Allison - Automatic Transmission

DTC P2771

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.

DTC Descriptor
DTC P2771
- Four Wheel Drive (4WD) Switch Circuit

Circuit/System Description

On vehicles equipped with 4 wheel drive, the output speed sensor is on the transfer case. The instrument panel mounted transfer case shift control switch consist of a 2HI position, 4HI and 4 Low. The transfer case shift control module continuously monitors the signal to determine the position selected by the vehicle operator. The transfer case shift control module sends the status of the transfer case range out on the serial data line to other control modules. The transmission control module (TCM) monitors the status of the transfer range. When the transfer case is in the 4 Low range, the TCM multiples the transfer case output shaft speed by the transfer case ratio in 4 Low to determine the transmission output shaft speed. The TCM uses this information to provide earlier upshifts and also to prevent an over-speed condition in 4 Low.

Conditions for Running the DTC

* The engine speed is greater than 200 RPM.
* The transmission output speed is greater than 60 RPM.
* The transmission fluid temperature is between 20°C (68°F) and 130°C (266°F).
* Shift is complete and range attained is not neutral.
* DTC P0721, P0722, or P2771 is not set.

Conditions for Setting the DTC

Condition 1

Transfer case four wheel drive switch indicates high range and the calculated transfer case range is low for 5 seconds.

Condition 2

Transfer case four wheel drive switch indicates low range and the calculated transfer case range is high for 5 seconds.

Action Taken When the DTC Sets

* DTC P2771 is a Type B DTC.
* The TCM detects four wheel drive high and low range with calculated speed ratios.

Conditions for Clearing the DTC

DTC P2771 is a Type B DTC.

Reference Information
Schematic Reference

Automatic Transmission Controls Schematics (Allison - Automatic Transmission)
Connector End View Reference

* Component Connector End Views (Connector End Views By Name)
* Inline Harness Connector End Views (Connector End Views By Number)

Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Powertrain Diagnostic Trouble Code (DTC) Type Definitions)
Scan Tool Reference

Control Module References (Control Module References) for scan tool information

Circuit/System Verification

1. Ignition ON, place the transfer case shift control switch in the 2 HI position. Verify the 2 high indicator light on the switch is ON and not flashing.
2. Ignition ON, transmission in Neutral, observe the scan tool TCM 4WD Low parameter. The parameter should change from Disabled, to Enabled when selecting the 4 Low position and the 4WD low indicator light is ON and not flashing.
3. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

Circuit/System Testing

1. If any other DTCs are set, diagnose them first. Refer to Diagnostic Trouble Code (DTC) List - Vehicle (Diagnostic Trouble Code (DTC) List - Vehicle).
2. Refer to Symptoms - Transfer Case (Symptoms - Transfer Case).

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

Control Module References (Control Module References) for TCM replacement, setup, and programming

Repair Verification

If another DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle (Diagnostic Trouble Code (DTC) List - Vehicle) and perform the appropriate diagnostic procedure.