LCT 1000 Allison Transmission

LCT 1000 Allison Transmission

DTC P2771

Circuit Description
The four-wheel drive (4WD) low signal circuit is used to notify the transmission control module (TCM) that the vehicle is in 4WD low range.

DTC P2771 is a type B DTC.

DTC Descriptor
This diagnostic procedure supports the following DTC:

DTC P2771 4WD Low Switch Circuit

Conditions for Running the DTC
^ DTCs P0721 and P0722 are not active.
^ The components are powered and ignition voltage is greater than 9.0 volts and less than 18 volts.
^ Engine speed is greater than 200 RPM and less than 7,500 RPM for 5 seconds.
^ Output speed is greater than 60 RPM.
^ Transmission fluid temperature (TFT) is between 20 - 130°C (68 - 266°F).
^ The shift is complete and NEUTRAL range is not selected.

Conditions for Setting the DTC
DTC P2771 sets when one of the following conditions occur:
^ The transfer case switch indicates high range, and the calculated range is low range for 5 seconds or greater.
^ The transfer case switch indicates low range, and the calculated range is high range for 5 seconds or greater.

Action Taken When the DTC Sets
^ The TCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the Conditions for Setting the DTC are met.
^ DTC 2771 is stored in TCM history.
^ The TCM inhibits torque converter clutch (TCC) engagement.

Conditions for Clearing the MIL/DTC
A scan tool can clear the code from TCM history. The TCM automatically clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without failure.

Diagnostic Aids
^ Inspect the wiring for poor electrical connections at the TCM. Inspect for the following conditions:
^ A bent terminal
^ A backed-out terminal
^ A damaged terminal
^ Poor terminal tension
^ A chafed wire
^ 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 condition.
^ Inspect for any transmission DTCs that may have set again.

Test Description

Step 1 - Step 14:

Step 15 - Step 17:

The numbers below refer to the step numbers on the diagnostic table.
2. This step tests the status of the 4WD low switch.
3. This step tests for a 4WD switch failure to ground.
4. This step tests for a short in the wiring harness.
9. This step tests for a 4WD switch failure to an open state.
10. This step tests for an open in the wiring harness.