Allison 3000 & 4000 Gen 4 Fault Code: P2637 Torque Management Feedback Signal – SEM

DIAGNOSTIC RESPONSE:
Inhibit SEM

CIRCUIT DESCRIPTION:
Shift Energy Management (SEM) allows the Transmission Control Module {TCM) to request torque reduction from the engine controller. By reducing torque, shifts can be made quicker and at a more consistent output torque, which reduces clutch temperatures allowing increased clutch life. When an engine torque rating exceeds certain limits, Low Range Torque Protection (LRTP) is used. This feature limits engine torque in lower ranges to protect the transmission from damage if a stall condition occurs.

This test detects:
Electronic Control Module (ECM) response to SAE J1939 transmission torque reduction commands during upshifts.

For high torque applications, the engine must support the required messages to allow SEM and LRTP. For low torque applications, SEM may or may not be active.

CONDITIONS FOR RUNNING THE DTC:
1. Upshift in process with driver’s demand engine torque is greater than 80 N·m (59 lb ft).
2. DTC test runs 1 second intervals.
3. This DTC and DTC UO 115 not active.

NOTE: The autodetect drive cycles counter is reset to 0 after TCM recalibration or SEM Reset Auto-Select.

CONDITIONS FOR SETTING THE DTC:
If a SEM engine does not respond to the transmission TSC1 commands to reduce engine torque during upshifts, DTC P2637 becomes active. Upshifts are not required to be consecutive but are in the same drive cycle.

ACTION TAKEN WHEN THE DTC SETS:
1. The CHECK TRANS light illuminates.
2. DTC is stored in the TCM history.
3. Clutch pressures are increased, which may result in shift quality issues.
4. Some shifts may be inhibited.
5. The TCM freezes shift adapts (DNA).
6. The TCM inhibits TCC engagement.
7. DTCs P0614 and/or P2641 may also be active.

CONDITIONS FOR CLEARING THE DTC:
The TCM will continue attempting to use SEM, and will self-clear the active DTC if engine response returns to within acceptable criteria for a number of shifts.

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:
DTC P2637 may be caused by:
1. Incorrect engine calibration.
2. Incorrect transmission calibration.
3. Engine calibration that does not support all required SAE J1939 SEM/LRTP messages.
4. Communication link problem.
5. Vehicle controller (such as a headway controller) preventing transmission control of the engine during the shift process.
6. Defective TCM.

Review applicable information in the OEM Technician’s Library, General Troubleshooting of Performance Complaints, Shift Selector Issues, Control System and Transmission Specifications, and other related information.