Allison 1000 & 2000 Gen 5 Fault Code:P0742 Torque Converter Clutch (TCC) System – Stuck On

CIRCUIT DESCRIPTION:
The transmission TCC solenoid is a pressure control solenoid used to control the rate of TCC apply and release. The TCM applies and releases the TCC by varying the amount of electrical current through a low side driver. The TCM uses data received from the engine speed sensor and the turbine speed sensor to calculate torque converter slip values. It can detect if the torque converter is stuck off (unlocked) or stuck on (locked) by comparing TCC slip speed to a preset value in the TCM calibration. DTC P0742 sets when the TCM detects minimal converter slip speed for a calibrated time when TCC state is commanded off. This DTC is only enabled with GM L96 applications.

ACTION TAKEN WHEN THE DTC SETS:
1. The CHECK TRANS light is illuminated.
2. The TCM forces Variable Modulated Main off.
3. Response is calibration dependent.
4. If response is disabled, no action is performed.
5. If enabled, reverse operation is disabled when active.

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without the DTC recurring.

DIAGNOSTIC AIDS:
1. This DTC sets when converter slip speed indicates TCC is staying applied when it should be released for a period of time that is calibration dependent. This may indicate an internal failure of the TCC or a stuck or sticking TCC valve.
2. This concern may indicate a torque converter flow valve that is stuck or sticking.