Road Test; Preliminary Inspection - Hummer H1 Service Manual

3-6 Fuel, Emissions, and Exhaust
POWERTRAIN CONTROL MODULE (PCM)
The PCM controls operation of all of the engine and transmis-
sion outputs. It is on a bracket attached to floor under the cen-
ter console. The data link connector is the access point for the
scan tool. The connector is under the instrument panel at the
left side of the steering column. It is a multi-pin connector with
integral mounting bracket.
Signal inputs used by the PCM include:
• Coolant temperature
• Intake air temperature
• Boost/baro pressure
• Optical/fuel temperature sensor
• Crankshaft position sensor
• Accelerator pedal position sensors (APP)
• Cruise control
• A/C request
• Vehicle speed sensor
• Automatic transmission fluid pressure manual valve po-
sition switch
• Transmission input speed sensor
• Transfer case low range switch
• Transmission fluid temperature sensor
• Diagnostic request
FUEL SYSTEM DIAGNOSIS
General Information
Fuel system diagnosis should begin with a preliminary inspec-
tion followed by a road test to confirm existence of a problem.
Preliminary inspection consists of visual checks designed to
rule out faults caused by leaks, fuel line damage, or loose elec-
trical connections.

Preliminary Inspection

1. Check fuel gauge readings for main and auxiliary tanks.
2. Visually inspect fuel lines at tanks, selector valve, fuel
pump, and at engine. Look for loose, damaged, leaking
lines.
3. Check vacuum actuator hose on turbo diesel models.
4. Check wiring for fuel lift pump.
5. Check glow plug and relay wiring. Also check electronic
accelerator pedal wiring on turbo diesel. Look for loose
connections at all points (including battery), or for
damaged, pinched wires.
6. Check serpentine drive belt tension and condition.
7. Note if any indicator lights are on. If engine runs, also
check voltage, oil pressure, and coolant temperature
readings.
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8. Check exhaust system components for damage, loose joint
connections, or grounding against body.
9. Inspect transmission linkage and wiring connections,
especially if vehicle is used for off road operation.
10. If vehicle is drivable, and no obvious problems are found,
road test vehicle. If vehicle is not drivable, refer to
diagnosis charts and scan tool diagnosis.

Road Test

1. Note battery voltage. Low voltage can cause PCM power-
up and initialization problems. Battery voltage should be
no lower than 11.5 volts for satisfactory operation.
2. Connect scan tool to diagnostic link connector. Set tool to
record fault codes, or have helper use tool to interrogate
system during test.
3. Start engine and check idle operation. Note any faults such
as rough idle, stalling, excessive smoke.
4. Apply service brake and shift transmission into drive
range. Note if engine stalls, runs rough, or another fault
occurs.
5. Drive vehicle on road at posted limits. Note operation
under light and heavy acceleration, and at normal cruise
speeds. Note faults such as surge, miss, vibration, noise,
low power, smoke (blue, black, white).
6. Shift into neutral at cruise speed and note engine
operation. If vibration or noise was experienced but it now
stops, problem may be with drive belt, engine mount,
accessory, U-joint, or torque converter.
7. Return to shop and note fault codes recorded by PCM . If a
fuel system fault was noted during road test, refer to
diagnosis charts, perform fuel system tests, and check
system electrical components with scan tool.
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