Powertrain, generic (extended)

P2643 Torque Management Feedback Signal B Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

With P2643 the transmission controller sees feedback signal B below its lower limit, so it cannot confirm that the engine is holding the torque level it asked for. No manufacturer describes this exact code in public documents. Where the feedback is a data-link message, as with Allison transmissions behind Cummins, Mack, Volvo or Navistar engines, a low value tends to come from a missing or defaulted message, an incompatible calibration or a bus fault; on a wired signal a short to ground is the classic cause. CARB requires comprehensive components to be tested against their range limits. Because Allison ties request B to Low Range Torque Protection, the TCM may lock out low ranges when it cannot verify the limit. P2644 is the high counterpart.

Likely causes

Torque message missing or sent with a default value on the data link
Engine and transmission calibrations incompatible
Bus or signal wiring damaged, short to ground

Symptoms

  • Transmission warning lamp on
  • Low ranges or reverse low not selectable
  • Harsh shifts because torque reduction is missing

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€80–600
Time0.5–3 h

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

Parts involved

tcmcommunication bus

Related codes

P2641P2642P2644P2639

References: law.cornell.edu, static.nhtsa.gov, static.nhtsa.gov.