16:07-00

Issue 2 en

General troubleshooting in
electrical systems and electronic
control systems

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ã Scania CV AB 2002, Sweden

1 711 498
Contents

Contents
General troubleshooting .................................................................................. 3

Cable marking .................................................................................. 4

To be considered ... .................................................................................. 5

Short circuit ................................................................................ 10

Open circuit ................................................................................ 12

Voltage drop ................................................................................ 13

Earth fault ................................................................................ 14

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Troubleshooting in the basic
electrical system ................................................................................ 18

Troubleshooting in systems
specific to the chassis ................................................................................ 22

Troubleshooting in the electronic

system ................................................................................ 26

Scania Diagnos, SD 2 ............................................. 27

Scania Programmer, SP 2 ....................................... 29

Faults without fault codes....................................... 30

Faults with fault codes............................................ 32

Hidden electrical faults........................................... 35

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 General troubleshooting

General
troubleshooting
according to the customer all the fuses are
intact.
If you study the specific fault and carry out
methodical troubleshooting using the wiring The vehicle is equipped with an ABS brake
diagram, test lamp, multimeter and in some system and it appears that the ABS system
cases also a PC and Scania Diagnos you will warning lamp is defective. The retarder and
find that troubleshooting in the electrical ABS system communicate with each other and
systems of modern vehicles and in their the retarder uses a test signal via the ABS
electronic control systems is not especially system warning lamp. If the signal is not
difficult. present, the retarder control unit disconnects
the retarder operation via the brake pedal and
Some basic knowledge of electrical systems downhill speed control.
and computer usage is however required.
It is therefore important to be familiar with the

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This booklet should help you to find a suitable design of different electronic control systems
procedure when troubleshooting traditional and their communication with each other.
electrical systems and electronic control
systems.
You will find the basic electrical system on
both the wiring and circuit diagrams.
Specific equipment which is not standard on all
vehicles, e g a tag axle lift, has its own wiring
diagram.
Electronic control systems communicate
frequently with their components using digital
signals and varying voltage analogue signals.
This makes it impossible to use a test lamp as a
tool when troubleshooting.
When faults occur in electronic control
systems, fault codes are generally generated,
which can be read off with a PC and Scania
Diagnos.
A fault in one control system can produce a
subsequent fault in other control systems.
Sometimes the subsequent faults are more
obvious than the original fault.
Example:
A customer comes in and complains that the
vehicle retarder is not operating correctly. It
cannot be controlled via the brake pedal and
the downhill speed control is not operating
either. Nor has any warning lamp come on and

16:07-00 ã Scania CV AB 2002, Sweden 3

Cable marking

Cable marking

Some basic information

Numeral mark
15 Voltage with key in drive position*
30 Constant voltage
31 Earth connection
*The circuit is supplied with voltage when the
key is in the drive position. When measuring a
component, it is necessary to know how it is
connected, e g there may be a switch that must
also be switched on for voltage to be supplied
to the component.

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Colour mark*
BK black YE yellow
BN brown RD red
OG orange GN green
BU blue VT violet
GY grey WH white
PK pink
*The abbreviation relates to the name in
English, BlacK, YEllow etc.

Example:

15HB.RD-2.5+C8-3
15HB. Function (15 = Voltage with
key in start position, HB =
ABS circuit)
RD Colour
-2.5 Cable area, mm² (2.5mm²)
+C8-3 Address, other end
C8 = connector
-3 = Connection 3

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 To be considered ...

To be considered ...

• You should never fit a fuse with a higher

amperage than permitted. The fuse is
designed to suit the electrical system and its
components.

• Avoid changing a fuse when the power

supply is still connected to it. This is to
avoid burns in the fuse holder.

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Sometimes it may not be appropriate to
disconnect the power supply to the vehicle
during troubleshooting, e g if the customer
has some equipment in the vehicle which is
dependent on a constant power supply for
memory functions etc. If this is the case,
you should remove the fuse from the fuse
holder during troubleshooting. You can do
this with a couple of self-made cables. Refer
to the illustration.

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To be considered ...

• Always try to carry out voltage tests on a

connector from the rear of the connector.
This avoids damaging the pin and you do
not need to disconnect the connector
unnecessarily. A connector which has been
frequently disconnected can, in time, give
rise to a loose contact.

IMPORTANT! If a connector to a currently

active electronic control system is disconnected,
fault codes are often generated. Therefore
remember to check that no new and misleading
fault codes have been generated after
troubleshooting and repair.

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 To be considered ...

