Allison 1000 & 2000 Gen 4 Fault Codes: P1688 Unmanaged Engine Torque Delivered To TCM Signal

CIRCUIT DESCRIPTION:
The GM 8.1 liter gasoline engine is used in some medium duty applications. The 8.1 liter engine uses a 3-wire system to communicate engine torque data/requests between the powertrain control module (PCM) and the transmission control module (TCM).

Unmanaged engine torque is sent from the PCM to the TCM over wire 109 (GM wire 2467).

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM).
2. Engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.
3. Pulse Width Modulation (PWM) driver demand torque option is selected in transmission calibration.

CONDITIONS FOR SETTING THE DTC:
DTC P1688 sets if unmanaged (gross) engine torque signal (circuit 2467) is under 1.5 percent or over 98.5 percent for more than a 2 second period.

ACTION TAKEN WHEN THE DTC SETS:
1. DTC P1688 is stored in the TCM history.
2. The CHECK TRANS light illuminates on the second occurrence.
3. The TCM defaults to a calculated unmanaged torque (gross) input value using throttle and engine speed.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool may be used to clear the code 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:
An Intermittent condition may be the cause of the problem. Return the vehicle to the OEM for further troubleshooting.