Chevrolet Silverado / GMC Sierra. Manual — part 831
2. With the ignition OFF, disconnect the 80-way connector at the transmission control module
(TCM).
3. Using a fused jumper wire, connect terminal R of the automatic transmission inline 20-way
connector to a known good ground.
4. Start the engine and move the range selector to REVERSE position.
The transmission should engage REVERSE range.
5. Move the range selector to the D position.
The transmission should engage THIRD range.
If REVERSE and THIRD range are available in Limp Home Mode, an electrical failure may be
indicated. If only 1 of the 2 ranges, or neither, was obtainable, this may indicate an internal
hydraulic failure such as one of the following:
Failed clutch
Stuck valve
Solenoid failure
The clutches that could possibly have an indicated failure in Limp Home are the 1-2-3-4 clutch,
the 3rd, 5th and reverse clutch, and the low and reverse clutch.
ROAD TEST
The following test provides a method of evaluating the condition of the automatic transmission.
The test is structured so that most driving conditions would be achieved. The test is divided into
the following parts:
Electrical Function Check
Upshift Control and Torque Converter Clutch (TCC) Apply
Part Throttle Detent Downshifts
Full Throttle Detent Downshifts
Manual Downshifts
Coasting Downshifts
IMPORTANT: The Road Test Procedure should be performed only as part of the
Functional Test Procedure. Refer to Functional Test.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
Manual Range Selection
REVERSE
Manual FIRST
Before the road test, ensure the following conditions are met:
The engine is performing properly.
Transmission fluid level is correct.
Tire pressure is correct.
During the road test:
Perform the test only when traffic conditions permit.
Operate the vehicle in a controlled, safe manner.
Observe all traffic regulations.
View the scan tool data while conducting this test.
Take along qualified help in order to operate the vehicle safely.
Observe any unusual sounds or smells.
After the road test, inspect the following conditions:
Transmission fluid level
Refer to Transmission Fluid Check.
Diagnostic trouble codes (DTCs) that may have set during the testing
Refer to the applicable DTC.
Scan tool data for any abnormal readings or data
ELECTRICAL FUNCTION CHECK
Perform this check first, in order to ensure the electronic transmission components are connected
and functioning properly. If these components are not checked, a simple electrical condition
IMPORTANT: Complete the test in the sequence given. Incomplete testing
cannot guarantee an accurate evaluation.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
could be mis-diagnosed.
1. Install the scan tool.
2. Ensure the shift selector is in PARK and set the parking brake.
3. Start the engine.
4. Verify that the following scan tool data can be obtained and is functioning properly. Refer
to Scan Tool Data List for typical data values. Data that is questionable may indicate a
concern.
Engine speed
Transmission input speed
Transmission turbine speed
Transmission output speed
Vehicle speed
TFP 1, 2, 3 and TFP 4 pressure switch states
C, D, E and REVERSE pressure switch states
Transmission range, engine list
4WD low
Commanded range, current range
PCS1 and PCS2 solenoid reference current
PCS1 and PCS2 solenoid actual current
PCS1 and PCS2 solenoid duty cycle
Brake switch
Engine coolant temperature
Transmission fluid temperature
Throttle angle
Ignition voltage
Shift solenoid 1
Shift solenoid 2
Shift solenoid 3
Modulated main G solenoid
TCC solenoid duty cycle
TCC slip speed
5. Monitor the brake switch signal while depressing and releasing the brake pedal. The scan
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
tool should display the following:
Closed when the brake pedal is released
Open when the brake pedal is depressed
6. Check the garage shifts.
1. Apply the brake pedal and ensure the parking brake is set.
2. Move the shift selector through the following ranges:
1. PARK to REVERSE
2. REVERSE to NEUTRAL
3. NEUTRAL to DRIVE
3. Pause 2 to 3 seconds in each selector position.
IMPORTANT: Harsh engagement may be caused by any of the
following conditions:
High idle speed
Compare engine idle speed to desired idle speed.
Commanded low PCS1 or PCS2 solenoid current
Compare PCS1 and PCS2 solenoid reference current
to PCS1 and PCS2 solenoid actual current.
A default condition caused by certain DTCs that
result in maximum full throttle shift points
Shift is not fully adapted
IMPORTANT: Soft or delayed engagement may be caused by any of the
following conditions:
Low idle speed
Compare engine idle speed to desired idle speed.
Low fluid level
Cold transmission fluid
Inspect for low transmission fluid temperature.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado
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