Circuit Description
The electronic brake control module (EBCM) and the engine control module (ECM) simultaneously control the traction control. The ECM 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 ECM. The duty cycle of the signal is used to determine how much engine torque the EBCM is requesting the ECM to deliver. Normal values are between 10-90 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 ECM supplies a pull up voltage of 12 volts that the EBCM switches to ground to create the signal.
The ECM sends a delivered torque message via a PWM signal to the EBCM. The duty cycle of the signal is used to determine how much engine torque the ECM is delivering. Normal values are between 10-90 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 ECM switches to ground to create the signal.
When certain ECM DTCs are set, the ECM 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 C1277 Requested Torque Signal Circuit Malfunction |
| • | DTC P0856 Traction Control Torque Request Circuit |
Conditions for Running the DTC
The engine is running.
Conditions for Setting the DTC
C1277
The ECM diagnoses the requested torque signal circuit and sends a serial data message to the EBCM indicating that a fault is present.
Action Taken When the DTC Sets
| • | The EBCM disables the EDC/traction control module (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 ECM 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 |
| • | A 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 ECM. |
| • | Loose or corroded EBCM ground or ECM 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 powertrain control module (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 - 3.8L | ||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | ||||
2 | Inspect the electronic brake control module (EBCM) ground and powertrain control module (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 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 frequency 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 |
Does the voltage measure near the specified value? | B+ | 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 | ||||
Replace the PCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 13 | -- | |||||
12 | Replace the EBCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 13 | -- | ||||
13 |
Does the DTC reset? | -- | Go to Step 2 | System OK | ||||
