Powertrain, generic (extended)

P2641 Torque Management Feedback Signal B

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

On Allison automatics, feedback B belongs to Low Range Torque Protection (LRTP): in the low ranges the TCM caps engine torque to spare gears and output shaft and checks that the engine respects the cap. P2641 sets when the engine reports more torque than permitted. Allison bulletin TT7401EN describes 2013 Cummins ISX12 engines with 4700/4800 transmissions: at idle in Low Reverse the ECM broadcast a wrong torque value, CHECK TRANS came on and Low Reverse was blocked until the ECM received calibration PH9.30.0.6 or later. Mack and Volvo tech tip TT-013-2015 sees the code under heavy load without drivability effects and also prescribes newer engine software. Navistar names it for Allison 1000/2000 calibrations. P2637 is the counterpart for request A.

Likely causes

Engine ECM software below the level required for the transmission (Cummins, Mack, Volvo)
Engine broadcasts torque above the low-range limit requested by the TCM
Transmission calibration not matched to the engine

Symptoms

  • CHECK TRANS lamp on
  • Low Reverse or other low ranges unavailable
  • Code appears under high load, often without noticeable drivability change

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€80–400
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

tcmecmcommunication bus

Related codes

P2637P2642P2643P2644P0614

References: static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov.