Limp Home Mode

Limp Home Mode

Principle of Operation
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, trim solenoid A, de-energized, allows C3 clutch to be applied. If the selector valve is moved to the REVERSE position, main pressure will be routed to C5 clutch, allowing REVERSE operation. If the selector valve is moved to the DRIVE position, main pressure will be routed to C1, allowing THIRD range operation.

Test Procedure
If a No Forward, No REVERSE, or Will Not Move condition occurs:
1. Disconnect the C1 connector, GREY, at the transmission control module (TCM).
2. Set the parking brake.
3. Apply the service brake.
4. Start the engine and move the range selector to REVERSE position.

The transmission should engage REVERSE range.

Next, move the range selector to the D4 position. The transmission should engage THIRD range

If REVERSE and THIRD range are available in Limp Home Mode, an electrical failure may be indicated. If only one of the two ranges, or neither, was obtainable, this may indicate an internal hydraulic failure such as:
^ Failed clutch
^ Stuck valve
^ Solenoid failure

The clutches that could possibly have an indicated failure in Limp Home are C1, C3, and C5.