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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2008 🢒 Sierra - 2WD 🢒 DTC 🢒 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

The numbers below refer to the step numbers on the diagnostic table.

  1. This step tests the status of the 4WD low switch.

    2.

  2. This step tests for a 4WD switch failure to ground.

    3.

  3. This step tests for a short in the wiring harness.

    4.

  4. This step tests for a 4WD switch failure to an open state.

    5.

  5. This step tests for an open in the wiring harness.

Step

Action

Yes

No

Schematic Reference: Automatic Transmission Controls Schematics

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Install the scan tool.
  2. Turn ON the ignition, with the engine OFF.

    3. Important: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records for reference. The Clear Info function will erase the data.

  3. Record the DTC Freeze Frame and Failure Records.
  4. Clear the DTC.
  5. Using the scan tool, monitor 4WD low status and select 4HI followed by 4LO with the transfer case selector.

Does the scan tool indicate 4WD low status as NO when 4HI is selected, and YES when 4LO is selected?

Go to Diagnostic Aids

Go to Step 3

3

Using the transfer case selector, select 4HI followed by 4LO.

Does the scan tool indicate 4WD low status as YES for both selector positions?

Go to Step 4

Go to Step 9

4

Remove the connector at the transfer case selector.

Does the scan tool indicate 4WD low status as YES?

Go to Step 5

Go to Step 7

5

Inspect the 4WD low signal circuit for a short to ground.

Did you find a condition?

Go to Step 6

Go to Step 15

6

Inspect and repair the short to ground in the 4WD low signal circuit. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 16

--

7

Inspect the transfer case switch for proper operation or a short to ground. Refer to Circuit Testing and Wiring Repairs .

Did you find a condition?

Go to Step 8

Go to Step 15

8

Replace the transfer case switch for the MP1222/1225/1226-NQG Transfer Case.

Replace the transfer case shift control module for the MP 1625/1626-NQF Transfer Case. Refer to Transfer Case Shift Control Module Replacement .

Did you complete the replacement?

Go to Step 16

--

9

Using the transfer case selector, select 4HI followed by 4LO.

Does the scan tool indicate 4WD low status as NO for both selector positions?

Go to Step 10

Go to Step 15

10

  1. Remove the connector at the transfer case switch.
  2. Ground the 4WD low signal circuit to a known ground.

Does the scan tool indicate 4WD low status as NO?

Go to Step 11

Go to Step 13

11

Inspect the short to ground in the brake switch signal circuit. Refer to Circuit Testing and Wiring Repairs .

Did you find a condition?

Go to Step 12

Go to Step 15

12

Repair the open condition in the 4WD low signal circuit. Refer to Circuit Testing and Wiring Repairs .

Did you complete the repair?

Go to Step 16

--

13

Inspect the transfer case switch for an open condition.

Did you find an open condition?

Go to Step 14

Go to Step 15

14

Replace the transfer case switch for the MP1222/1225/1226-NQG Transfer Case.

Replace the transfer case switch for the MP 1625/1626-NQF Transfer Case. Refer to Transfer Case Shift Control Switch Replacement .

Did you complete the replacement?

Go to Step 16

--

15

Important: In most cases, the TCM is not at fault. Investigate thoroughly before replacing the TCM.

Replace the TCM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 16

--

16

Perform the following procedure in order to verify the repair:

  1. Clear the DTC.
  2. Drive the vehicle under normal operating conditions. Monitor the 4WD low status with the scan tool. The 4WD low status must indicate NO when 4WD HI is selected, and YES when 4WD LO is selected.
  3. Select Specific DTC.
  4. Enter DTC P2771.

Has the test run and passed?

Go to Step 17

Go to Step 2

17

With the scan tool, observe the stored information, capture info and DTC info.

Does the scan tool display any DTCs that you have not diagnosed?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK