P0856

DTC C0241 or P0856

Circuit Description

The electronic brake control module (EBCM) and the powertrain control module (PCM) simultaneously control the traction control. The PCM reduces the amount of torque supplied to the drive wheels by retarding spark timing and selectively turning off fuel injectors. The EBCM actively applies the brakes to the front wheels in order to reduce torque.

The EBCM sends a requested torque message via a pulse width modulated (PWM) signal to the PCM. The duty cycle of the signal is used to determine how much engine torque the EBCM is requesting the PCM to deliver. Normal values are between 10 and 90 percent duty cycle. The signal should be at 90 percent when traction control is not active and at lower values during traction control activations. The PCM supplies a pull up voltage of 5 volts that the EBCM switches to ground to create the signal.

The PCM sends a delivered torque message via a pulse width modulated (PWM) signal to the EBCM. The duty cycle of the signal is used to determine how much engine torque the PCM is delivering. Normal values are between 10 and 90 percent duty cycle. The signal should be at low values (around 10 percent) at idle and higher values under driving conditions. The EBCM supplies a pull up voltage of 12 volts that the PCM switches to ground to create the signal.

When certain PCM DTCs are set, the PCM will not be able to perform the torque reduction portion of traction control. A serial data message is sent to the EBCM indicating that traction control is not allowed.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

* DTC C0241 Powertrain Control Module (PCM) Indicated Requested Torque Malfunction
* DTC P0856 Traction Control Input Signal

Conditions for Running the DTC

The engine is running.

Conditions for Setting the DTC

The PCM diagnoses the requested torque signal circuit and sends a serial data message to the EBCM indicating that torque reduction is not allowed.

Action Taken When the DTC Sets

* The EBCM disables the engine drag control (EDC)/Traction Control System (TCS) for the duration of the ignition cycle.
* The Traction Off indicator turns ON.
* The Antilock Brake System (ABS) remains functional.

Conditions for Clearing the DTC

* The condition for the DTC is no longer present and you used the scan tool Clear DTC function.
* The EBCM automatically clears the history DTC when a current DTC is not detected in 100 consecutive drive cycles.
* The PCM automatically clears the history DTC when a current DTC is not detected in 40 consecutive warm-up cycles.

Diagnostic Aids

The following conditions can cause this concern:

* An open in the delivered torque circuit.
* An short to ground or voltage in the delivered torque circuit.
* A wiring problem, terminal corrosion, or poor connection in the delivered torque circuit.
* A communication frequency problem.
* A communication duty cycle problem.
* The EBCM is not receiving information from the engine control module (ECM).
* Loose or corroded EBCM ground or ECM ground.

Test Description

The numbers below refer to the step numbers on the diagnostic table.
2. Use the scan tool in order to determine if the requested torque signal has a valid duty cycle.
3. Measure the requested torque signal in order to determine if the signal has a valid duty cycle.
4. Measure the requested torque signal in order to determine if the signal has a valid frequency.
10. This vehicle is equipped with an ECM which uses an electrically erasable programmable read only memory (EEPROM). When replacing the ECM, the replacement ECM must be programmed.

Step 1 - Step 7:

Step 8 - Step 12: