C1277

DTC C1277 or P1571 (w/NW9)





Step 1 - 6





Step 7 - 13

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.
3. Use the scan tool in order to determine if the requested torque signal has a valid duty cycle.
4. Measure the requested torque signal in order to determine if the signal has a valid duty cycle.
5. Measure the requested torque signal in order to determine if the signal has a valid frequency.
11. 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.