
TEST ACTION APPLICABILITY
4 If there are no potential causes remaining, view repair. All
Repair
Replace the Controller Antilock Brake.
Perform ABS VERIFICATION TEST - VER 1.
5 Turn the ignition off.
Visually inspect the related wiring harness. Look for any chafed, pierced, pinched, or
partially broken wires.
Visually inspect the related wire harness connectors. Look for broken, bent, pushed
out, or corroded terminals.
Refer to any Hotline letters or Technical Service Bulletins that may apply.
Were any problems found?All
Ye s®Repair as necessary.
Perform ABS VERIFICATION TEST - VER 1.
No®Test Complete.
31
BRAKES (CAB)
SYSTEM UNDERVOLTAGE ÐContinued

TEST ACTION APPLICABILITY
3 Turn the ignition off.
Remove the ABS Fuse 21 from the IPM.
Disconnect the CAB harness connector.
Note: Check connector - Clean/repair as necessary.
Using a test light connected to 12 volts, probe the Fused B(+) Circuit.
Does the test light illuminate brightly?All
Ye s®Repair the Fused B(+) Circuit short to ground.
Perform ABS VERIFICATION TEST - VER 1.
No®Go To 4
4 Turn the ignition off.
Remove the ABS Fuse 21 from the IPM.
The CAB must be connected for the results of this test to be valid.
Using a test light connected to 12 volts, probe the Fused B(+) Circuit at the IPM fuse
terminal.
Does the test light illuminate brightly?All
Ye s®Replace the Controller Antilock Brake in accordance with the
Service Information.
Perform ABS VERIFICATION TEST - VER 1.
No®Go To 5
5 Turn the ignition off.
If there are no potential causes remaining, view repair.All
Continue
Replace the Fuse.
Perform ABS VERIFICATION TEST - VER 1.
6 Remove the ABS Fuse 21 from the IPM.
Turn the ignition on.
Measure the voltage of the Fused B+ supply to Fuse 21 in the IPM.
Is the voltage above 10 volts?All
Ye s®Go To 7
No®Repair the B+ Supply circuit for an open.
Perform ABS VERIFICATION TEST - VER 1.
7 Turn the ignition off.
Remove the ABS Fuse 21 from the IPM.
Disconnect the CAB harness connector.
Note: Check connector - Clean/repair as necessary.
Measure the resistance of the Fused B(+) circuit between IPM Fuse terminal 21 and
the CAB connector.
Is the resistance below 5 ohms?All
Ye s®Go To 8
No®Repair the Fuse B+ circuit for an open.
Perform ABS VERIFICATION TEST - VER 1.
8 If there are no possible causes remaining, view repair. All
Repair
Replace the Controller Antilock Brake in accordance with the
Service Information.
Perform ABS VERIFICATION TEST - VER 1.
33
BRAKES (CAB)
VALVE POWER FEED FAILURE ÐContinued

TEST ACTION APPLICABILITY
9 Turn the ignition off.
Visually inspect the related wiring harness. Look for any chafed, pierced, pinched, or
partially broken wires.
Visually inspect the related wire harness connectors. Look for broken, bent, pushed
out, or corroded terminals.
Refer to any Hotline letters or Technical Service Bulletins that may apply.
Were any problems found?All
Ye s®Repair as necessary.
Perform ABS VERIFICATION TEST - VER 1.
No®Test Complete.
34
BRAKES (CAB)
VALVE POWER FEED FAILURE ÐContinued

Symptom:
*BRAKE LAMP SWITCH
POSSIBLE CAUSES
CHECK BRAKE LAMP SWITCH OUTPUT
BRAKE LAMP SWITCH B+ OPEN
BRAKE LAMP SWITCH OUTPUT CIRCUIT SHORT OR OPEN
BRAKE LAMP SWITCH OPEN
CAB -- INTERNAL OPEN
TEST ACTION APPLICABILITY
1 With the DRBIIItin Inputs/Outputs, read the Brake Lamp Switch state.
Press and release the brake pedal.
Does the DRBIIItdisplay PRESSED and RELEASED?All
Ye s®The Brake Lamp Switch is OK.
Perform ABS VERIFICATION TEST - VER 1.
No®Go To 2
2 Disconnect the Brake Lamp Switch harness connector.
Using a 12-volt test light connected to ground, check the Brake Lamp Switch Fused
B+ circuit.
Does the test light illuminate brightly ?All
Ye s®Go To 3
No®Repair the Brake Lamp Switch Fused B+ circuit for an open.
Perform ABS VERIFICATION TEST - VER 1.
3 Disconnect the Brake Lamp Switch harness connector.
Connect a jumper wire between the Brake Lamp Switch B+ and Output circuits.
With the DRBIIItin Inputs/Outputs, read the Brake Lamp Switch state.
Does the DRBIIItdisplay PRESSED?All
Ye s®Replace the Brake Lamp Switch in accordance with the Service
Information.
Perform ABS VERIFICATION TEST - VER 1.
No®Go To 4
4 Disconnect the CAB harness connector.
Disconnect the Brake Lamp Switch harness connector.
Check the Brake Lamp Switch Output circuit for a short to voltage and an open.
Is the Brake Lamp Switch Output circuit shorted or open?All
Ye s®Repair the Brake Lamp Switch Output circuit for a short to
voltage or an open.
Perform ABS VERIFICATION TEST - VER 1.
No®Replace the Controller Anti-Lock Brake in accordance with the
Service Information.
Perform ABS VERIFICATION TEST - VER 1.
35
BRAKES (CAB)

TEST ACTION APPLICABILITY
4 Turn the ignition off.
Disconnect the TRAC OFF Switch harness connector.
Disconnect the CAB harness connector.
Turn the ignition on.
Check the TRAC OFF Switch Sense circuit for short to B+ and for an open.
Is the Sense circuit shorted or open?All
Ye s®Repair the TRAC OFF Switch Sense circuit for a short to battery
or for an open.
Perform ABS VERIFICATION TEST - VER 1.
No®Replace the Body Control Module in accordance with the Service
Information.
Perform ABS VERIFICATION TEST - VER 1.
5 Turn the ignition off.
NOTE: The purpose of this test is to perform the Instrument Cluster self
test.
Depress and hold the Odometer Reset Button.
Turn the Key from OFF to ON and then back to OFF.
Release the Odometer Reset Button.
Do the Instrument Cluster Indicators and Gauges activate and deactivate?All
Ye s®Replace the Body Control Module in accordance with the Service
Information.
Perform ABS VERIFICATION TEST - VER 1.
No®Replace the Instrument Cluster in accordance with the Service
Information.
Perform ABS VERIFICATION TEST - VER 1.
6NOTE: The purpose of this test is to perform the Instrument Cluster self
test.
Depress and hold the Odometer Reset Button.
Turn the Key from OFF to ON and then back to OFF.
Release the Odometer Reset Button.
Do the Instrument Cluster Indicators and Gauges activate and deactivate?All
Ye s®Go To 7
No®Replace the Instrument Cluster in accordance with the Service
Information.
Perform ABS VERIFICATION TEST - VER 1.
7 Turn the ignition off.
Disconnect the TRAC OFF Switch harness connector.
Turn the ignition on.
Does the TRAC OFF Indicator light and then go out?All
Ye s®Replace the TRAC OFF switch.
Perform ABS VERIFICATION TEST - VER 1.
No®Go To 8
37
BRAKES (CAB)
*TRAC OFF INDICATOR NEVER/ALWAYS ON ÐContinued

TEST ACTION APPLICABILITY
8 Turn the ignition off.
Disconnect the TRAC OFF Switch harness connector.
Disconnect the BCM C4 harness connector.
Check the TRAC OFF Switch Sense circuit for short to ground.
Is the Sense circuit shorted to ground?All
Ye s®Repair the TRAC OFF Switch Sense circuit for a short to ground.
Perform ABS VERIFICATION TEST - VER 1.
No®Replace the Body Control Module in accordance with the Service
Information.
Perform ABS VERIFICATION TEST - VER 1.
38
BRAKES (CAB)
*TRAC OFF INDICATOR NEVER/ALWAYS ON ÐContinued

Symptom:
*TRAC ON INDICATOR NEVER/ALWAYS ON
POSSIBLE CAUSES
CAB -- INTERNAL FAULT
CHECK TRACTION CONTROL SYSTEM
CLUSTER FAILS SELF TEST
CAB - NO TRAC MESSAGE
TEST ACTION APPLICABILITY
1NOTE: The DRBIIItmust be able to communicate with the CAB prior to
performing this test.
NOTE: Note: If any CAB DTC's are present, they must be repaired prior to
performing this test.
Perform the KEY-ON bulb test.
Did the TRAC Indicator illuminate and then go out?All
Ye s®Go To 2
No®Go To 3
2 Make sure the Traction Control system has not been deactivated with the TRAC OFF
switch.
NOTE: The purpose of this test is to determine if the Traction Control
system is operating.
With the DRBIIItin Inputs/Outputs, read the ABS Pump Motor voltage state.
Accelerate sufficient to cause drive wheel slip.
Does the TRAC Indicator illuminate and the DRBIIItdisplay approximately 9 volts?All
Ye s®The traction control system is functioning normally.
Perform ABS VERIFICATION TEST - VER 1.
No®Replace the Controller Antilock Brake in accordance with the
Service Information.
Perform ABS VERIFICATION TEST - VER 1.
3NOTE: The purpose of this test is to perform the Instrument Cluster self
test.
Depress and hold the Odometer Reset Button.
Turn the Key from OFF to ON and then back to OFF.
Release the Odometer Reset Button.
Do the Instrument Cluster Indicators and Gauges activate and deactivate?All
Ye s®Replace the Controller Antilock Brake in accordance with the
Service Information.
Perform ABS VERIFICATION TEST - VER 1.
No®Replace the Instrument Cluster in accordance with the Service
Information.
Perform ABS VERIFICATION TEST - VER 1.
39
BRAKES (CAB)

