Chevrolet Silverado / GMC Sierra. Service manual — part 829

3. Examine the drained fluid for evidence of engine coolant.

4. Examine the drained fluid for evidence of metal. Metal particles in the fluid, other than

minute particles normally trapped in the oil filter, indicate internal transmission damage.
When this occurs, inspect the inside of the oil pan for excessive metal particles. Refer to Oil
Pan Replacement.

5. Metal contamination requires complete transmission disassembly.

6. Clean all internal and external hydraulic circuits, cooler, and all other areas where the

particles could lodge. Refer to Transmission Fluid Cooler Flushing and Flow Test.

LINE PRESSURE CHECK

immediate action. Failure to clean or replace all contaminated
components may result in premature transmission failure.

NOTE:

Flush the transmission cooling system and inspect for
restrictions following a transmission failure. Failure to repair
or replace restricted oil cooling system components may result
in premature transmission failure.

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

Fig. 1: Transmission Pressure Gage
Courtesy of GENERAL MOTORS CORP.

Checking main line pressure helps to determine if a transmission malfunction is due to a
mechanical or an electrical condition. Properly making these pressure checks requires
transmission and vehicle, or test stand, preparation, recording of data, and comparing recorded
data against specifications provided.

1. Remove the oil pressure tap plug. Refer to Automatic Transmission Fluid Pressure Test

Hole Plug Replacement.

2. Install J 45056 pressure test adapter fitting prior to connecting the oil pressure gage.

CAUTION: Keep the brakes applied at all times in order to prevent

unexpected vehicle motion. Personal injury may result if the
vehicle moves unexpectedly.

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

3. All transmission fluid level and pressure checks must be made at normal operating

temperatures 71-93°C (160-200°F) sump; 82-104°C (180-220°F) converter-out. Check the
transmission fluid level.

4. Connect a 0-2070 kPa (0-300 PSI) oil pressure gage to the pressure test adapter fitting. Use

the scan tool to check the engine RPM. Refer to Line Pressure for pressure level
specifications.

5. With the brakes applied, record the line pressure values at 600 RPM engine speed in

NEUTRAL and REVERSE range. The transmission will be in converter mode, torque
converter clutch not applied.

6. With the brakes applied, record the line pressure values with the engine running at 2,100

RPM in NEUTRAL. The transmission will be in converter mode, torque converter clutch
not applied.

7. With the brakes applied, use the following scan tool settings to check pressures in FIRST

through FIFTH gear ranges at 600 RPM. The transmission will be in converter mode, torque
converter clutch not applied, at 600 RPM.

1. Select F0: Diagnostics and press ENTER.

2. Select Model Year.

3. Select LDTrk, MPV, Incomplete.

4. Select F0: Powertrain, press ENTER.

5. Select Engine size, press ENTER.

6. Select 6 Speed Automatic, press ENTER.

7. Select F2: Special Functions, press ENTER.

8. Select F1: Transmission Output Controls.

9. Select Shift transmission. This will allow the technician to shift the transmission and

check line pressure in each forward gear range.

8. Compare the data recorded to the line pressure specifications. Refer to Line Pressure .

9. Disconnect the oil pressure gage and remove the J 45056 pressure test adapter fitting.

10. Install the oil pressure tap plug again. Refer to Automatic Transmission Fluid Pressure

Test Hole Plug Replacement.

OBTAINING DIAGNOSTIC TROUBLE CODES

A microcomputer controls the transmission by receiving and processing signals from various
switches and sensors. The microcomputer determines shift sequences, shift timing, and clutch
apply and release characteristics. The microcomputer is an independent controller and is referred
to as a transmission control module (TCM). The pressure switch manifold (PSM) and the

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

park/neutral position (PNP) switch provide operator input to the TCM. Other data sent to the
TCM include throttle position, engine, turbine, and output speeds, and sump temperature. Any
active special function, such as anti-lock brakes or power take-off, is also an input to the TCM.
The TCM processes this data to determine proper shift points, to monitor the current range, to
perform ratio tests, and to compile diagnostic data. The TCM is programmed to protect the
transmission and other vehicle driveline components by inhibiting actions, such as full-throttle
NEUTRAL-to-range shifts and high-speed direction changes. The TCM determines if a system
malfunction exists and stores diagnostic codes related to the malfunction. The codes accessed by
the service technician are used in diagnosing persistent or intermittent trouble in the system.

To obtain transmission diagnostic trouble codes (DTCs), install the scan tool and follow the menu
selections listed below:

1. From the title screen, press ENTER.

2. Select F0: Diagnostics. Press ENTER.

3. Select Model Year. Press ENTER.

4. Select LDTrk, MPV, Incomplete. Press ENTER.

5. Select Product Make, i.e. (C) Chevrolet Truck, (T) GMC Truck. Press ENTER.

6. Select Product Line, i.e. (C) or (K). Press ENTER.

7. Select F0: Powertrain. Press ENTER.

8. Select Engine size. Press ENTER.

9. Select 6 Speed Automatic. Press ENTER.

10. Select F0: Diagnostic Trouble Codes (DTC). Press ENTER.

11. Select F1: DTC (s) (Transmission). Press ENTER.

12. Select F0: DTC Information. Press ENTER.

13. Select F0: DTC Info. Press ENTER.

CLUTCH TEST

Clutch Test allows a technician to manually select each of the 6 forward ranges with the vehicle
stopped. The intent of this procedure is to verify the ability of all 5 clutches to transmit torque
without slipping. Refer to the table in Adapt Function, which shows the clutches applied in each
gear range.

IMPORTANT: Before clearing the DTC, use the scan tool in order to record the

Freeze Frame and Failure Records for reference. The Clear Info
function will erase the data.

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado