pressure port to main pressure. PCS2 pressure is directed through shift valve 1, shift valve 3, and
the manual selector valve to the 1-2-3-4 clutch, applying the 1-2-3-4 clutch. The transmission
control module (TCM ) provides current to PCS2, which controls the rate of pressure buildup of
the 1-2-3-4 clutch, assuring a smooth transition to first range. The combination of the 1-2-3-4
clutch and the low and reverse clutch application produces first range operation.
After the shift to first range is complete, SS3 is de-energized followed closely by SS1 (timed to
prevent SS3 from becoming "latched" down), allowing both valves to de-stroke. PCS2 is also de-
energized, exhausting PCS2 signal pressure, allowing the pressure control valve 2 to de-stroke
which exhausts PCS2 pressure. In the de-stroked position, shift valve 3 routes main pressure to
the manual selector valve which directs the main pressure to the 1-2-3-4 clutch, keeping the 1-2-
3-4 clutch applied. With the 1-2-3-4 clutch and the low and reverse clutch applied, the
transmission stays in first range.
Control main pressure exhaust for pressure switch 2 is blocked by shift valve 2, keeping the
switch turned on and providing feedback to the TCM that shift valve 2 is stroked. With shift valve
1 in the de-stroked position, the pressure feed to pressure switch 1 is eliminated and with the
pressure exhausted, the pressure switch turns off, providing feedback that shift valve 1 is in the
de-stroked position. With shift valve 3 de-stroked, the control main pressure to pressure switch 3
is exhausted and pressure switch 3 turns off, providing feedback that shift valve 3 is in the de-
stroked position.
If electrical power is interrupted while the transmission is in first range, SS2 is de-energized.
Normally closed PCS1 continues to block the exhaust of PCS1 signal pressure (signal pressure
remains control main) which results in maximum PCS1 pressure being maintained. PCS1 pressure
continues to be directed to shift valve 2, which stays stroked due to the "latching" effect of
pressure on two different diameter lands and fluid continues to be directed to the low and reverse
clutch, keeping the low and reverse clutch applied. Main pressure continues to be directed
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 combination of the 1-2-3-4 clutch and the low and reverse clutch application
allows the transmission to stay in first range for limp home capability.
If electrical power is interrupted while the transmission is in first range, moving the manual
selector valve to Neutral (N) will cut off main pressure to the 1-2-3-4 clutch and the clutch will
be exhausted. The low and reverse clutch remains applied. Because only one clutch is applied, the
transmission goes to neutral.
If electrical power is interrupted while the transmission is in first range, moving the manual
selector valve to Reverse (R) cuts off main pressure to the 1-2-3-4 clutch, exhausting the 1-2-3-4
clutch. The manual selector valve directs main pressure through the TCC valve to the bottom of
shift valve 2, de-stroking the valve and opening the path for main pressure to feed the low and
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &
Silverado