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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2006 🢒 GMC K Sierra - 4WD 🢒 Transmission/Transaxle 🢒 Automatic Transmission - Allison 🢒 DTC P0878

Circuit Description

The pressure switch manifold (PSM) is a multiple-switch assembly made up of 3 normally open (N/O) pressure switches and 1 normally closed (N/C) pressure switch for reverse pressure. Fluid pressure is supplied to the pressure switch 4 (PS4) when the manual selector valve is in any position except REVERSE. When the manual selector valve is moved to REVERSE, pressure to PS4 is cut off, causing the switch to close.

DTC P0878 is a type B DTC.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0878 Transmission Pressure Switch 4 Circuit High

Conditions for Running the DTC
    • 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.
    • The transmission fluid temperature (TFT) is greater than 0°C (32°F).
    • DTC P0708 and P0877 is not active.

Conditions for Setting the DTC

DTC P0878 sets when P (PARK), N (NEUTRAL), or a forward range is selected and the PS4 state remains in the mechanically closed/electrically ON position for greater than 1.0 second.

Action Taken When the DTC Sets
    • The transmission control module (TCM) illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the Conditions for Setting the DTC are met.
    • The transmission will lock in NEUTRAL.
    • DTC P0878 is stored in TCM history.
    • The TCM freezes shift adapts.
    • The TCM inhibits torque converter clutch (TCC) engagement.

Conditions for Clearing the 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.
    • This DTC could indicate a hydraulic leak path exhausting pressure from the reverse PS4.

Test Description

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

  1. This step tests the TCM response function.

    2.

  2. This step tests the transmission wiring harness integrity.

    3.

  3. This step tests and evaluates the internal wiring harness.

    4.

  4. This step tests the PSM for a failed condition.

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

Measure the transmission fluid level. Refer to Transmission Fluid Check .

Is the transmission fluid level correct?

Go to Step 3

Go to Transmission Fluid Check

3

Measure line pressure. Refer to Line Pressure Check .

Is the line pressure within the specified value?

Go to Step 4

Go to Symptoms - Automatic Transmission

4
  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. Start the engine.
  6. Using the scan tool, monitor PS4 status.

Does the scan tool indicate PS4 status is HIGH?

Go to Diagnostic Aids

Go to Step 5

5

  1. Turn the ignition OFF.
  2. Disconnect the 80-way connector at the TCM.
  3. Install the J 39700 breakout box, the J-47275 breakout box adapter and the J 47275-1 magnetic overlay.
  4. Turn ON the ignition, with the engine OFF.
  5. Connect terminal 77 to ground.

Does the scan tool indicate PS4 status is LOW when grounded and HIGH when open?

Go to Step 6

Go to Step 12

6

  1. Turn the ignition OFF.
  2. Disconnect the AT inline 20-way connector. Additional DTCs may set.
  3. Install the J 44152 jumper harness (20 pins) on the engine side of the AT inline 20-way connector.
  4. Turn ON the ignition, with the engine OFF.
  5. Using the J 35616 GM terminal test kit and a fused jumper wire, connect terminal K of the J 44152 to ground. Refer to Automatic Transmission Inline Harness Connector End View .

Does the scan tool indicate PS4 status is LOW when grounded and HIGH when open?

Go to Step 7

Go to Step 11

7

  1. Connect the AT inline 20-way connector.
  2. Remove the transmission oil pan. Refer to Oil Pan Replacement .
  3. Disconnect the AT internal wiring harness from the PSM and ground terminal D.

Does the scan tool indicate PS4 status is LOW when grounded and HIGH when open?

Go to Step 8

Go to Step 10

8

  1. Remove the PSM from the control valve body and leave the internal harness connected. Refer to Pressure Switch Manifold Replacement .
  2. Provide a ground connection for the PSM to either the control valve body or the transmission main case.
  3. Using the eraser end of a pencil, push gently on pressure switch R membrane.

Does the scan tool indicate PS4 status is LOW when grounded and HIGH when open?

Go to Step 9

Go to Diagnostic Aids

9

Replace the PSM. Refer to Pressure Switch Manifold Replacement .

Did you complete the replacement?

Go to Step 13

--

10

Replace the internal wiring harness. Refer to Wiring Harness Replacement .

Did you complete the replacement?

Go to Step 13

--

11

Replace the AT internal wiring harness. Refer to Wiring Harness Replacement .

Did you complete the replacement?

Go to Step 13

--

12

Important: Prior to replacing the TCM, swap the TCM with a known good TCM. 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 13

--

13

Perform the following procedure in order to verify the repair:

  1. Clear the DTC.
  2. Start the engine.
  3. Using the scan tool, monitor PS4 status.
  4. Select Specific DTC.
  5. Enter DTC P0878.

Has the test run and passed?

Go to Step 14

Go to Step 2

14

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

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

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK