C129B
DTC C0277, C120C, C129A, C129B, C129D, C129E, C12B1, or C12F8
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (with HP2) (Initial Inspection and Diagnostic Overview)Diagnostic System Check - Vehicle (without HP2) (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview) provides an overview of each diagnostic category.
DTC Descriptors
DTC C0277 03
- BMC Primary Piston Position Sensor A and B Circuit Performance Voltage Below Threshold
DTC C129A
- BMC Primary Piston Position Sensor A and B Circuit Performance Voltage Below Threshold
DTC C129D
- BMC Primary Piston Position Sensor A and B Circuit Performance Voltage Below Threshold
DTC C0277 07
- BMC Primary Piston Position Sensor A and B Circuit Voltage Above Threshold
DTC C129B
- BMC Primary Piston Position Sensor A and B Circuit Voltage Above Threshold
DTC C129E
- BMC Primary Piston Position Sensor A and B Circuit Voltage Above Threshold
DTC C0277 5A
- BMC Primary Piston Position Sensor A and B Circuit Plausibility Failure
DTC C12B1
- BMC Primary Piston Position Sensor A and B Circuit Plausibility Failure
DTC C12F8
- BMC Primary Piston Position Sensor A and B Circuit Plausibility Failure
DTC C120C
- BMC Primary Piston Position Sensor A and B Circuit Plausibility Failure
Diagnostic Fault Information
Circuit/System Description
The electronic brake control module (EBCM) uses input from the brake master cylinder piston position sensor and the brake switch to determine when the brake pedal is being applied and with how much force. These inputs are used to determine the driver's braking intent which is used to the control amount of pressure applied to the wheels.
Conditions for Running the DTC
* Ignition ON.
* Ignition voltage is greater than 10 volts.
Conditions for Setting the DTC
C0277 03, C129A, or C129D
The EBCM detects voltage below threshold on signal circuit.
C0277 07, C129B, or C129E
The EBCM detects voltage above threshold on signal circuit.
C0277 5A, C12B1, C12F8, or C120C
This EBCM detects the master cylinder pressure is high when brake master cylinder piston position sensor is reading low travel.
Action Taken When the DTC Sets
* The EBCM disables the antilock brake system (ABS), traction control and the vehicle stability enhancement system (VSES) for the duration of the ignition cycle.
* A DIC message and/or a warning indicator may be displayed.
Conditions for Clearing the DTC
* The condition for the DTC is no longer present.
* The EBCM clears the history DTC when a current DTC is not detected in 40 consecutive drive cycles.
Reference Information
Schematic Reference
Antilock Brake System Schematics (Electrical Diagrams)
Connector End View Reference
Component Connector End Views (Connector Views)
Description and Operation
ABS Description and Operation (Description and Operation)
Electrical Information Reference
* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)
Scan Tool Reference
Control Module References (Programming and Relearning) for scan tool information
Circuit/System Verification
1. Verify the 12V battery state of charge is above 12V.
• If the battery state of charge is low, charge the 12V battery. Refer to Battery Charging (Service and Repair)
2. Using the scan tool, perform the EBCM relearn.
3. Start the engine and allow it to idle.
4. Place the transmission in the PARK position.
Note: To ensure the complete brake modulator and high pressure accumulator (HPA) pressure relief procedure is allowed to occur; the brake controller must see one brake pedal apply with the engine running followed by a proper shutdown without applying the brake pedal.
5. Apply and release the brake pedal.
Note: The ignition must transition to the OFF position without pausing at ACCESSORY and without applying the brake pedal. This ensures that the brake modulator and HPA pressure relief occurs. This process will take approximately 1-3 minutes.
6. Turn the ignition OFF without pausing at ACCESSORY and without applying the brake pedal. Remove the key from the ignition.
7. Wait 3 minutes until the brake modulator and HPA pressure relief is complete.
Note: The ignition must remain in the ON position with the engine OFF during this entire procedure.
8. Ignition ON, brakes unapplied, verify the following EBCM scan tool parameters are within the specified values:
* Master Cylinder Pressure Sensor Input -Less than 100 KPA (15 PSI)
* Rear Brake Pressure Sensor -Less than 100 KPA (15 PSI)
* Brake Pressure Sensor Input -Less than 100 KPA (15 PSI)
* Accum. Pressure Sensor -Less than 100 KPA (15 PSI)
• If any of the parameters are not within the specified range, replace the EBCM.
9. Brakes unapplied, verify the following EBCM scan tool parameters are within the specified values:
* BMC Prim. Piston Pos. Sen. 5V Reference Position -Between 4.75 V and 5.25 V
* BMC Primary Piston Position Sensor A -Less than.8V
* BMC Primary Piston Position Sensor B -Greater than 4.2V
• If any of the parameters are not within the specified range, refer to Circuit/System Testing.
Circuit/System Testing
1. Inspect master cylinder fluid level.
• If fluid is low, refer to Symptoms - Hydraulic Brakes (Symptoms - Hydraulic Brakes)
2. Ignition OFF, disconnect the harness connector at the brake master cylinder piston position sensor.
3. Ignition OFF, test for less than 5 ohm between the low reference circuit terminal 4 and ground.
• If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the EBCM.
4. Ignition ON, test for 4.8-5.2 volts between the 5-volt reference circuit terminal 2 and ground.
• If less than the specified range, test the 5-volt reference circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the EBCM.
• If greater than the specified range, test the 5-volt reference circuit for a short to voltage. If the circuit tests normal, replace the EBCM.
5. Verify the scan tool BMC Primary Piston Position Sensor A parameter is less than 1 volt.
• If greater than the specified range, test the signal circuit terminal 1 for a short to voltage. If the circuit tests normal, replace the EBCM.
6. Install a 3A fused jumper wire between the signal circuit terminal 3 and the 5-volt reference circuit terminal 2. Verify the scan tool BMC Primary Piston Position Sensor A parameter is greater than 4.5 volts.
• If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the EBCM.
7. Verify the scan tool BMC Primary Piston Position Sensor B parameter is less than 1 volt.
• If greater than the specified range, test the signal circuit terminal 3 for a short to voltage. If the circuit tests normal, replace the EBCM.
8. Install a 3A fused jumper wire between the signal circuit terminal 1 and the 5-volt reference circuit terminal 2. Verify the scan tool BMC Primary Piston Position Sensor B parameter is greater than 4.5 volts.
• If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the EBCM.
9. If all circuits test normal, replace the brake master cylinder piston position sensor.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Brake Master Cylinder Primary Piston Position Sensor Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for EBCM replacement, setup, and programming