• When you are searching for a open circuit

between connectors, the following applies:
You should never make a hole in a cable
which is located outside the cab to check
whether it is live. Moisture and salt can
penetrate even a very small hole and in time
the cable will form verdigris within the
insulation. Such an open circuit is almost
impossible to see. It is better then to cut the
cable and make a waterproof joint
afterwards.

! WARNING!

Never cut a cable with several internal leads

when it is live. There is a risk of a short
circuit which can result in personal injuries

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and costly consequential damage.

16:07-00 ã Scania CV AB 2002, Sweden 7

To be considered ...

• Do not use a test lamp with an LED to check

whether there is a power supply to
components such as lamps, magnets,
motors, etc which are operated with
24 volts. A bad earth connection to the
circuit concerned is enough to switch on an
LED which then gives an incorrect result. A
test lamp fails to come on or comes on at a
very much reduced output in such a test.

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• Troubleshooting in electronic control
systems requires access to a multimeter and/
or PC with the Scania Diagnos program.
• Electronic control systems generally store a
fault code in their control unit. The fault
code can be read off using Scania Diagnos.
It is generally possible to locate faults and
test various components relatively easily
using Scania Diagnos.

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 To be considered ...

• Control Area Network, CAN

Certain electronic control systems operate

in networks with other control units and
components, CAN communication.
It is not possible to carry out
troubleshooting with a test lamp in
electronic control systems which use CAN
communication. In these control systems
you troubleshoot with a PC and the Scania
Diagnos and Scania Programmer programs.
Cables which form part of CAN
communication controlled circuits are
marked with the letters CAN on their
connections.

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Short circuit

Short circuit

There are different types of short circuits:

• Short circuit to earth on live cables.

This often results in a fuse blowing or a

function being absent and a fault code is
generated in an electronic control system.
• Short circuit to earth on an earth circuit.

E g the cable to the brake lamp switch is

earthed because a screw is screwed through
the cable. Normally that cable is earthed via

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the brake lamp switch. The short circuit
does not cause any fuse to blow in this case,
but fault codes can be generated in an
electronic control system. Also different
electronic control systems can lose
functions since several functions are
required at the same time. These faults are
more difficult to find and it is necessary to
understand how the electronic system
operates.

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 Short circuit

• Short circuit from one live circuit to another

circuit which is not currently live.

These types of short circuits may be that a

screw is screwed into a cable with several
leads, or that there is a touch condition
between two pins in a trailer connection, so
that normal lighting turns on the direction
indicator, activates the tilt function on a
connected tipper, etc.
These short circuits do not necessarily cause
any fuse to blow, but fault codes can be
generated in an electronic control system.

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Open circuit

Open circuit

When there are open circuits, the fuses do not

generally blow. What is known as a current
spike may be generated by this if the cable or
lead is loaded just when it is pulled, torn or cut
off. Then a fuse may blow but when a new fuse
is fitted it will hold, since there is no longer any
load there.

Fault codes are however often generated in

electronic control systems when there is an open
circuit on their cables. This is because electronic
control systems often keep watch and
communicate with their components.

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 Voltage drop

Voltage drop

When resistance testing the cable in a currently

load-free circuit, you can obtain a misleading
measurement result which indicates that the
cable and its connections are undamaged.
Example:
A work lamp is not working. You remove the
bulb and measure directly in the bulb holder.
There you obtain a value of 24 volts and think
that it was the bulb that was defective. But it still
does not work with a new bulb.
You remove the new bulb and resistance test the
cables and obtain a measurement result which
shows that the cables and their connections are

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undamaged.
This is a misleading measurement result. With
such a measurement, the load on a cable is so
low that it is sufficient if just one copper wire in
the cable is intact or the connection is quite poor
to obtain a correct measurement result. Under
load, however, the conductivity becomes much
too poor and the bad cable or connection then
functions as a large resistance and a voltage
drop occurs. The greater the load the greater the
heat released at the voltage drop point.

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Earth fault

Earth fault

Earth faults in the light circuits or circuits with

warning lamps are frequently recognised
because the lamps do not come on at full output.

Good earth connection to the test lamp. Correct

voltage to L1, but the lamp is glowing.

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Good earth connection to the test lamp. Faulty
earth connection to L1 and both the test lamp
and L1 are glowing.

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 Earth fault

A good earth connection is always dead. Always

make sure that there is a good earth connection
to the test equipment.

Good earth connection to the test lamp and L2.

The test lamp does not come on.

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Faulty earth connection to L1 and the test lamp.
L1 is glowing and the test lamp comes on faintly.
This gives a misleading voltage value for L1.

