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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 1077 (Automatic Transmission - ALLISON, Diagnosis (DTC P0873 To DTC P2810)) >> Diagnosis >> DTC P2771 >> Test Description

Test Description

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

  1. 2:  This step tests the status of the 4WD Low switch.
  2. 3:  This step tests for a 4WD switch failure to ground.
  3. 4:  This step tests for a short in the wiring harness.
  4. 9:  This step tests for a 4WD switch failure to an open state.
  5. 10:  This step tests for an open in the wiring harness.
DTC P2771

Step Action Value Yes No
Schematic Reference: Automatic Transmission Controls Schematics
1 Did you perform the Diagnostic System Check - Automatic Transmission? - Go to Step 2  Go to Diagnostic System Check - Automatic Transmission
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  Select 4HI followed by 4LO using the transfer case selector.
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 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 4WD Low signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
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 in Wiring Systems.
Did you find a condition?		 - Go to Step 8  Go to Step 15 
8 Replace the transfer case switch. Refer to Transfer Case Shift Control Module Replacement in Transfer Case - NVG 263 - NP1.
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 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 in Wiring Systems.
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 in Wiring Systems.
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. Refer to Transfer Case Shift Control Module Replacement in Transfer Case - NVG 263 -NP1.
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 Transmission Control Module (TCM) Replacement .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 4WDHI is selected, and YES when 4WDLO is selected.
  3. Select Specific DTC.
  4. Enter DTC P2771.

Has the test run and passed?		 - System OK Go to Step 1 
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