Allison 1000 & 2000 Gen 4 Fault Codes: P2771 Four-Wheel Drive Switch Circuit

Allison 1000 & 2000 Gen 4 Fault Codes: P2771 Four-Wheel Drive Switch Circuit

CIRCUIT DESCRIPTION:
Detects abnormal conditions for the four-wheel drive indication switch input. Four wheel-drive statuses are detected using speed and ratios.

CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM) or greater than 18V and less than 32V (24V TCM).
2. Engine speed is greater than 200 rpm and less than 7500 rpm for 5 seconds.
3. DTCs P0721 and P0722 (output speed sensor circuits) are not active.
4. Output speed is above 60 rpm.
5. Transmission fluid temperature it between 20°C (68°F) and 130°C (266°F).
6. Shift is complete and the range attained is not neutral.

CONDITIONS FOR SETTING THE DTC:
P2771 sets when one of the following conditions occurs:
1. The transfer case switch indicates high range, and the calculated range is low range for 5 seconds or more.
2. The transfer case switch indicates low range, and the calculated range is high range for 5 seconds or more.

ACTION TAKEN WHEN THE DTC SETS:
1. DTC P2771 will be stored in the TCM memory.
2. The CHECK TRANS light illuminates on the second occurrence.
3. The TCM inhibits TCC engagement.

CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool can be used to clear the code from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without a failure.

DIAGNOSTIC AIDS:
Inspect the wiring for poor electrical connections at the TCM. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.

You may have to drive the vehicle in order to experience a fault.

Inspect for any transmission DTCs that may have reset.

When testing for a short or ground, the transmission should be in a range where the switch or solenoid is in the OFF state. When testing for an open, the transmission should be in a range where the switch or solenoid is in the ON state.