GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Blazer - 4WD 🢒 Restraints 🢒 SIR 🢒 DTC C0387

Circuit Description

The transfer case encoder is a switch that converts a shaft position, representing a mode or range position, into electrical signals for use by the transfer case shift control module. The encoder houses 4 hall effect sensors that are used for channels P, A, B, and C. These sensors provide a path to ground when a magnet, part of the motor encoder, passes over them.

The transfer case shift control module supplies an 8-volt reference and a ground circuit for the encoder hall effect sensors to function.

The transfer case shift control module supplies current limited 5 volts on all the channels. As these channels are pulled to ground, the module can interpret the location of the transfer case shift position.

This DTC detects that the transfer case is unable to successfully complete a shift to a requested mode or range.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC C0387 Unable to Perform Shift

Conditions for Running the DTC
    • The ignition is ON.
    • The system voltage is 9-16 volts.

Conditions for Setting the DTC
    • The transfer case shift control module tests the motor circuits and determines that there are no motor circuit faults existing.
    • The transfer case shift control module has attempted to shift to a requested mode/range and has failed to reach the desired position.
    • The transfer case shift control module has performed a default shift to 4LO or 4HI.

Action Taken When the DTC Sets
    • The SERVICE 4WD indicator remains illuminated for the remainder of the current ignition cycle.
    • All shifting is disabled.
    • The range/mode lights are disabled.
    • The next ignition cycle, the transfer case shift control module will power-up as if the 4HI was the mode present when the ATC was powered down.

Conditions for Clearing the DTC
    • The transfer case shift control module clears the DTC if the condition for setting the DTC no longer exists.
    • A history DTC clears after 100 consecutive ignition cycles without a fault present.
    • History DTCs can be cleared using a scan tool.

Test Description

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

  1. This step tests whether the transfer case shift control module is sensing any change in all of the encoder channels.

    2.

  2. This step helps determine whether the shift is prevented due to a mechanical fault.

    3.

  3. This step tests whether the transfer case shift control module is sensing any change in any of the encoder channels.

    4.

  4. This step tests if proper reference voltage is being supplied to the encoder from the suspect circuit.

    5.

  5. This step helps determine whether a short between 2 or more encoder circuits exist.

    6.

  6. This step helps determine whether the shift is prevented due to a mechanical fault.

    7.

  7. This step tests for the electrical energizing of the encoder motor.

    8.

  8. This step tests the suspect circuits for an open or a short to power.

    9.

  9. This step tests for a short between the suspect circuits.

Step

Action

Value(s)

Yes

No

Schematic Reference: Transfer Case Control Schematics

Connector End View Reference: Transfer Case Control Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

Does DTC C0387 set as current?

--

Go to Step 6

Go to Step 3

3

  1. Install a scan tool.
  2. Start the engine.
  3. Place the transmission into Neutral.
  4. Set the parking brake.
  5. Using the scan tool, command through all the shift modes.
  6. Monitor the states of the Encoder P, A, B, and C.

Do all the Encoder Circuits change between ON and OFF?

--

Go to Step 4

Go to Step 5

4

  1. Turn the ignition OFF.
  2. Remove the motor encoder. Refer to Transfer Case Motor/Encoder Replacement .
  3. Place a wrench on the end of the transfer case sector shaft and inspect for binding.

Does the sector shaft bind?

--

Go to Step 12

Go to Step 14

5

Do any of the Encoder Circuits P, A, B, or C change states?

--

Go to Step 6

Go to Step 9

6

  1. Identify which encoder circuits did not change.
  2. Turn ON the ignition, with the engine OFF.
  3. Disconnect the six wire connector from the transfer case encoder.
  4. Test the voltage in the suspect circuit or circuits to a good ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Is the voltage reading within the specified range?

4.8-5.2 V

Go to Step 7

Go to Step 10

7

  1. Using the scan tool, monitor the states of the encoder circuit P, A, B, and C.
  2. Individually jumper each of encoder circuit signal wires at the encoder motor harness connector, one circuit at a time, to ground.

Do each of the Encoder Circuit states display ON when the encoder circuit is shorted to ground?

--

Go to Step 8

Go to Step 11

8

  1. Turn OFF the ignition.
  2. Remove the motor encoder. Refer to Transfer Case Motor/Encoder Replacement .
  3. Place a wrench on the end of the transfer case sector shaft and inspect for binding.

Does the sector shaft bind?

--

Go to Step 12

Go to Step 9

9

  1. Place the transmission in Neutral.
  2. Start the engine.
  3. Install a scan tool.
  4. Install a DMM on motor control A and motor control B circuits at the transfer case connector, on the module side of the harness.
  5. Use the scan tool in order to command the Encoder Motor A and B ON.

Does the DMM show battery voltage?

--

Go to Step 13

Go to Step 14

10

Test the suspect circuit or circuits for an open or a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 14

11

Test for a short between the suspect circuits. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 14

12

Remove the transfer case and overhaul. Refer to Transfer Case Assembly Replacement .

Did you complete the repair?

--

Go to Step 15

--

13

Replace the transfer case encoder motor. Refer to Transfer Case Motor/Encoder Replacement .

Did you complete the repair?

--

Go to Step 15

--

14

Replace the transfer case shift control module. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

Did you complete the repair?

--

Go to Step 15

--

15
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC set?

--

Go to Step 2

System OK