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Earth fault

Earth faults often result in circuits, which have

no common connection apart from the earth
connection, quite suddenly having an effect on
each other.
If a common earth point for several different
components comes loose, e g from a chassis, the
current will be conducted to the nearest earth
point.
Earth faults in electronic control systems do not
always generate fault codes.
Examples 1 and 2:
Example 1
An earth bolt comes loose but is still held in the
ring cable terminal connectors of other circuits.
Now the current cannot be conducted to earth as
intended but the current is conducted to another
earth point. Then the current is conducted back

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into another circuit and in this way circuits are
affected by each other, which they normally are
not.

1. Current via switch, through lamp, to earth

point, faulty earth point, on to motor, back
through the motor, to output on switch, on to
lamp and earths through the lamp and its earth
point. This means that the lamps are glowing
and the motor runs slowly and in the wrong
direction.

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 Earth fault

Example 2

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2. Current via switch, through lamp, to earth
point, faulty earth point, on to relay, back
through the relay, to output on switch, on to
lamp and earths through the lamp and its earth
point. This means that the relay operates and the
engine is running at full output, but the lamps
are glowing.

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Troubleshooting in the basic electrical system

Troubleshooting in
the basic electrical
system

Example
A customer comes in and says that the brake
light fuse is defective.

1 An initial inspection of the brake lights

shows no visible damage.

2 Check that a fuse with the correct amperage

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is fitted. If a fuse with too low an amperage
is fitted, it may be enough to fit one with the
correct amperage for it to operate.

Note: Make sure that the power supply is

disconnected to avoid burns to the fuse holder,
or remove the fuse holder.

3 If the fault remains it is now advisable to

refer to 16:01-01 Wiring diagram handbook
to find the various connection points for the
brake lamp.

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 Troubleshooting in the basic electrical system

4 At the beginning of the manual there is a list

of components with explanations. Select
section L in this case and look for brake
light.

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5 You will now find a reference to the
appropriate wiring diagram. When
troubleshooting in the basic electrical
system, and if you are unfamiliar with it, it
may be advisable to choose the circuit
diagram where you can follow one line
which corresponds to one cable.
If you have learned how to interpret the
cable markings and addresses on the wiring
diagram you will obtain an overview of the
system by using the connection diagram.

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Troubleshooting in the basic electrical system

6 In the wiring diagram you will see which

different components and connection points
that belong to the brake light circuit.
In this case C50 is the nearest component to
the brake light. But what is C50 and where
is it located?

7 Go back to 16:01-01 Wiring Diagram

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handbook.

8 Refer to section C, C50. There it is specified

that this is a "junction box in rear
crossmember" and is found in location
drawing 9.

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 Troubleshooting in basic electrical system

9 Go to the "Location drawings" section in the

booklet and select the configuration which
corresponds to the chassis you are working
on.

10 Now it is advisable to disconnect the brake

light cable terminal on connection 3 in the
junction box and either resistance test the

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wiring to the respective brake light or fit a
new fuse and check its function. If it is now
free from defects the fault has been located
to one of the brake light circuits after the
junction box. If the fault remains, continue
using the same method until the fault is
located.
Study the whole system and what the
vehicle looks like with a view to possible
damage points, e g where the wiring runs
around the cab, etc.

16:07-00 ã Scania CV AB 2002, Sweden 21

Troubleshooting in systems specific to the chassis

Troubleshooting in
systems specific to
the chassis

Example
1 A customer comes in and has problems with
the tag axle lift on his leaf sprung vehicle.
The tag axle lift cannot be lowered.

2 First check the fuse in the cab central

electric unit. Even if the customer says that

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the fuse is intact, you should check that this
is the case yourself.

3 The next step is to find the tag axle lift on

the wiring diagram and study the whole
system. Also check what the vehicle looks
like with a view to possible damage points,
e g where the wiring runs around the cab,
connection points, etc. Refer to 16:01-01,
Wiring Diagram handbook Connection
Diagram section.

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 Troubleshooting in systems specific to the chassis

4 Go on to "Optional equipment" since the tag

axle lift is not part of the basic electrical
system.

5 In the subgroup "Frame, chassis,

suspension" you will find tag axle lift, axle
weight limiter and will find a reference
there to diagram No. 16:04-45. Look for the
diagram in the Workshop Manual, group 16.

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6 Now you have the diagram which relates to
the tag axle lift and its components.

