to the vehicle. A meter is then
primarily used to send a voltage signal to a module or controller used to check for voltage at various parts of the cir-
to request that a device be turned on or off. The switch makes cuit to help determine if there is a fault. The diagnosis
momentary contact and then returns to the open position. A can be made easier if the parts of the circuit are first
horn switch is a commonly used momentary switch. The symbol color-coded using markers or color pencils.
that represents a momentary switch uses two dots for the The colors represent voltage conditions in
contact with a switch above them. A momentary switch can be various parts of a circuit. Once the circuit has been
either normally open or normally closed. ● see Figure 11–23. color-coded, it can be tested using the factory wire
A momentary switch, for example, can be used to lock colors as a guide. ● see Figure 11–24.
or unlock a door or to turn the air conditioning on or off. If
the device is currently operating, the signal from the momen-
tary switch will turn it off, and if it is off, the switch will signal
the module to turn it on. The major advantage of momentary
switches is that they can be lightweight and small, because the
switch does not carry any heavy electrical current, just a small
voltage signal. Most momentary switches use a membrane
constructed of foil and plastic.

150 Ch AP TER 11
 86 87 87a

MOVABLE ARM
(ARMATURE)

INSULATED
STOP

CONTACT
85 30 (MOSTLY RELAY COILS POINTS
COIL
HAVE BETWEEN
86—POWER SIDE OF THE COIL 60–100 
30 87
85—GROUND SIDE OF THE COIL OF RESISTANCE)

30—COMMON POWER FOR RELAY CONTACTS

87—NORMALLY OPEN OUTPUT (N.O.)
87a—NORMALLY CLOSED OUTPUT (N.C.) 86 85
Figure 11–25 A relay uses a movable arm to complete a Figure 11–26 A cross-sectional view of a typical four-
circuit whenever there is a power at terminal 86 and a ground terminal relay. Current flowing through the coil (terminals
at terminal 85. A typical relay only requires about 1/10 ampere 86 and 85) causes the movable arm (called the armature)
through the relay coil. The movable arm then closes the to be drawn toward the coil magnet. The contact points
contacts (#30 to #87) and can relay 30 amperes or more. complete the electrical circuit connected to terminals
30 and 87.

relay Terminal
idenTifiCaTion
definiTion A relay is a magnetic switch that uses a ■■ The ISO identification of the coil terminals are 86 and
movable armature to control a high-amperage circuit by using 85. The terminal number 86 represents the power to
a low-amperage electrical switch. the relay coil and the terminal labeled 85 represents
the ground side of the relay coil.
iSo relay Terminal idenTifiCaTion Most ■■ The relay coil can be controlled by supplying either
automotive relays adhere to common terminal identification. power or ground to the relay coil winding.
The primary source for this common identification comes ■■ The coil winding represents the control circuit
from the standards established by the International Standards which uses low current to control the higher current
Organization (ISO). Knowing this terminal information will help through the other terminals of the relay. ● see
in the correct diagnosis and troubleshooting of any circuit Figure 11–27.
containing a relay. ● see Figures 11–25 anD 11–26. 2. Other terminals used to control the load current
Relays are found in many circuits because they are capa- ■■ The higher amperage current flow through a relay
ble of being controlled by computers, yet are able to handle flows through terminals 30 and 87, and often 87a.
enough current to power motors and accessories. Relays ■■ Terminal 30 is usually where power is applied to a
include the following components and terminals. relay. Check service information for the exact opera-
tion of the relay being tested.
relay operaTion ■■ When the relay is at rest without power and ground
1. coil (terminals 85 and 86) to the coil, the armature inside the relay electrically
■■ A coil provides the magnetic pull to a movable arma- connects terminals 30 and 87a if the relay has five
ture (arm). terminals. When there is power at terminal 85 and
■■ The resistance of most relay coils ranges from 50 to a ground at terminal 86 of the relay, a magnetic
150 ohms, but is usually between 60 and 100 ohms.

