Powertrain, generic (extended)

P2639 Torque Management Feedback Signal A Low

Check engine lightUsually off

EmissionsNot relevant

Drive cycleNot required

Limp modePossible

P2639 means the torque management feedback signal A stays below its valid range, so the transmission controller cannot read a usable torque confirmation from the engine. No manufacturer explains this exact code publicly. The closest documented relatives are Allison's P2637, where the TCM checks over the SAE J1939 data link whether the engine honours shift torque reductions, and Honda's P2638 on its nine-speed gearbox. If the feedback travels as a network message rather than a wire, a low value usually stems from a calibration mismatch between engine and transmission software or a missing message; on a hard-wired signal, check for a ground short. CARB requires such inputs to be tested against range limits. Shift quality may suffer and torque reduction may be disabled.

Likely causes

Engine and transmission software versions not matched

Torque feedback message missing or corrupted on the data bus

Hard-wired feedback line shorted to ground

Symptoms

Transmission warning or stored code
Firm or flared shifts

Repair estimate

DifficultyShop_onlyWorkshop job

Parts and labour€80–500

Time0.5–2.5 h

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

Parts involved

tcmcommunication bus

Related codes

P2637 P2638 P2643

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