16:07-00 ã Scania CV AB 2002, Sweden 23

Troubleshooting in systems specific to the chassis

7 Each component has its code and

designation on the wiring diagram in
English. If you require the designation in
your own language you should refer to
16:01-01, Wiring Diagram handbook, List
of components.

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8 You will find solenoid valve V49 on the
wiring diagram which should be activated
when lowering the tag axle lift.

9 One of the cable designations at the

connection to the V49 is:
15GD. RD-1 +C76-2
The designation then gives you the
following important information in three
stages:
15GD = 15 means that the circuit must have
starting voltage in order to be activated by
switches, etc.
RD-1 = The cable is red (RD) and has
1 mm² cable area
+C76-2 = The other end of the cable is
found on component C76, connection 2

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 Troubleshooting in systems specific to the chassis

10 The designation of the other cable on the

connection to the V49 is:

31.BK-1 +M11-2
31 = Earth connection
BK-1 = The cable is black (BK) and has
1 mm² cable area
+M11-2 = The other end of the cable is
found on component M11, connection 2
11 So now you can look for the tag axle lift
machine box on the vehicle and check using
a test lamp whether there is voltage between
the red and black connections on the V49
when the switch is pressed. By reading the
wiring diagram and looking at the cable
colour you can now figure out which

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components you should test. Particularly if
there are several similar components side by
side.

12 Continue to troubleshoot calmly and

methodically in the same way. Then the
fault will be located and the correct action
will be taken so that components are not
replaced unnecessarily.

08_1529

16:07-00 ã Scania CV AB 2002, Sweden 25

Troubleshooting in the electronic system

Troubleshooting in
the electronic
system

When troubleshooting in electronic control

systems, it is important to think in system terms
and to consider how different systems
communicate with each other.
A fault in one control system can produce a
subsequent fault in other control systems.
Sometimes the subsequent faults are more
obvious than the original fault.
When troubleshooting it is advisable to connect

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the control system to a PC with the program
Scania Diagnos 2, SD2.
Faults can be read off as flashing codes via the
diagnostics lamp in the vehicle but if you

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connect the computer it will read off the control
systems that are located in the vehicle and any
fault codes at the same time. At the same time
you can troubleshoot and obtain the wiring
diagram, component location diagrams,
proposed remedies, etc using Scania Diagnos.
Scania Diagnos is therefore an extremely
effective tool with which to carry out
troubleshooting. As long as you work calmly
and methodically and read what appears on the
screen, you will resolve most faults. The risk of
replacing undamaged components is reduced
and at the same time unnecessary complaint
disputes are avoided.

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 Scania Diagnos, SD 2

Scania Diagnos, SD 2
You can troubleshoot in the various electronic
control systems with the Scania Diagnos
program. The program is sometimes called just
SD or SD2.
Scania Diagnos contains a number of windows,
three of which are main windows.
A detailed explanation about the program is
provided under the ? symbol found in the menu
bar.
There is a Help button in each window. This
provides information about all the functions and
buttons in the current window.
1 In the first window you select the vehicle
category you wish to work with.

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In this window you can also opt to run
Scania Diagnos in demo mode. This option
is available in the Demo menu bar. The
choice of control systems which you can run
in demo mode is set out under File.
2 In the second window you select the system
group that you wish to work with.

16:07-00 ã Scania CV AB 2002, Sweden 27

Scania Diagnos, SD 2

3 When you come to the third window you are

inside the control system and can start to
carry out active troubleshooting by clicking
on the different buttons.
1. Fault codes: Here there are explanations,
causes and proposed remedies for the
specific faults. In addition there is an option
to delete fault codes and obtain direct
information about selected components and
the wiring diagram which is related to the
fault code.
2. Read/Activate: Here you can read off and
activate signals and functions in the current
system.
3. Components: When you know a
component designation, you can select this

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button directly to obtain information on the
component.
4. Wiring diagrams: Here you can study the
control system wiring diagram.
5. Location: Here you obtain the component
location diagrams for the control system.
6. Configuration: Here you can see how the
control unit is configured. The configuration
can also provide information about how the
present control system communicates with
other control systems. However, sometimes
you must use Scania Programmer to see
further configuration information. Refer to
the section Scania Programmer.

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 Scania Programmer, SP2

Scania Programmer, SP2

You can reprogram and configure control units
in the various electronic control systems with
the Scania Programmer program. The program
is sometimes called just SP or SP2.
You navigate through the Scania Programmer
program in the same way as through the
Scania Diagnos program.

IMPORTANT! Remember that reprogramming

a control unit is only possible if the specified
control unit does not have any fault codes
stored. Fault codes are remedied and deleted
with Scania Diagnos.

