GM Service Manual Online
For 1990-2009 cars only

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.

P1571

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 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 requested torque circuit
    • An short to ground or voltage in the requested torque circuit
    • A wiring problem, terminal corrosion, or poor connection in the requested torque circuit
    • A communication frequency problem
    • A communication duty cycle problem
    • The PCM is not receiving information from the EBCM
    • Loose or corroded EBCM ground or PCM ground

A DTC P1571 may set along with several other PCM DTCs if the key is held in the CRANK position while the engine is running. The starter lockout function of the PCM is enabled several seconds after the engine is running and prevents the starter from engaging while the engine is running. This will cause a partial loss of power to some components and systems.

Test Description

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

  1. Use the scan tool in order to determine if the requested torque signal has a valid duty cycle.

  2. Measure the requested torque signal in order to determine if the signal has a valid duty cycle.

  3. Measure the requested torque signal in order to determine if the signal has a valid frequency.

  4. 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

Values

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

1

Did you perform the ABS Diagnostic System Check?

--

Go to Step 2

Go to Diagnostic System Check - ABS

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

3

  1. Install a scan tool.
  2. Start the engine.
  3. With the scan tool, observe the Torque Request Signal parameter in the Powertrain Control Module data list.

Does the scan tool display less than the specified value?

90%

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Important: Removing battery voltage or ground from the EBCM will result in the following conditions:

       • Loss of the TIM learned tire inflation configuration parameters in the EBCM
       • The EBCM sets DTC C1245
    When the diagnosis is complete, inspect the tire pressures and perform the TIM reset. Refer to Tire Pressure Monitor Procedure in Tire Pressure Monitoring.

  3. Disconnect the EBCM harness connector.
  4. Install the J 39700 universal breakout box using the J 39700-300 cable adapter to the EBCM harness connector and the EBCM connector.
  5. Start the engine.
  6. Measure the DC duty cycle between the requested torque signal circuit and a good ground.

Is the duty cycle within the specified range?

5-95%

Go to Step 5

Go to Step 6

5

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

  1. Turn OFF the ignition.
  2. Disconnect the J 39700-300 cable adapter from the EBCM connector.
  3. 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 C1245 or DTC C1298.

  4. Turn ON the ignition, with the engine OFF.
  5. Measure the voltage from the requested torque signal circuit to a good ground.

Does the voltage measure within the specified range?

4-6 V

Go to Step 10

Go to Step 7

7

  1. Turn OFF the ignition.
  2. Disconnect the powertrain control module (PCM) harness connector.
  3. Test the requested torque signal circuit for the following conditions:
  4. • A short to voltage
    • A short to ground

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

  1. Turn OFF the ignition.
  2. Disconnect the powertrain control module (PCM) harness connector.
  3. Test the requested torque signal circuit for the following conditions:
  4. • An open
    • A high resistance

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

11

Important: The replacement PCM must be programmed.

Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls - 4.0L.

Did you complete the repair?

--

Go to Step 13

--

12

Important: Perform the setup procedure for the EBCM. An unprogrammed EBCM will result in the following conditions:

   • Inoperative or poorly functioning system operations
   • The EBCM sets DTC C1248 and DTC C1255m3.

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

Did you complete the repair?

--

Go to Step 13

--

13

  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK