Circuit Description
The EBCM and the 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.
Conditions for Running the DTC
The engine is running.
Conditions for Setting the DTC
C1277
The PCM diagnoses the requested torque signal circuit and sends a serial data message to the EBCM indicating a fault is present.
P0856
One of the following conditions exists:
| • | The PCM detects that requested torque signal is out of the valid range. |
| • | The PCM does not receive the requested torque signal. |
Action Taken When the DTC Sets
| • | The EBCM disables the TCS for the duration of the ignition cycle. |
| • | The PCM will store conditions which were present when the DTC set as Fail Records data only. |
| • | The Traction Off indicator turns ON. |
| • | The message center displays the Service Traction System message. |
| • | The 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 PCM. |
| • | Loose or corroded EBCM ground or PCM ground. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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Use the scan tool in order to determine if the requested torque signal has a valid duty cycle.
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Measure the requested torque signal in order to determine if the signal has a valid duty cycle.
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Measure the requested torque signal in order to determine if the signal has a valid frequency.
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This vehicle is equipped with a PCM which uses an Electrically Erasable Programmable Read Only Memory (EEPROM). When replacing the PCM, the replacement PCM must be programmed.
Step | Action | Value(s) | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
Schematic Reference: Antilock Brake System Schematics Connector End View Reference: Antilock Brake System Connector End Views , or Powertrain Control Module Connector End Views in Engine Controls-3.4 L, or Powertrain Control Module Connector End Views in Engine Controls-3.8 L | ||||||||
1 | Did you perform the ABS Diagnostic System Check? | -- | Go to Step 2 | |||||
2 | Inspect the EBCM ground and PCM ground, making sure each ground is clean and torqued to the proper specification. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 3 | ||||
Does the scan tool display less than the specified value? | 100% | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems | Go to Step 4 | |||||
Is the duty cycle within the specified range? | 5-95% | Go to Step 5 | Go to Step 6 | |||||
Measure the DC Hz between the requested torque signal circuit and a good ground. Does the frequency measure within the specified range? | 121-134 Hz | Go to Step 8 | Go to Step 6 | |||||
6 |
Important: Disconnecting the EBCM connector and turning ON the ignition could cause other modules to set loss of communication DTCs (Uxxxx). Once the EBCM is reconnected, the EBCM may set DTC C1298. Does the voltage measure within the specified range? | 4-6 V | Go to Step 10 | Go to Step 7 | ||||
7 |
Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 10 | ||||
8 |
Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 9 | ||||
9 | Inspect for poor connections 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 13 | Go to Step 11 | ||||
10 | Inspect for poor connections 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 13 | Go to Step 12 | ||||
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Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls - 3.4L or Powertrain Control Module Replacement in Engine Controls - 3.8L. Did you complete the repair? | -- | Go to Step 13 | -- | |||||
12 | Replace the EBCM. Refer to Electronic Brake Control Module Replacement . Did you complete the repair? | -- | Go to Step 13 | -- | ||||
13 |
Does the DTC reset? | -- | Go to Step 2 | System OK | ||||