1.0 INTRODUCTION
The procedures contained in this manual include
all the specifications, instructions and graphics
needed to diagnose engine control module (ECM)
and sentry key immobilizer system (SKIS) prob-
lems; they are no start, diagnostic trouble code
(DTC), and no trouble code problems for the ECM.
The diagnostics in this manual are based on the
trouble condition or symptom being present at the
time of diagnosis.
When repairs are required, refer to the appropri-
ate service information for the proper removal and
repair procedure.
Diagnostic procedures change every year. New
diagnostic systems may be added; carryover sys-
tems may be enhanced. IT IS RECOMMENDED
THAT YOU REVIEW THE ENTIRE MANUAL TO
BECOME FAMILIAR WITH ALL NEW AND
CHANGED DIAGNOSTIC PROCEDURES.
This manual is designed to begin all diagnosis at
the DTC TEST, which is located at the beginning of
Section 7.0. This will cover all the necessary re-
quirements to begin a logical diagnostic path for
each problem. If there is a diagnostic trouble code
(DTC) detected, it will direct you to the trouble code
test. If there are no DTCs present, it will direct you
by symptom to a no trouble code test.
This book reflects many suggested changes from
readers of past issues. After using this book, if you
have any comments or recommendations, please fill
out the form at the back of the book and mail it back
to us.
1.1 SYSTEM COVERAGE
This diagnostic procedures manual covers all
2001 RG body vehicles equipped with the 2.5L VM
diesel engine.
1.2 SIX-STEP TROUBLESHOOTING
PROCEDURE
Diagnosis of the engine control module (ECM)
and sentry key immobilizer system (SKIS) is done
in six basic steps:
²verification of complaint
²verification of any related symptom
²symptom analysis
²problem isolation
²repair of isolated problem
²verification of proper operation
NOTE: All tests in this manual should be per-
formed with the engine at operating temperature,
unless specified within a particular test.
2.0 IDENTIFICATION OF
SYSTEM
The ECM is located in the left side of the engine
compartment between the left front headlamp and
the intelligent power module. The sentry key immo-
bilizer module (SKIM) is located below the steering
column behind the steering wheel.
3.0 SYSTEM DESCRIPTION AND
FUNCTIONAL OPERATION
3.1 GENERAL DESCRIPTION
The 2.5L VM diesel engine system is equipped
with the latest in technical advances. The on-board
diagnostics incorporated in the engine control mod-
ule and SKIM are intended to assist the field
technician in repairing vehicle problems by the
quickest means.
The engine system incorporates a common rail
fuel delivery design. This design utilizes electroni-
cally controlled solenoid valve type fuel injectors.
Each injector is controlled individually by the ECM.
Injector timing and fuel quantity are controlled by
the ECM based on inputs from the various sensors.
The precision control of the injectors by the ECM
helps to reduce the engine noise, odor and smoke.
3.2 FUNCTIONAL OPERATION
3.2.1 ECM ON-BOARD DIAGNOSTICS
The ECM has been programmed to monitor many
different circuits of the diesel fuel injection system.
This monitoring is called on-board diagnostics.
Certain criteria must be met for a trouble code to
be entered into the ECM memory. The criteria may
be a range of: engine rpm, engine temperature, time
or other input signals to the ECM. If all of the
criteria for monitoring a system or circuit are met,
and a problem is sensed, then a DTC will be stored
in the ECM memory.
It is possible that a DTC for a monitored circuit
may not be entered into the ECM memory, even
though a malfunction has occurred. This may hap-
pen when the monitoring criteria has not been met.
The ECM compares input signal voltages from
each input device with specifications (the estab-
lished high and low limits of the input range) that
are programmed into it for that device. If the input
voltage is not within the specifications and other
trouble code criteria are met, a DTC will be stored
in the ECM memory.
1
GENERAL INFORMATION