P2640: Torque Management Feedback Signal A High

P2640 is the high-range fault of torque management feedback signal A: the confirmation that the transmission controller receives after asking the engine for a torque change carries a value above the plausible upper limit. On Allison-equipped trucks this feedback is not a separate wire but part of the engine data broadcast on the J1939 link, so a high reading usually means the engine claims more torque than is physically possible or than was requested. No manufacturer documents P2640 publicly. For the related P2637 Cummins cured the fault with new ECM calibrations, a hint that software mismatches are a frequent root. P2637, P2638 and P2639 cover the general, plausibility and low faults of signal A. Expect harsher shifts and torque reductions that no longer work.

Medium severity. Can I keep driving?Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.

Symptoms

Check transmission lamp or MIL on, depending on calibration
Firm or abrupt upshifts
Torque reduction during shifts no longer active

Most likely causes

Engine ECM software reports torque values the transmission calibration does not accept
Outdated or wrong transmission controller calibration
Hard-wired feedback line shorted to voltage (where used)

What to check, in order

Start with the simplest. You can tick off each step.

Check the fluid level and condition
Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
Read the input and output speed in live data
Check the connector and wiring
Check for other related engine codes
Consult the manufacturer's procedure for the transmission

Good to know

It can trigger limp mode.

What each character of P2640 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
6: Control modules and outputs
40: Specific fault in that system

Frequently asked questions

Can I keep driving with code P2640?

Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.

What causes code P2640?

The most common causes are: engine ECM software reports torque values the transmission calibration does not accept; outdated or wrong transmission controller calibration; hard-wired feedback line shorted to voltage (where used).

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.