Chevrolet Silverado / GMC Sierra. Manual — part 1003

which allows the pressure control valve 1 to de-stroke and the 3rd, 5th, and reverse clutch
pressure is exhausted. PCS2 is energized, raising the PCS2 signal and PCS2 pressures. The PCS2
pressure is directed through shift valve 1, shift valve 3, shift valve 2, and the manual selector
valve to the 4-5-6 clutch, applying the clutch. Main pressure continues to flow through shift valve
3 and the manual selector valve to the 1-2-3-4 clutch, keeping the 1-2-3-4 clutch applied. The
TCM supplies current to PCS2, which controls the rate at which pressure control valve 2 supplies
pressure to the 4-5-6 clutch, and current to PCS1, which controls the rate at which pressure
control valve 1 exhausts the 3rd, 5th, and reverse clutch, assuring a smooth transition to fourth
range.

The combination of the 1-2-3-4 clutch and the 4-5-6 clutch application produces fourth range
operation.

After the shift to fourth range is completed, SS3 is energized, directing control main pressure to
the top of shift valve 3, stroking the valve. In the stroked position, main pressure is redirected
through shift valve 2 and the manual selector valve to the 4-5-6 clutch, keeping the 4-5-6 clutch
applied. The flow from pressure control valve 2 to shift valve 3 is directed through the manual
selector valve to the 1-2-3-4 clutch, keeping the 1-2-3-4 clutch applied.

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

If electrical power is interrupted while the transmission is in fourth range, PCS1, PCS2, the
torque converter clutch (TCC) solenoid, SS1, and SS3 are de-energized. Main pressure continues
to flow through shift valve 3 and shift valve 2 and the manual selector valve to the 4-5-6 clutch,
keeping the 4-5-6 clutch applied. When de-energized, normally open PCS2 exhausts the PCS2
signal pressure, allowing pressure control valve 2 to de-stroke and the 1-2-3-4 clutch exhausts.
After shift valve 1 de-strokes, the 1-2-3-4 clutch exhausts to exhaust backfill. Normally closed
PCS1 blocks the exhaust of PCS1 signal pressure, allowing the PCS1 signal and PCS1 pressures
to rise. Pressure control valve 1 directs full pressure through shift valve 2 to the 3rd, 5th, and
reverse clutch, applying the clutch. The combination of the 4-5-6 clutch and the 3rd, 5th, and
reverse clutch applied produces fifth range operation for a limp home capability. If the TCC is
applied, de-energizing the TCC solenoid causes the TCC 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 to the cooler. The TCC is now released.

If electrical power is interrupted while the transmission is in fourth 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

2008 Chevrolet Silverado 1500

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

Silverado

to neutral.

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

Fig. 527: Identifying Hydraulic Circuits Used In Fourth Range
Courtesy of GENERAL MOTORS CORP.

FIFTH RANGE

Before making a shift from fourth to fifth range, the transmission control module (TCM) makes
certain all of the shift valves are in the correct position with shift valve 1 and shift valve 3 stroked
and shift valve 2 de-stroked.

Pressure control solenoid 2 (PCS2) is de-energized, exhausting PCS2 signal pressure and pressure
control valve 2 de-strokes, which allows the 1-2-3-4 clutch to exhaust through the manual selector

2008 Chevrolet Silverado 1500

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

Silverado