GENERAL FAULT FINDING
Below procedure is a guideline to facilitate fault
finding in electrical circuits. Following the entire
procedure is not always practical. Use common
sense. For instance, if only one light out of a set of
two has failed, you will first check the bulb of the
failed light.
The procedure is not suited for electronic circuits
like CANbus, which requires specific knowledge
and software. Fortunately, defective electric compo-
nents and connections that are part of a CANbus
circuit will generate a fault code in the controller.
The fault code guide provides further detail about
the defect. Always verify presence of fault codes.
Preparation
1.
Have the electrical Schematic available. See Di-
agrams SRM.
2.
Identify the symptoms of the defect.
3.
Establish how the relevant switches and levers
should operate. See the Operating Manual.
4.
Observe the condition of the truck. Look for
signs of mechanical damage, overheating, un-
usual sounds, burnt smells.
Define the Problem Area
1.
Determine the schematic location of the failed
function. See Table 11.
2.
Establish which sections of the failed function
operate correctly. Identify these sections on the
schematic.
Identify Possible Causes of Malfunction
1.
Make a list of every possible fault. Use your ini-
tial observations to help you writing down
these faults.
Determine the Most Probable Cause
1.
Prioritize each of the listed possible faults.
2.
Perform a Fuse Check, a Wiring Check and a
Component Check for each of the listed faults.
Fuse Check
Determine which fuse protects the component in
the failed circuit. See Table 11 to look up the rela-
ted page of the schematic and find the fuse refer-
ence number.
Investigate if an overload condition or a short cir-
cuit caused the fuse to fail. First make repairs to
solve the short circuit or overload condition. See
Wiring Check.
Replace the fuse after the repair has been made.
The actual location of the fuse is indicated on Fig-
ure 4.
Wiring Check
Check signal presence at the component.
If no signal is present, check the signal at the next
component closer to the battery (relay, fuse, switch,
ignition switch). See electric diagram.
Check for damaged wire insulation especially when
there are erratic failures.
Check connection to battery ground. Establish vir-
tually zero resistance for battery ground connec-
tions.
Component Check
Isolate the component and check electrical func-
tionality according the electric schematic.
Switches and connectors must have virtually zero
resistance when closed.
Check for open circuits, short circuits and insula-
tion breakdown.
Check if coils and relays do not overheat, which in-
dicates a defect.
Repair and Test
Remove the ground cable from the battery when do-
ing repairs. Replace or repair the defective compo-
nents after the cause of failure has been estab-
lished. Replace the fuse found defective.
2200 SRM 1592
Electrical Schematic and System Description
17