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16:07-00 ã Scania CV AB 2002, Sweden 29

Faults without fault codes

Perceptible faults which do not

give any fault warning or fault
codes
When one of several voltage supplies is missing
from an electronic control system, faults may
arise which do not give a fault warning or fault
codes in Scania Diagnos.

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Example:

1 A customer says that it is not possible to

program the starting gear on his Opticruise-
equipped vehicle.

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2 No fault codes are found when
Scania Diagnos is connected and attempts to
program another starting gear are
successful.
3 It is easy then to believe that everything is
working, but when the power is turned on
with the key next time, the newly
programmed starting gear is not stored.

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 Faults without fault codes

4 A far too common hasty conclusion is that

there may be a fault on the control unit.
Make a habit of always checking the voltage
supplies to the control system concerned. In
this case there are two fuses for the
Opticruise. One fuse that protects the
ignition on supply (pin 15) and one that
protects the permanent supply (pin 30).

101 292
Either look directly at the label on the
central electric unit cover or use
Scania Diagnos/Opticruise/Components or
the Wiring diagram to see which fuses
belong to the control system.

5 In this case fuse No. 41 was defective. This

fuse provides a permanent supply (pin 30)
to the control unit. When the permanent
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supply (pin 30) is missing, neither fault
codes nor previous settings are stored.
Conclusion: Always check first that all voltage
supplies are present in the specific control
system you wish to work with.

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Faults with fault codes

Perceptible faults which give a

fault warning and/or fault codes
Example:
1 A customer comes in and has problems with
the ABS system on his vehicle. The ABS
system warning lamp has come on.
2 Connect the computer and start Scania
Diagnos. Follow the instructions on the
screen.
3 When you have selected the vehicle
category and system group, the system you
are working on and the number of fault
codes will be displayed. Click on OK.

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 Faults with fault codes

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4 In the next window you have an overview of
the system and a number of function
windows. At this point select Fault codes.

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Faults with fault codes

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5 At the top of the next window there is a
scroll bar with stored fault codes, the
marked fault code is explained in the large
screen. Fault code 140....
Alongside the scroll bar a display indicates
whether the fault is ACTIVE or
INACTIVE.
In this window you can then obtain
information on the respective fault code
with Cause, Comments and Action for the
fault. This is a great help when
troubleshooting in electronic control
systems.

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 Hidden electrical faults

Perceptible faults which do not at

first appear to be faults in
electronic control systems
Example:
80 100
60 120
1 A customer complains that his vehicle has
difficulty in reaching its working
temperature. It is then easy to think that
there is a fault on the engine thermostat or a
fault in the cooling fan.

113 173

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2 During troubleshooting, the cooling fan
appears to run permanently. The engine is a
16 litre engine. When checking in the
Workshop Manual, cooling system, you
note that there are engines which are
equipped with electronically controlled
cooling fans.

16:07-00 ã Scania CV AB 2002, Sweden 35

Hidden electrical faults

3 When Scania Diagnos is connected, you

note that the engine control system is
EDC MS6.2 DC16 with a coordinator. As
you noted earlier, this engine should be
equipped with an electronically controlled
cooling fan, but in Scania Diagnos you see
no option to check the cooling fan. You
should be able to do this under the Read/
Activate button.

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4 Now you may start to suspect that the
vehicle control system is configured
incorrectly. You look under the
Configuration button but do not obtain
enough information there. The next step is
then to exit Scania Diagnos and start the
Scania Programmer program instead.

Note: If the system has fault codes stored you

must rectify these before it is possible to
program a new configuration.

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 Hidden electrical faults

5 Select Programming/View configuration or

Fan control in Scania Programmer. You can
obtain information about the current setting
under the option Fan control and can
reprogram it at the same time.

In this case it was the setting Mechanically

controlled which caused the cooling fan on
the vehicle to be connected the whole time.
The cooling fan control operates in such a
way that cooling fan is fully connected if it
does not receive any signal from the EDC
control unit. This is done to ensure that the
engine is cooled even if there is a fault with
fan control.

After reprogramming the Cooling fan

option is then also displayed under Read/
Activate in Scania Diagnos.

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In this way an incorrectly configured control
unit can generate a subsequent fault which does
not leave any fault codes and which also gives
fault symptoms which do not even appear to be
electrical faults.
It is therefore important to study faults and fault
symptoms and at the same time think in system
terms and consider how components and
systems communicate with each other.

16:07-00 ã Scania CV AB 2002, Sweden 37