W I RI N G SC h E MAT I C S AN D C I R C U IT T ES T IN G 151
 RELAY RELAY SOCKET
Tech Tip

Divide the Circuit in Half

When diagnosing any circuit that has a relay, start
testing at the relay and divide the circuit in half.
• high current portion: Remove the relay and check
that there are 12 volts at the terminal 30 socket.
If there is, then the power side is okay. Use an
ohmmeter and check between terminal 87 socket
and ground. If the load circuit has continuity, there
should be some resistance. If OL, the circuit is
electrically open.
• control circuit (low current): With the relay
removed from the socket, check that there is
12 volts to terminal 86 with the ignition on and the
control switch on. If not, check service information
to see if power should be applied to terminal 86,
then continue troubleshooting the switch power
and related circuit.
• check the relay itself: Use an ohmmeter and
measure for continuity and resistance. Figure 11–27 A typical relay showing the schematic of the
• Between terminals 85 and 86 (coil), there should wiring in the relay.
be 60 to 100 ohms. If not, replace the relay.
• Between terminals 30 and 87 (high-amperage
switch controls), there should be continu-
ity (low ohms) when there is power applied to NORMALLY OPEN
(N.O.) RELAY
terminal 85 and a ground applied to terminal
86 that operates the relay. If OL is displayed on
the meter set to read ohms, the circuit is open
which requires that the relay be replaced. NORMALLY CLOSED
(N.C.) RELAY
• Between terminals 30 and 87a (if equipped), with
the relay turned off, there should be low resis-
Figure 11–28 All schematics are shown in their normal,
tance (less than 5 ohms).
nonenergized position.

relay VolTage Spike ConTrol Relays contain a

field is created in the coil winding, which draws the coil and when power is removed, the magnetic field surrounding
armature of the relay toward the coil. The armature, the coil collapses, creating a voltage to be induced in the coil
when energized electrically, connects terminals 30 winding. This induced voltage can be as high as 100 volts or
and 87. more and can cause problems with other electronic devices in
The maximum current through the relay is determined by the vehicle. For example, the short high-voltage surge can be
the resistance of the circuit, and relays are designed to safely heard as a “pop” in the radio. To reduce the induced voltage,
handle the designed current flow. ● see Figures 11–28 some relays contain a diode connected across the coil. ● see
anD 11–29. Figure 11–30.

152 Ch AP TER 11
 RESISTOR
15 A
FUSE
COIL
WINDING
2
HORN Figure 11–31 A resistor used in parallel with the coil
RELAY
windings is a common spike reduction method used in
many relays.
1 3

4 2E 5 2A

10
HORN
SWITCH
HORN
1
HORN
1
? FrequenTLy asKeD quesTiOn

LH RH
What Is the Difference Between a Relay and a
Solenoid?

Figure 11–29 A typical horn circuit. Note that the relay Often, these terms are used differently among vehicle
contacts supply the heavy current to operate the horn when manufacturers, which can lead to some confusion.
the horn switch simply completes a low-current circuit to
relay: A relay is an electromagnetic switch that
ground, causing the relay contacts to close.
uses a movable arm. Because a relay uses a mov-
able arm, it is generally limited to current flow not
exceeding 30 amperes.
solenoid: A solenoid is an electromagnetic switch
TRANSISTOR that uses a movable core. Because of this type
(OFF)
B+ of design, a solenoid is capable of handling 200
amperes or more and is used in the starter motor
circuit and other high-amperage applications, such
as in the glow plug circuit of diesel engines.
Figure 11–30 When the relay or solenoid coil current is
turned off, the stored energy in the coil flows through the
clamping diode and effectively reduces voltage spike.

When the current flows through the coil, the diode Most relays use a resistor connected in parallel with the
is not part of the circuit because it is installed to block cur- coil winding. The use of a resistor, typically about 400 to 600
rent. however, when the voltage is removed from the coil, the ohms, reduces the voltage spike by providing a path for the
resulting voltage induced in the coil windings has a reversed voltage created in the coil to flow back through the coil wind-
polarity to the applied voltage. Therefore, the voltage in the coil ings when the coil circuit is opened. See ● Figure 11–31.
is applied to the coil in a forward direction through the diode,
which conducts the current back into the winding. As a result,
the induced voltage spike is eliminated.

