P1688

ALLISON AUTOMATIC TRANSMISSION

DTC P1688





Circuit Description for Allison Automatic Transmission
The Transmission Control Module (TCM) receives input from the Engine Control Module (ECM) on Throttle Knockdown Signal Circuit connector C1 pin 9 for unmanaged gross engine torque, and Delivered Torque Signal Circuit connector C1 pin 16 for managed net engine torque. The TCM monitors and compares the engine input torque signals for proper torque requested values. The TCM sends a signal via Requested Torque Signal Circuit connector C1 pin 22 to the engine if a torque reduction is needed.
DTC P1688 is a type X DTC for the 6.6 L engine and a type B DTC for the 8.1 L engine.

Conditions for Running the DTC
^ The components are powered and ignition voltage is greater than 9 V and less than 18 V.
^ Engine speed is greater than 200 RPM and less than 7,500 RPM for 5 seconds.
^ Pulse Width Modulation (PWM) driver demand torque option is selected in transmission calibration.
^ DTC 1688 must not be active.

Conditions for Setting the DTC
DTC P1688 sets if unmanaged engine torque (gross) signal is under 1.5 percent for 2 seconds, or over 98.5 percent for 2 seconds.

Action Taken When the DTC Sets
Value is defaulted with a managed engine torque value. If P1688 and P1779 are active, value is defaulted with a look-up torque value based on throttle and engine speed. If P1688 and P1779 are active and throttle is defaulted (U1016 or U1300 or U1301 active), the TCC is disengaged and inhibited.

Conditions for Clearing the MIL/DTC
A scan tool can 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 failure.

Test Description
The number below refers to the step number on the diagnostic table.
2. This step tests for open/shorts in input connector C1 pin 9 Throttle Kickdown Signal Circuit.