P077D: output-speed sensor voltage is electrically too high

P077D detects a high output-speed sensor circuit voltage in the cited TCM. The raw input must exceed 4.7500 volts, corresponding to the documented short-to-power signature.

High voltage does not mean excessive shaft speed

The code describes the electrical input. It cannot be translated into a diagnosis that the transmission output is rotating too fast. A believable speed PID also cannot independently validate the raw circuit voltage that this test monitors.

The table associates the fault-detection design with less than 0.5 ohm between signal and controller power. That specification is not an instruction to probe or resistance-test an energized transmission connector.

The result depends on more than one brief sample

A failure interval longer than fifty milliseconds advances a count, which must exceed sixteen in the shown logic. The source lists separate update rates for the raw check and count handling; combining them into an invented single stopwatch duration would be unreliable.

Both battery and run/crank voltage must remain above 10 volts for more than five seconds. The monitor also excludes service fast learn, code-clear activity and an active P077C. Preserve those conditions alongside the voltage record. Verification should show a passing enabled high-circuit diagnostic and appropriate speed behavior using the exact service procedure. The code does not by itself identify whether the sensor, wiring or controller interface requires repair.

Source and applicability

Documented monitors in gm-2025-03a, model year 2025. These are module/calibration examples; vehicle, engine and transmission applicability must be confirmed independently. GM 2025-03a, page 1119.