WI RI N G SCh E MAT I C S AN D C I R C U IT T E S T IN G 153

 ● see Figure 11–32 for an example of lights being powered
loCaTing an open CirCuiT by one fuse (power source).
■■ Underhood light
Terminology An open circuit is a break in the electrical ■■ Inside lighted mirrors
circuit that prevents current from flowing and operating an ■■ Dome light
electrical device. Examples of open circuits include: ■■ Left-side courtesy light
■■ Blown (open) lightbulbs ■■ Right-side courtesy light
■■ Cut or broken wires
Therefore, if a customer complains about one or more of
■■ Disconnected or partially disconnected electrical the items listed, check the fuse and the common part of the
connectors circuit that feeds all of the affected lights. Check for a com-
■■ Electrically open switches mon ground if several components that seem unrelated are not
■■ Loose or broken ground connections or wires functioning correctly.

■■ Blown fuse

proCedure To loCaTe an open CirCuiT The

case sTuDy
typical procedure for locating an open circuit involves the
following steps. the electric Mirror Fault Story
Step 1 perform a thorough visual inspection. Check the A customer noticed that the electric mirrors stopped
following: working. The service technician checked all electrical
• Look for evidence of a previous repair. Often, an components in the vehicle and discovered that the
electrical connector or ground connection can be interior lights were also not working.
accidentally left disconnected. The interior lights were not mentioned by the
• Look for evidence of recent body damage or body customer as being a problem most likely because the
repairs. Movement due to a collision can cause driver only used the vehicle in daylight hours.
metal to move, which can cut wires or damage con- The service technician found the interior light
nectors or components. and power accessory fuse blown. Replacing the fuse
Step 2 print out the schematic. Trace the circuit and check restored the proper operation of the electric outside
for voltage at certain places. This will help pinpoint the mirror and the interior lights. however, what caused
location of the open circuit. the fuse to blow? A visual inspection of the dome
Step 3 check everything that does and does not light, next to the electric sunroof, showed an area
work. Often, an open circuit will affect more than one where a wire was bare. Evidence showed the bare
component. Check the part of the circuit that is com- wire had touched the metal roof, which could cause
mon to the other components that do not work. the fuse to blow. The technician covered the bare

Step 4 check for voltage. Voltage is present up to the loca- wire with a section of vacuum hose and then taped

tion of the open circuit fault. For example, if there is the hose with electrical tape to complete the repair.

battery voltage at the positive terminal and the negative Summary:

(ground) terminal of a two-wire lightbulb socket with ■■ complaint—The electric power mirrors stopped
the bulb plugged in, then the ground circuit is open.
working.
■■ cause—A blown fuse due to a fault in the wiring at

Common poWer ■■
the dome light.
correction—Repaired the wiring at the dome light
or ground which restored the proper operation of the electric
mirrors that shared the same fuse as the dome light.

When diagnosing an electrical problem that affects more

than one component or system, check the electrical sche-
matic for a common power source or a common ground.

154 Ch AP TER 11
 HOT AT ALL TIMES ADDED
ADDED RELAY
FUSE
#14 COURTESY
FUSE (15 A)
LIGHT SWITCH
(NORMALLY CLOSED
UNDERHOOD LIGHT WITH HOOD OPEN)

S201 SPLICE

RIGHT INSIDE
LIGHTED MIRROR
EXISTING
LIGHT CIRCUIT
S319
ADDED
LIGHTS

LEFT INSIDE
LIGHTED MIRROR Figure 11–33 To add additional lighting, simply tap into an
existing light wire and connect a relay. Whenever the existing
light is turned on, the coil of the relay is energized. The arm of
the relay then connects power from another circuit (fuse) to
the auxiliary lights without overloading the existing light circuit.
RIGHT SIDE
COURTESY LIGHT

Tech Tip
S316

Do It Right—Install a Relay

LEFT SIDE Often the owners of vehicles, especially owners of

COURTESY LIGHT pickup trucks and sport utility vehicles (SUVs), want
to add additional electrical accessories or lighting.
LIGHT It is tempting in these cases to simply splice into an
SWITCH
existing circuit. however, when another circuit or
S364
component is added, the current that flows through
RIGHT-HAND FRONT the newly added component is also added to the
DOOR JAMB
current for the original component. This additional
SWITCH
current can easily overload the fuse and wiring. Do
S320 not simply install a larger amperage fuse; the wire
C101 gauge size was not engineered for the additional
current and could overheat.
DOME LIGHT
The solution is to install a relay, which uses a
small coil to create a magnetic field that causes a
LIFT-HAND FRONT movable arm to switch on a higher current circuit.
DOOR JAMB The typical relay coil has 50 to 150 ohms (usually
SWITCH
60 to 100 ohms) of resistance and requires just
Figure 11–32 A typical wiring diagram showing multiple
0.24 to 0.08 ampere when connected to a 12 volt
switches and bulbs powered by one fuse.
source. This small additional current will not be
enough to overload the existing circuit. ● see
Figure 11–33 for an example of how additional
lighting can be added.

