Principle of Operation

WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

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 will provide R/N/D, REVERSE/NEUTRAL/DRIVE capability regardless of the range, wherein the failure occurred. In this state, pressure control solenoid A, de-energized, allows the 3rd, 5th and reverse clutch to be applied. If the selector valve is moved to the REVERSE position, main pressure will be routed to the low and reverse clutch, allowing REVERSE operation. If the selector valve is moved to the DRIVE position, main pressure will be routed to the 1-2-3-4 clutch, allowing THIRD range operation.