Chevrolet Silverado / GMC Sierra. Service manual — part 738

Shift Solenoid 1 and 2

3. Place the gear selector in the DRIVE position.

4. Accelerate the vehicle using the chosen throttle position. Hold the throttle steady.

5. As the transmission upshifts, there should be a noticeable shift feel or engine speed change

within 1 to 2 seconds of the commanded gear change. The TCC feel may not be noticeable.
Look for 100-300 RPM engine speed change.

6. Compare the shift speeds to the Shift Speed table. Refer to Shift Speed . Shift speeds may

vary slightly due to TFT or other operating variables, including hydraulic delays in
responding to electronic controls.

Note any harsh, soft or delayed shifts or slipping.

Note any noise or vibration.

7. Monitor TCC PC solenoid current while driving and make sure the current rises when the

TCC is commanded to apply.

When the TCC applies there should be a noticeable drop in engine speed and a drop in
slip speed to below 100 RPM. If the TCC apply can not be detected:

Check for DTCs.

Refer to Torque Converter Diagnosis.

Apply and release the brake pedal. The TCC will release on most applications.

PART THROTTLE DETENT DOWNSHIFT

IMPORTANT: This transmission is equipped with an electronically

controlled capacity clutch (ECCC), which does not allow the
clutch to fully lock to the torque converter cover. The clutch
maintains a small amount of slippage, approximately 20
RPM, in 2nd, 3rd, 4th, 5th, and 6th gears, depending on the
vehicle application. ECCC was developed to reduce the
possibility of noise, vibration or chuggle caused by TCC
apply. Full lockup is available at highway speeds on some
applications.

IMPORTANT: The TCC will not engage until the engine is in closed loop

operation and the vehicle speed is as shown in the Shift
Speed table. Refer to Shift Speed . The vehicle must be in a
near-cruise condition, not accelerating or coasting, and on a
level road surface.

2008 Chevrolet Silverado 1500

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

Sierra & Silverado

1. Place the gear selector in the DRIVE position.

2. Accelerate the vehicle to 64-88 km/h (40-55 mph) in 6th gear.

3. Quickly increase throttle angle to between 25-30 percent.

4. Verify the following:

The TCC releases.

The transmission downshifts immediately.

MANUAL DOWNSHIFTS

Manual downshift testing is not required for vehicles equipped with Driver Shift Control (DSC).
The TCM and shift solenoids will automatically override DSC downshifts to protect the
transmission from damage.

COASTING DOWNSHIFTS

1. Place the gear selector in the DRIVE position.

2. Accelerate the vehicle to 6th gear with the TCC applied.

3. Release the throttle and apply the brakes

4. Verify the following:

The TCC releases (on most applications).

Downshifts occur as commanded.

MANUAL GEAR RANGE SELECTION

This application does not utilize manual forward gear ranges.

Reverse

Perform the following test using a 10-15 percent throttle position.

1. With the vehicle stopped, move the gear selector to REVERSE.

2. Slowly accelerate the vehicle.

3. Verify that there is no noticeable slip, noise or vibration.

Driver Shift Control (DSC)

Refer to the owner's manual for specific instructions on DSC. The TCM will upshift
automatically when maximum engine speed is achieved and will protect from any downshift
which may cause excessive engine RPMs.

2008 Chevrolet Silverado 1500

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

Sierra & Silverado

TORQUE CONVERTER DIAGNOSIS

The torque converter clutch (TCC) is applied by fluid pressure, which is controlled by a TCC
pressure control (PC) solenoid. This solenoid is part of the control solenoid valve assembly,
which is located inside the automatic transmission assembly. The solenoid is controlled through a
combination of computer controlled switches and sensors.

TORQUE CONVERTER STATOR

The torque converter stator roller clutch can have 2 different malfunctions.

The stator assembly freewheels in both directions.

The stator assembly remains locked up at all times.

POOR ACCELERATION AT LOW SPEED

If the stator is freewheeling at all times, the vehicle tends to have poor acceleration from a
standstill. At speeds above 50-55 km/h (30-35 mph), the vehicle may act normally. For poor
acceleration, you should first determine that the exhaust system is not blocked, and the
transmission is in First gear when starting out.

If the engine freely accelerates to high RPM in NEUTRAL, you can assume that the engine and
the exhaust system are normal. Check for poor performance in DRIVE and REVERSE to help
determine if the stator is freewheeling at all times.

POOR ACCELERATION AT HIGH SPEED

If the stator is locked up at all times, performance is normal when accelerating from a standstill.
Engine RPM and vehicle speed are limited or restricted at high speeds. Visual examination of the
converter may reveal a blue color from overheating.

If the converter has been removed, you can check the stator roller clutch by inserting a finger into
the splined inner race of the roller clutch and trying to turn the race in both directions. You
should be able to freely turn the inner race clockwise, but you should have difficulty in moving
the inner race counterclockwise or you may be unable to move the race at all.

NOISE

IMPORTANT: Do not confuse this noise with pump whine noise, which is

usually noticeable in PARK, NEUTRAL and all other gear ranges.
Pump whine will vary with line pressure.

2008 Chevrolet Silverado 1500

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

Sierra & Silverado

You may notice a torque converter whine when the vehicle is stopped and the transmission is in
DRIVE or REVERSE. This noise will increase as you increase the engine RPM. The noise will
stop when the vehicle is moving or when you apply the torque converter clutch, because both
halves of the converter are turning at the same speed.

Perform a stall test to make sure the noise is actually coming from the converter:

1. Place your foot on the brake.

2. Put the gear selector in DRIVE.

3. Depress the accelerator to approximately 1,200 RPM for no more than six seconds.

A torque converter noise will increase under this load.

TORQUE CONVERTER CLUTCH SHUDDER

The key to diagnosing torque converter clutch (TCC) shudder is to note when it happens and
under what conditions.

TCC shudder which is caused by the transmission should only occur during the apply or the
release of the converter clutch. Shudder should never occur after the TCC plate is fully applied.

IF SHUDDER OCCURS DURING TCC APPLY OR RELEASE

If the shudder occurs while the TCC is applying, the problem can be within the transmission or
the torque converter. Something is causing one of the following conditions to occur:

Something is not allowing the clutch to become fully engaged.

Something is not allowing the clutch to release.

The clutch is releasing and applying at the same time.

One of the following conditions may be causing the problem to occur:

Leaking turbine shaft seals

A restricted release orifice

A distorted clutch or housing surface due to long converter bolts

Defective friction material on the TCC plate

NOTE:

You may damage the transmission if you depress the
accelerator for more than 6 seconds.

2008 Chevrolet Silverado 1500

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

Sierra & Silverado