WI RI N G SCh E MAT I C S AN D C I R C U IT T E S T IN G 155

 ? FrequenTLy asKeD quesTiOn

Where to Start?
The common question is, where does a technician
start the troubleshooting when using a wiring diagram
(schematic)?
hint 1 If the circuit contains a relay, start your diag-
nosis at the relay. The entire circuit can be
tested at the terminals of the relay.
hint 2 The easiest first step is to locate the unit on
the schematic that is not working at all or not
working correctly.
a. Trace where the unit gets its ground
connection.
b. Trace where the unit gets its power
connection.
Often a ground is used by more than one component.
Therefore, ensure that everything else is working
correctly. If not, then the fault may lie at the common
ground (or power) connection.
hint 3 Divide the circuit in half by locating a connec-
tor or a part of the circuit that can be accessed
easily. Then check for power and ground at this
midpoint. This step could save you much time.
Figure 11–34 Always check the simple things first. Check
hint 4 Use a fused jumper wire to substitute a ground
the fuse for the circuit you are testing. Maybe a fault in another
circuit controlled by the same fuse could have caused the fuse or a power source to replace a suspected
to blow. Use a test light to check that both sides of the fuse switch or section of wire.
have voltage.

CirCuiT TroubleShooTing
proCedure If voltage is available at the socket, the problem is either a
defective bulb or a poor ground at the socket or a ground wire
Follow these steps when troubleshooting wiring problems. connection to the body or frame. If no voltage is available at the
socket, consult a wiring diagram for the type of vehicle being
Step 1 Verify the malfunction. If, for example, the backup
tested. The wiring diagram should show all of the wiring and
lights do not operate, make certain that the ignition
components included in the circuit. For example, the backup
is on (key on, engine off), with the gear selector in re-
light current must flow through the fuse and ignition switch to
verse, and check for operation of the backup lights.
the gear selector switch before traveling to the rear backup
Step 2 Check everything else that does or does not oper-
light socket. As stated in the second step, the fuse used for the
ate correctly. For example, if the taillights are also
backup lights may also be used for other vehicle circuits.
not working, the problem could be a loose or broken
The wiring diagram can be used to determine all other
ground connection in the trunk area that is shared by
components that share the same fuse. If the fuse is blown
both the backup lights and the taillights.
(open circuit), the cause can be a short in any of the circuits
Step 3 Check the fuse for the backup lights. ● see Figure sharing the same fuse. Because the backup light circuit cur-
11–34. rent must be switched on and off by the gear selector switch,
Step 4 Check for voltage at the backup light socket. This can an open in the switch can also prevent the backup lights from
be done using a test light or a voltmeter. functioning.

156 Ch AP TER 11
 units in the circuit. Visual inspection of all the wiring or further
loCaTing a ShorT CirCuiT disconnecting will be necessary to locate the problem.

TeST lighT meThod To use the test light method,

Terminology A short circuit usually blows a fuse, and
simply remove the blown fuse and connect a test light to the
a replacement fuse often also blows in the attempt to locate
terminals of the fuse holder (polarity does not matter). If there is
the source of the short circuit. A short circuit is an electrical
a short circuit, current will flow from the power side of the fuse
connection to another wire or to ground before the current flows
holder through the test light and on to ground through the short
through some or all of the resistance in the circuit. A short-
circuit, and the test light will then light. Unplug the connectors
to-ground will always blow a fuse and usually involves a wire
or components protected by the fuse until the test light goes
on the power side of the circuit coming in contact with metal.
out. The circuit that was disconnected, which caused the test
Therefore, a thorough visual inspection should be performed
light to go out, is the circuit that is shorted.
around areas involving heat or movement, especially if there is
evidence of a previous collision or previous repair that may not
buzzer meThod The buzzer method is similar to the test
have been properly completed.
light method, but uses a buzzer to replace a fuse and act as
A short-to-voltage may or may not cause the fuse to blow
an electrical load. The buzzer will sound if the circuit is shorted
and usually affects another circuit. Look for areas of heat or
and will stop when the part of the circuit that is grounded is
movement where two power wires could come in contact with
unplugged.
each other. Several methods can be used to locate the short.

ohmmeTer meThod The fifth method uses an

fuSe replaCemenT meThod Disconnect one ohmmeter connected to the fuse holder and ground. This is
component at a time and then replace the fuse. If the new fuse the recommended method of finding a short circuit, as an
blows, continue the process until you determine the location of ohmmeter will indicate low ohms when connected to a short
the short. This method uses many fuses and is not a preferred circuit. however, an ohmmeter should never be connected to
method for finding a short circuit. an operating circuit. The correct procedure for locating a short
using an ohmmeter is as follows:
CirCuiT breaker meThod Another method is to 1. Connect one lead of an ohmmeter (set to a low scale) to a
connect an automotive circuit breaker to the contacts of the good clean metal ground and the other lead to the circuit
fuse holder with alligator clips. Circuit breakers are available (load) side of the fuse holder.
that plug directly into the fuse panel, replacing a blade-type
fuse. The circuit breaker will alternately open and close the CAUtIoN: Connecting the lead to the power side
circuit, protecting the wiring from possible overheating damage of the fuse holder will cause current to flow through

while still providing current flow through the circuit. and damage the ohmmeter.

2. The ohmmeter will read zero or almost zero ohms if the

Note: A heavy-duty (HD) flasher can also be used in
circuit or a component in the circuit is shorted.
place of a circuit breaker to open and close the cir-
3. Disconnect one component in the circuit at a time and
cuit. Wires and terminals must be made to connect the
watch the ohmmeter. If the ohmmeter reading shoots to a
flasher unit where the fuse normally plugs in.
high value or infinity, the component just unplugged was
All components included in the defective circuit should be the source of the short circuit.
disconnected one at a time until the circuit breaker stops click- 4. If all of the components have been disconnected and the
ing. The unit that was disconnected and stopped the circuit ohmmeter still reads low ohms, then disconnect electrical
breaker clicking is the unit causing the short circuit. If the circuit connectors until the ohmmeter reads high ohms. The loca-
breaker continues to click with all circuit components unplugged, tion of the short-to-ground is then between the ohmmeter
the problem is in the wiring from the fuse panel to any one of the and the disconnected connector.

WI RI N G SCh E MAT I C S AN D C I R C U IT T E S T IN G 157

 (a) (b)
Figure 11–35 (a) After removing the blown fuse, a pulsing circuit breaker is connected to the terminals of the fuse. (b) The
circuit breaker causes current to flow, then stop, then flow again, through the circuit up to the point of the short-to-ground. By
observing the Gauss gauge, the location of the short is indicated near where the needle stops moving due to the magnetic field
created by the flow of current through the wire.

Note: Some meters, such as the Fluke 87, can be set

to beep (alert) when the circuit closes or when the cir-
cuit opens—a very useful feature.

gauSS gauge meThod If a short circuit blows a fuse,

a special pulsing circuit breaker (similar to a flasher unit) can
be installed in the circuit in place of the fuse. Current will flow FUSE
through the circuit until the circuit breaker opens the circuit. CIRCUIT BREAKER
As soon as the circuit breaker opens the circuit, it closes CAUSING PULSING
CURRENT FLOW IN
again. This on-and-off current flow creates a pulsing magnetic
AFFECTED CIRCUIT
field around the wire carrying the current. A gauss gauge is
a handheld meter that responds to weak magnetic fields. It is GAUSS GAUGE
OSCILLATING BACK
used to observe this pulsing magnetic field, which is indicated AND FORTH UNTIL
on the gauge as needle movement. This pulsing magnetic field GAUGE REACHES
POINT OF SHORT
will register on the Gauss gauge even through the metal body CIRCUIT
of the vehicle. A needle-type compass can also be used to Figure 11–36 A Gauss gauge can be used to determine the
observe the pulsing magnetic field. ● see Figures 11–35 location of a short circuit even behind a metal panel.
anD 11–36.

eleCTroniC Tone generaTor TeSTer An

electronic tone generator tester can be used to locate a short- The tone will be generated as long as there is a continuous
to-ground or an open circuit. Similar to test equipment used electrical path along the circuit. The signal will stop if there is a
to test telephone and cable television lines, a tone generator short-to-ground or an open in the circuit. ● see Figure 11–38.
tester generates a tone that can be heard through a receiver The windings in the solenoids and relays will increase the
(probe). ● see Figure 11–37. strength of the signal in these locations.

