What causes low power and unwanted braking?
GM has issued a service bulletin PIT5206D to address multiple issues: low power, unwanted braking, transmission shifting and trouble codes P057B, P057C or P057D on the vehicles listed below. GM has identified a possible brake pedal calibration issue as the cause.
Vehicles affected by bulletin PIT5206D
2012-2016 Cadillac Escalade
2012-2013 Chevrolet Avalanche
2012-2016 Chevrolet Express
2012-2016 GMC Savana
Symptoms covered by service bulletin PIT5206D
Unwanted grade braking when NOT going down a grade
Reduced/Low engine power with no DIC messages (Brake Pedal Override Feature)
Transmission busy or frequent up and down shifts
Trouble codes P057B, P057C or P057D may or may not be set
Cause of low power, unwanted braking
On older vehicles the brake pedal switch was a simple on/off mechanism. In later model vehicles, car makers incorporated several on/off switches in the same brake pedal switch; one to control brake lights and the second switch to operate the shift interlock solenoid and alert the computer that the brake pedal was applied and to allow starting.
But in the vehicle listed above, the brake pedal position (BPP) sensor is NOT a typical switch. It’s more like a throttle position sensor in that it detects the POSITION of the brake pedal and reports that position as a varying voltage. So the BPP uses a 5-volt reference signal, low reference and position signal, just like a TPS sensor. In addition, the vehicles listed above have 6 separate circuits inside the BPP.
Three of the BPP circuits supply brake pedal position information to the body control module (BCM) and the other 3 circuits supply brake pedal position information to the ECM.
What is brake override?
The ECM monitors the vehicle speed and BPP sensor data. Based on input from the BPP and vehicle speed sensors, the ECM determines if the vehicle is decelerating at the proper speed and rate of deceleration with the brake pedal applied. The ECM can override the driver’s brake pedal application and reduce engine torque to help speed vehicle slowdown.
What is transmission braking?
The BPP supplies data to the BCM for stop light operation. When the BPP voltage reaches a threshold point, the BCM provides voltage to brake lights. But it also communicates to the transmission control module (TCM) so the module can control downshifts and apply transmission braking. The BCM also uses data from the BPP to power the center mount stop light.
What are the trouble codes?
P057B Brake Pedal Position Sensor Performance trouble code will set if the ECM determines the BPP sensor is stuck in a range
P057C Brake Pedal Position Sensor Circuit Low Voltage will set if the ECM sees voltage less than 0.25 volts for 1-sec or more.
P057D Brake Pedal Position Sensor Circuit High Voltage will set if the ECM sees voltage greater than 4.75 volts for 1-sec or more.
Diagnose and fix a lower power, unwanted braking condition
Using a scan tool with live data check the BPP information in the Auto Transmission data section. Then view the BPP parameters the ECM has learned for brake pedal released versus the actual BPP voltage. With the brake pedal released, the actual value and released value should be the same or close to the same.
• BPP sensor percentage ranges from 0% to 100%. BPP should read 0% with pedal released.
• BPP sensor voltage ranges should be approximately 1 volt with pedal released
• BPP sensor learned released position should equal the BPP sensor voltage with pedal released
NOTE: Different engines may have different values
Once the ECM has learned the BPP Sensor Learned Released Position voltage, that voltage should never change. However, an intermittent wiring or connection issue can cause the BPP voltage to drop below the learned voltage. The ECM will then learn the new voltage. Once the wiring issue corrects itself, the BPP voltage will no longer equal the BPP Sensor Learned Released Position voltage.
This condition will cause the ECM to think the driver is applying the brake pedal even when the pedal is not applied. That can cause the vehicle to go into an unwanted grade braking condition, resulting in low engine power due to the brake override feature and cause the transmission to engage is multiple shifts.
If you perform a BPP relearn but do not fix the underlying intermittent wiring problem, the symptoms will repeat again and again.
Service bulletin PIT5206D identifies common wiring issues that cause low power and unwanted braking
Check the condition of the these connectors of 2014 and newer vehicles
• Inline connector X115
• Inline connector X119
On 2014 and older vehicles, check
• Inline connector X205
Perform a BPP relearn procedure
NOTE: There is an ECM BPP relearn procedure AND a BCM BPP relearn procedure.
©, 2018 Rick Muscoplat