Chevrolet Silverado / GMC Sierra. Service manual — part 737

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, check the following:

Inspect for proper transmission fluid level. Refer to Transmission Fluid Check.

Inspect for any diagnostic trouble codes (DTCs) that may have set during the testing. Refer
to the applicable DTC.

Monitor the scan tool data for any abnormal readings or data.

Inspect for fluid leaks. Refer to Fluid Leak Diagnosis.

ELECTRICAL FUNCTION CHECK

Perform this procedure first in order to ensure the electronic transmission components are
functioning properly. If these components are not checked, a simple electrical condition could be
misdiagnosed.

1. Connect the scan tool.

2. Ensure the gear 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 Control Module References for typical data values. Data that is questionable may
indicate a concern.

Engine Speed

Transmission ISS

Transmission OSS

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - 6L50/6L80/6L90 - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado

Vehicle Speed

IMS

Commanded Gear

Gear Ratio

Line PC Sol. Pressure Cmd.

TCC Brake Switch

ECT, Engine Data List

Trans. Fluid Temp.

TCM Temperature

Calc. Throttle Position

Ignition Voltage

TFP Switch 1

TFP Switch 3

TFP Switch 4

TFP Switch 5

PC Sol. 2 Pressure Cmd.

PC Sol. 3 Pressure Cmd.

PC Sol. 4 Pressure Cmd.

PC Sol. 5 Pressure Cmd.

Shift Solenoid 1

Shift Solenoid 2

TCC PC Sol. Duty Cycle

TCC Slip Speed

5. Monitor the TCC Brake Switch parameter while applying and releasing the brake pedal.

The scan tool should display:

Open when the brake pedal is applied.

Closed when the brake pedal is released.

6. Check the garage shifts.

1. Apply the brake pedal and ensure the parking brake is set.

2. Move the gear selector through the following ranges:

1. PARK to REVERSE

2. REVERSE to NEUTRAL

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - 6L50/6L80/6L90 - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado

3. NEUTRAL to DRIVE

3. Pause 2 to 3 seconds in each gear position.

4. Verify the gear engagements are immediate and not harsh.

7. Monitor transmission range on the scan tool, engine data list.

1. Apply the brake pedal and ensure the parking brake is set.

2. Move the gear selector through all ranges.

3. Pause 2 to 3 seconds in each range.

4. Return gear selector to PARK.

5. Verify that all selector positions match the scan tool display.

8. Check throttle position input.

1. Apply the brake pedal and ensure the parking brake is set.

IMPORTANT: Harsh engagement may be caused by any of the following

conditions:

High engine idle speed-Compare engine idle speed to
desired idle speed.

Commanded low pressure control (PC) solenoid current-
Investigate PC Sol. Pressure Cmd. kPa (psi) for all
solenoids. High pressure will cause harsh shifts.

A default condition caused by certain DTCs that result in
maximum line pressure to prevent clutch slippage.

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

Commanded high PC solenoid current-Investigate PC
Sol. Pressure Cmd. kPa (psi) for all solenoids. Low
pressure will cause soft or delayed shifts.

Cold transmission fluid temperature (TFT)-Use the scan
tool to determine TFT.

Selector linkage-Inspect and adjust as necessary.

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - 6L50/6L80/6L90 - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado

2. Ensure the gear selector is in PARK.

3. Monitor the scan tool Calc. Throttle Position while increasing and decreasing engine

speed with the throttle pedal. The scan tool Calc. Throttle Position percentage should
increase and decrease with engine speed.

If any of the above checks do not perform properly, record the result for reference after
completion of the road test.

UPSHIFT CONTROL AND TORQUE CONVERTER CLUTCH (TCC) APPLY

The TCM calculates the upshift points based primarily on 2 inputs: throttle position and vehicle
speed. When the TCM determines that conditions are met for a shift to occur, the TCM
commands the shift by closing or opening the ground circuit for the appropriate solenoid.

Perform the following steps:

1. Refer to Shift Speed table in this section and choose a throttle position of 12.5 percent or

25 percent. All throttle positions shown should be tested to cover the normal driving range.

2. Monitor the following scan tool parameters:

Calc. Throttle Position

Vehicle Speed

Engine Speed

Transmission ISS

Transmission OSS

Commanded Gear

TCC PC Sol. Pressure Cmd.

TCC Pressure Actual

TCC Slip Speed

TFP Switch 1

TFP Switch 3

TFP Switch 4

TFP Switch 5

PC Sol. 2 Pressure Cmd.

PC Sol. 3 Pressure Cmd.

PC Sol. 4 Pressure Cmd.

PC Sol. 5 Pressure Cmd.

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - 6L50/6L80/6L90 - Cab & Chassis Sierra, Cab & Chassis Silverado,

Sierra & Silverado