158 Ch AP TER 11
 Tech Tip

Heat or Movement
Electrical shorts are commonly caused either by
movement, which causes the insulation around the
wiring to be worn away, or by heat melting the insu-
lation. When checking for a short circuit, first check
the wiring that is susceptible to heat, movement, and
damage.
1. heat. Wiring near heat sources, such as the
exhaust system, cigarette lighter, or alternator
2. Wire movement. Wiring that moves, such as in
Figure 11–37 A tone generator–type tester used to locate
areas near the doors, trunk, or hood
open circuits and circuits that are shorted-to-ground. Included
3. Damage. Wiring subject to mechanical injury,
with this tester is a transmitter (tone generator), receiver
probe, and headphones for use in noisy shops. such as in the trunk, where heavy objects can
move around and smash or damage wiring;
can also occur as a result of an accident or a
previous repair

LOAD SIDE
OF FUSE TERMINAL

RED LEAD BLACK LEAD

GOOD CHASSIS
GROUND
TONE
LIGHT GENERATOR
SWITCH

LOCATION OF  
SHORT-TO-GROUND VEHICLE
BATTERY

Figure 11–38 To check for a short-to-ground using a tone generator, connect the black transmitter lead to a good chassis
ground and the red lead to the load side of the fuse terminal. Turn the transmitter on and check for tone signal with the receiver.
Using a wiring diagram, follow the strongest signal to the location of the short-to-ground. There will be no signal beyond the fault,
either a short-to-ground as shown or an open circuit.

WI RI N G SCh E MAT I C S AN D C I R C U IT T E S T IN G 159

 Tech Tip
STep­by­STep
Wiggle test TroubleShooTing
Intermittent electrical problems are common yet
difficult to locate. To help locate these hard-to-find
proCedure
problems, try operating the circuit and then start
wiggling the wires and connections that control the Knowing what should be done and when it should be done
circuit. If in doubt where the wiring goes, try moving is a major concern for many technicians trying to repair an
all the wiring starting at the battery. Pay particular electrical problem. The following field-tested procedure
attention to wiring running near the battery or the provides a step-by-step guide for troubleshooting an electri-
windshield washer container. Corrosion can cause cal fault.
wiring to fail, and battery acid fumes and alcohol-
Step 1 Determine the customer concern (complaint) and get
based windshield washer fluid can start or contribute
as much information as possible from the customer or
to the problem. If you notice any change in the oper-
service advisor.
ation of the device being tested while wiggling the
a. When did the problem start?
wiring, look closer in the area you were wiggling until
b. Under what conditions does the problem occur?
you locate and correct the actual problem.
c. have there been any recent previous repairs to the
vehicle which could have created the problem?
Step 2 Verify the customer’s concern by actually observing
the fault.
Step 3 Perform a thorough visual inspection and be sure to
eleCTriCal Trouble­ check everything that does and does not work.

ShooTing guide Step 4 Check for technical service bulletins (TSBs).

Step 5 Locate the wiring schematic for the circuit being
When troubleshooting any electrical component, remember the diagnosed.
following hints to identify the problem faster and more easily. Step 6 Check the factory service information and follow the
1. For a device to work, it must have two things: power and troubleshooting procedure.
ground. a. Determine how the circuit works.
b. Determine which part of the circuit is good, based
2. If there is no power to a device, an open power side (blown
on what works and what does not work.
fuse, etc.) is indicated.
c. Isolate the problem area.
3. If there is power on both sides of a device, an open ground
is indicated. Note: Split the circuit in half to help isolate the
4. If a fuse blows immediately, a grounded power-side wire problem and start at the relay (if the circuit has
is indicated. a relay).

5. Most electrical faults result from heat or movement. Step 7 Determine the root cause and repair the vehicle.
6. Most noncomputer–controlled devices operate by open- Step 8 Verify the repair and complete the work order by list-
ing and closing the power side of the circuit (power-side ing the three Cs (complaint, cause, and correction).
switch).
7. Most computer-controlled devices operate by opening
and closing the ground side of the circuit (ground-side
switch).

