Chevrolet Silverado / GMC Sierra. Service manual — part 1004

valve, shift valve 3 and shift valve 1, and pressure control valve 2. PCS1 is de-energized, raising
the PCS1 signal and pressure control valve 1 pressures. PCS1 pressure is directed through shift
valve 2 to the 3rd, 5th, and reverse clutch, applying the clutch. The TCM reduces the current to
PCS1, which controls the rate at which pressure control valve 1 supplies pressure to the 3rd, 5th,
and reverse clutch, and reduces the current to PCS2, which controls the rate at which pressure
control valve 2 exhausts the 1-2-3-4 clutch, assuring a smooth transition from fourth to fifth range.
The 4-5-6 clutch continues to be applied by main pressure flowing through shift valve 3, shift
valve 2, and the manual selector valve.

The combination of the 4-5-6 clutch and the 3rd, 5th, and reverse clutch application produces
fifth range operation.

Pressure switch 1 and pressure switch 2 are turned OFF, providing feedback that shift valve 1 and
shift valve 2 are in the de-stroked position, and pressure switch 3 remains ON, providing
feedback that shift valve 3 is in the stroked position.

If electrical power is interrupted while the transmission is in fifth range, PCS2, shift solenoid 3
(SS3), and the torque converter clutch (TCC) solenoid (if TCC is applied) de-energize. Although
SS3 no longer supplies control main pressure to the top of shift valve 3, control main pressure
flows through shift valve 1 to the top of shift valve 3, keeping the valve stroked. Normally closed
PCS1 continues to block the exhaust of the signal pressure (signal remains control main). The
signal pressure on top of pressure control valve 1 produces maximum control pressure, which is
routed through shift valve 2 to the 3rd, 5th, and reverse clutch, keeping the clutch applied. Main
pressure feeds through shift valve 3 and shift valve 2 and the manual selector valve to the 4-5-6
clutch, keeping the clutch applied.

The combination of the 4-5-6 clutch and the 3rd, 5th, and reverse clutch application produces
fifth range operation for limp home capability.

If the TCC is applied, the TCC solenoid de-energizes, causing the TCC pressure control valve to
de-stroke, exhausting pressure from the top of the converter flow valve. The converter flow valve
de-strokes, redirecting main overage to converter in and converter out flow to the cooler. The
TCC is now released.

If electrical power is interrupted while the transmission is in fifth range (causing fifth range
converter operation) and the manual selector valve is moved to Neutral (N), the 4-5-6 clutch
exhausts through the manual selector valve to exhaust backfill, releasing the 4-5-6 clutch. The
3rd, 5th, and reverse clutch remains applied. With only one clutch applied, the transmission goes
to neutral.

If electrical power is interrupted while the transmission is in fifth range, (fifth range converter

2008 Chevrolet Silverado 1500

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

Silverado

operation) and the manual selector valve is moved to Reverse (R), the 4-5-6 clutch exhausts
through the manual selector valve, releasing the clutch. The 3rd, 5th, and reverse clutch remains
applied. With only one clutch applied, the transmission goes to neutral.

If electrical power is interrupted while the transmission is in fifth range, resulting in only fifth and
neutral operation, the engine may be shut down and restarted to attain neutral, third and reverse
operation.

Fig. 528: Identifying Hydraulic Circuits Used In Fifth Range
Courtesy of GENERAL MOTORS CORP.

SIXTH RANGE

Before making a shift from fifth to sixth range, the transmission control module (TCM) makes
certain that all shift valves are in the correct position. Shift solenoid 1 (SS1) and SS2 are
electrically off and shift valve 1 and shift valve 2 are in the de-stroked position. SS3 is
electrically on and shift valve 3 is in the stroked position.

Pressure control solenoid 1 (PCS1) is energized, opening PCS1 pressure to exhaust, which allows
pressure control valve 3 to de-stroke, exhausting the 3rd, 5th, and reverse clutch pressure and
releasing the 3rd, 5th, and reverse clutch. PCS2 is energized, raising the PCS2 signal and PCS2
pressure. PCS2 pressure is directed through shift valve 1, shift valve 3, shift valve 2, and again

2008 Chevrolet Silverado 1500

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

Silverado

through shift valve 3 to the 2-6 clutch, applying the clutch. Main pressure continues to flow
through shift valve 3, shift valve 2, and the manual selector valve to the 4-5-6 clutch, keeping the
4-5-6 clutch applied. The TCM supplies current to the PCS2, which controls the rate at which the
pressure control valve 2 supplies pressure to the 2-6 clutch, and current to PCS1, which controls
the rate at which the pressure control valve 1 exhausts the 3rd, 5th, and reverse clutch, assuring a
smooth transition to sixth range.

The combined application of the 4-5-6 clutch and the 2-6 clutch produces sixth range operation.

Pressure switch 1 and pressure switch 2 remain open (OFF) providing feedback that shift valve 1
and shift valve 2 remain in the de-stroked position, and pressure switch 3 remains closed (ON)
providing feedback that shift valve 3 remains in the stroked position.

If electrical power is interrupted while the transmission is in sixth range, PCS1 and PCS2, SS3
and the torque converter clutch (TCC) solenoid (if TCC is applied) are de-energized. Although
SS3 supplies control main pressure to the top of shift valve 3, control main pressure flows
through shift valve 1 to the top of shift valve 3, keeping the valve in the stroked position.
Normally open PCS2 exhausts PCS2 signal pressure, allowing the pressure control valve 2 to de-
stroke, exhausting the 2-6 clutch. Normally closed PCS1 blocks exhausting of PCS1 signal
pressure, allowing PCS1 signal and PCS1 pressure to rise.

PCS1 pressure is directed through shift valve 2 to the 3rd, 5th, and reverse clutch, applying the
clutch. Main pressure continues to flow through shift valve 3, shift valve 2, and the manual
selector valve to the 4-5-6 clutch, keeping the 4-5-6 clutch applied. The combination of 4-5-6
clutch and 3rd, 5th, and reverse clutch application produces fifth range operation for limp home
capability.

If the TCC is applied, the TCC solenoid de-energizes, causing the TCC valve to de-stroke,
exhausting pressure form the top of the converter flow valve. De-stroking the converter flow
valve redirects main overage pressure to "converter in" path, then to "converter out" proceeding
on to the transmission oil cooler. The TCC is now released.

If electrical power is interrupted while the transmission is in sixth range (causing sixth range
converter operation [TCC OFF]) and the manual selector valve is moved to Neutral (N), the 4-5-
6 clutch exhausts through the manual selector valve to exhaust backfill pressure, releasing the 4-5-
6 clutch. The 3rd, 5th, and reverse clutch remains applied. With only one clutch applied the
transmission shifts to neutral

If electrical power is interrupted while the transmission is in sixth range (causing fifth range
converter operation [TCC OFF]) and the manual selector valve is moved to Reverse (R), the 4-5-
6 clutch exhausts through the manual selector valve to exhaust backfill pressure, releasing the 4-5-

2008 Chevrolet Silverado 1500

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

Silverado

6 clutch. The 3rd, 5th, and reverse clutch remains applied. With only one clutch applied, the
transmission shifts to neutral.

If electrical power is interrupted while the transmission is in sixth range, resulting in only fifth
range and neutral operation, the engine may be shut down and restarted to attain neutral, drive,
and reverse operation.

Fig. 529: Identifying Hydraulic Circuits Used In Sixth Range
Courtesy of GENERAL MOTORS CORP.

REVERSE

When the manual selector valve is moved from Neutral (N) to Reverse (R), shift solenoid 1 (SS1)
de-energizes, turning OFF pressure switch 1. SS2 and SS3 remain energized, keeping pressure
switch 2 and pressure switch 3 on. Reverse signal pressure is exhausted, removing pressure from
pressure switch 4, turning on the normally closed pressure switch 4.

The transmission control module (TCM) energizes pressure control solenoid 2 (PCS2) to control
the rate at which PCS2 pressure supplies oil to the oncoming 3rd, 5th, and reverse clutch,
assuring a smooth transition to reverse. As PCS2 is energized, PCS2 signal pressure increases.
Initially, the pressure control valve 2 gain valve spring keeps the gain valve away from pressure
control valve 2, so that only PCS2 signal pressure acts on pressure control valve 2. As PCS2

2008 Chevrolet Silverado 1500

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

Silverado