GM Service Manual Online
For 1990-2009 cars only

Circuit Description

C0240

The powertrain control module (PCM) and the electronic brake control module (EBCM) communicate on the serial data line whenever the ignition is ON.

P0856

The PCM supplies 5 volts through an internal resistor, to the EBCM on the requested torque signal circuit. The EBCM toggles this voltage to ground to create the requested torque signal.

Conditions for Running the DTC

C0240

The ignition is ON.

P0856

    • The ignition is ON.
    • The engine is running at a speed greater than 450 RPM for 5-20 seconds.

Conditions for Setting the DTC

C0240

The EBCM receives a serial data message stating that the PCM has lost the ability to reduce engine torque.

P0856

The PCM receives an invalid requested torque signal for 3 seconds.

Action Taken When the DTC Sets

C0240

    • The EBCM disables the VSES.
    • The stability indicator turns ON.

P0856

    • The PCM sends a serial data message to the EBCM stating that the PCM has lost the ability to reduce engine torque.
    •  The EBCM sets DTC C0240 as a current DTC for as long as the malfunction is present.

Test Description

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

  1. This step tests for voltage supplied to the EBCM from the PCM.

  2. This step tests for a shorted resistor in the PCM or a short to voltage within the requested torque circuit by verifying that a large voltage drop occurs when a test lamp is connected in parallel with the DMM.

Step

Action

Values

Yes

No

Schematic Reference: Antilock Brake System Schematics

Connector End View Reference: Antilock Brake System Connector End Views and Engine Controls Connector End Views.

1

Did you perform the ABS Diagnostic System Check?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Is DTC P0856 set?

--

Go to Step 3

Go to Diagnostic Aids

3

  1. Use the scan tool to clear the DTCs.
  2. Turn OFF the ignition for 5 seconds.
  3. Turn ON the ignition.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does DTC P0856 reset?

--

Go to Step 4

Go to Diagnostic Aids

4

  1. Turn OFF the ignition.
  2. Disconnect from the electronic brake control module (EBCM), the harness connector containing the requested torque signal circuit.
  3. Turn ON the ignition.
  4. Use a DMM in order to measure the voltage between the requested torque signal circuit and a good ground.

Does the voltage measure greater than the specified value?

4.75 V

Go to Step 5

Go to Step 8

5

  1. With the DMM still connected to monitor the requested torque signal circuit, connect one end of a test lamp to a good ground.
  2. Connect the other end of the test lamp to the positive lead of the DMM.

Does the voltage measure less than the specified value?

0.15 V

Go to Step 6

Go to Step 9

6

  1. Turn OFF the ignition.
  2. Install a J 39700 Universal Breakout Box and a J 39700-650 Adapter Cable. Install the equipment between the EBCM and the EBCM harness connector.
  3. Start the engine.
  4. Use the DMM in order to measure the Hz frequency of the requested torque signal.

Does the Hz frequency of the requested torque signal measure within the specified range?

121-134 Hz

Go to Step 7

Go to Step 11

7

Use the DMM in order to measure the duty cycle of the requested torque signal circuit.

Does the duty cycle of the requested torque signal measure within the specified range?

40-95%

Go to Step 12

Go to Step 13

8

Test the requested torque signal circuit for an open or a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 10

9

Test the requested torque signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

 

Go to Step 14

Go to Step 12

10

Inspect for poor connections at the harness connector of the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 12

11

Inspect for poor connections at the harness connector of the EBCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 13

12

Important: Perform the set up procedure for the PCM.

Replace the PCM. Refer to Control Module References.

Did you complete the replacement?

--

Go to Step 14

--

13

Important: Following EBCM replacement, perform the set-up procedure for the EBCM. Use the scan tool to perform the Tire Size Calibration procedure.

Replace the EBCM. Refer to Electronic Brake Control Module Replacement .

Did you complete the replacement?

--

Go to Step 14

--

14

  1. Use the scan tool Clear All Class 2 DTCs function to clear all of the DTCs from all modules.
  2. Turn OFF the ignition for 5 seconds.
  3. Turn ON the ignition.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does DTC P0856 reset?

--

Go to Step 4

System OK