160 Ch AP TER 11
 case sTuDy

Shocking experience
A customer complained that after driving for a while,
he got a static shock whenever he grabbed the
door handle when exiting the vehicle. The customer
thought that there must be an electrical fault and that
the shock was coming from the vehicle itself. In a
way, the shock was caused by the vehicle, but it was
not a fault. The service technician sprayed the cloth
seats with an antistatic spray and the problem did not
reoccur. Obviously, a static charge was being cre-
ated by the movement of the driver’s clothing on the
seats and then discharged when the driver touched
the metal door handle. ● see Figure 11–39.

Summary: Figure 11–39 Anti-static spray can be used by customers

to prevent being shocked when they touch a metal object like
■■ complaint–Vehicle owner complained that he got
the door handle.
shocked when the door handle was touched.
■■ cause–Static electricity was found to be the cause
and not a fault with the vehicle.
■■ correction–The seats and carpet were sprayed
with an anti-static spray and this corrected the
concern.

Summary
1. Most wiring diagrams include the wire color, circuit num- 4. A typical relay uses a small current through a coil
ber, and wire gauge. (terminals 85 and 86) to operate the higher current part
2. The number used to identify connectors, grounds, and (terminals 30 and 87).
splices usually indicates where they are located in the 5. A short-to-voltage affects the power side of the circuit and
vehicle. usually involves more than one circuit.
3. All switches and relays on a schematic are shown in their 6. A short-to-ground usually causes the fuse to blow and
normal position either normally closed (N.C.) or normally usually affects only one circuit.
open (N.O.). 7. Most electrical faults are a result of heat or movement.

reVieW QueSTionS
1. List the numbers used on schematics to indicate grounds, 3. List three methods that can be used to help locate a short
splices, and connectors and where they are used in the circuit.
vehicle. 4. how can a tone generator be used to locate a short
2. List and identify the terminals of a typical ISO type relay. circuit?

W I RI N G SC h E MAT I C S AN D C I R C U IT T ES T IN G 161
 ChapTer Quiz
1. On a wiring diagram, S110 with a “0.8 BRN/BLK” means 7. Technician A says that there is often more than one cir-
. cuit being protected by each fuse. Technician B says that
a. Circuit #.8, spliced under the hood more than one circuit often shares a single ground con-
b. A connector with 0.8 mm2 wire nector. Which technician is correct?
c. A splice of a brown with black stripe, wire size being a. Technician A only
0.8 mm2 (18 gauge AWG) b. Technician B only
d. Both a and b c. Both Technicians A and B
2. Where is connector C250? d. Neither Technician A nor B
a. Under the hood 8. Two technicians are discussing finding a short-to-ground
b. Under the dash using a test light. Technician A says that the test light,
c. In the passenger compartment connected in place of the fuse, will light when the circuit
d. In the trunk that has the short is disconnected. Technician B says
3. All switches illustrated in schematics are . that the test light should be connected to the positive (+ )
a. Shown in their normal position and negative (- ) terminals of the battery during this test.
b. Always shown in their on position Which technician is correct?
c. Always shown in their off position a. Technician A only
d. Shown in their on position except for lighting switches b. Technician B only
c. Both Technicians A and B
4. When testing a relay using an ohmmeter, which two ter-
d. Neither Technician A nor B
minals should be touched to measure the coil resistance?
a. 87 and 30 c. 87a and 87 9. A short circuit can be located using a .
b. 86 and 85 d. 86 and 87 a. Test light
b. Gauss gauge
5. Technician A says that a good relay should measure
c. Tone generator
between 60 and 100 ohms across the coil terminals.
d. All of the above
Technician B says that OL should be displayed on an
ohmmeter when touching terminals 30 and 87. Which 10. For an electrical device to operate, it must have .
technician is correct? a. Power and a ground
a. Technician A only b. A switch and a fuse
b. Technician B only c. A ground and fusible link
c. Both Technicians A and B d. A relay to transfer the current to the device
d. Neither Technician A nor B
6. Which relay terminal is the normally closed (N.C.) terminal?
a. 30 c. 87
b. 85 d. 87a

162 Ch AP TER 11