Chevrolet Silverado / GMC Sierra. Service manual — part 830

CLUTCH TEST

Use the scan tool in order to perform the clutch test. Select the following information when
setting up the tool for running the test. Use the Shift Transmission feature on the scan tool to
complete the clutch test.

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 allows the technician to shift the transmission and observe

the integrity of each clutch/range in the transmission.

Attach the transmission pressure gage as described in the Line Pressure Check Procedure. Refer
to Line Pressure Check. Apply the parking brake. Hold the service brakes with your left foot
and move the gear selector to DRIVE. Increase engine speed to 1,400 RPM with your right foot.
With soft keys, select UPSHIFT and shift the transmission from FIRST gear to SIXTH gear while
observing turbine speed. Turbine speed should increase momentarily and then drop to 0 RPM
when each gear is attained. Line pressure should drop momentarily, then gain pressure again when
each gear is attained.

Maintain 1,400 RPM and select the DOWNSHIFT soft key. Downshift the transmission from
SIXTH gear sequentially to FIRST gear while observing turbine speed. Turbine speed should
increase momentarily and then drop to 0 RPM when each gear is attained.

To further investigate a suspected leak in a clutch circuit, the clutch test should be repeated,
keeping the engine at idle speed. While performing the clutch test at 600 RPM, main pressure
should drop momentarily, then gain pressure again when each gear range is attained. If main
pressure remains low, suspect damaged seal in that range. If low pressure is in FIRST range only,
suspect Low and Reverse piston seal damage.

FASTLEARN PROCEDURE

OVERVIEW

2008 Chevrolet Silverado 1500

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

Silverado

In general, FastLearn is a procedure for Allison 1000 Series transmissions in which a series of
tests are run to allow the transmission control module (TCM) to learn individual clutch
characteristics. Once the clutch data is learned, FastLearn translates it to the adaptive data cells,
which the TCM uses for clutch control during range shifts. FastLearn is used at GM assembly
plants, and allows the vehicle to be driven out of the assembly plant in a near-fully-adapted state
to minimize any customer shift concerns. The scan tool version of FastLearn is intended to
provide the same benefit following transmission repair or replacement at GM Dealerships.

FastLearn must be used when one of the following repairs have been made to the vehicle:

Transmission replacement or internal service/overhaul

Valve body repair or replacement, including replacement of the pressure control solenoids

TCM replacement

TCM software/calibration update

Any service in response to a shift quality concern

The scan tool is used to initiate FastLearn by selecting the following commands:

F3: Special Functions

F1: Transmission Output Controls

FastLearn

Block the drive wheels.

Apply the parking brake.

Apply the service brake during DRIVE and REVERSE.

0 percent throttle and engine at idle RPM

Transmission sump temperature is between 40-100°C (104-212°F) - Ref temp bargraph on
screen.

If equipped, 4WD transfer case in 2WD

Three steps are required to successfully complete the FastLearn procedure:

IMPORTANT: When performing FastLearn, the following conditions must be

met:

IMPORTANT: If at any time during the procedure, required conditions are not

met, FastLearn may abort and the process will need to be started
again from the beginning.

2008 Chevrolet Silverado 1500

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

Silverado

1. PARK Mode

2. DRIVE Mode

3. REVERSE Mode

PARK MODE

While the transmission is in PARK, with the engine idling, FastLearn will cycle through a series
of tests where the 3rd, 5th and reverse clutch and the 2nd clutch is repeatedly applied to learn
their clutch characteristics. During the 3rd, 5th and reverse clutch and 2-6 clutch apply/release
procedure, FastLearn is able to characterize the pressure control solenoid 1 (PCS1) and pressure
control solenoid 2 (PCS2) pressures corresponding to clutch return springs, and is also able to
learn the volumes for the 3rd, 5th and reverse clutch and 2-6 clutch packs. In addition, the low
and reverse clutch is repeatedly applied and released in PARK to purge out air for later learning
of the clutch volume.

DRIVE MODE

Once all of the PARK test data have converged, the scan tool instructs the driver to select
DRIVE. Once DRIVE is selected, the TCM engages the 1-2-3-4 clutch to obtain DRIVE and
learn 1-2-3-4 clutch volume. The TCM repeats this test until the volume learned for the 1-2-3-4
clutch has converged. The TCM then engages 4-5-6 clutch until the volume is learned and 4-5-6
clutch has converged.

REVERSE MODE

Next, the scan tool instructs the driver to select REVERSE. Once REVERSE is selected, the
TCM engages the low and reverse clutch to obtain REVERSE and to learn the low and reverse
clutch volume. The TCM repeats this test until the volume learned for the low and reverse clutch

CAUTION: Block the wheels before selecting DRIVE. While in DRIVE, if

the Scan Tool loses communication or becomes
disconnected, the vehicle could move forward. Failure to
block the wheels could result in personal injury or property
damage.

CAUTION: Block the wheels before selecting REVERSE. While in

REVERSE, if the Scan Tool loses communication or becomes
disconnected, the vehicle could move backward. Failure to
block the wheels could result in personal injury or property
damage.

2008 Chevrolet Silverado 1500

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

Silverado

has converged.

After learning the 1-2-3-4 and the low and reverse clutch volume, FastLearn updates the adaptive
volume data for all shifts with either the 1-2-3-4 or the low and reverse on-coming clutch.

TROUBLESHOOTING

If FastLearn will not run and the above stated conditions have been met, ensure the following:

Transmission fluid temperature is between 40-100°C (104-212°F).

Closed throttle and engine at warm idle RPM

No active DTCs

All speed sensors are connected and functioning properly.

Internal mode switch is functioning properly

Main pressure is within specification.

TCC slip speed less than 100 RPM at idle in PARK/NEUTRAL

LIMP HOME MODE

PRINCIPLE OF OPERATION

The Allison transmission incorporates control logic that enables the transmission to revert to a
limited total hydraulic operation. The hydraulic system provides a Limp Home Mode, allowing
the operator to drive the vehicle to a service location. If power is interrupted while the
transmission is operating in R, REVERSE, or N, NEUTRAL, the transmission defaults to or
continues NEUTRAL operations. The operator may turn off the engine and start the engine again.
The shift selector provides REVERSE/NEUTRAL/DRIVE capability regardless of the range
where the failure occurred. In this state, pressure control solenoid 1 (PCS1), de-energized, allows
the 3rd, 5th and reverse clutch to be applied. If the selector valve is moved to the REVERSE
position, main pressure is routed to the low and reverse clutch, allowing REVERSE operation. If
the selector valve is moved to the DRIVE position, main pressure is routed to the 1-2-3-4 clutch,
allowing THIRD range operation.

TEST PROCEDURE

If a No Forward, No REVERSE, or Will Not Move condition occurs:

1. Set the parking brake.

IMPORTANT: All shifts will be left in fast adaptive mode at this point.

2008 Chevrolet Silverado 1500

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

Silverado