ATRA + - September - 2023

Stop That! Why Is This Transmission Doing These Strange Things?

One of the things we strive to do at ATRA is to provide information not only regarding repairs for the common conditions you are seeing in your shops but to also provide you with information regarding vehicle systems and their operation so you have an understanding of the components as well as how they interface in today’s vehicles. With as complex as the vehicles are becoming, it is very apparent to most that many of the issues you face today have very little to do with the transmission itself, but instead, the condition may be caused by some component used by another system on the vehicle.

In 2012 GM started using a brake pedal position sensor (BPP) (sensor id # B22) and updated the software to use the sensor as an input for grade braking as well as engine and transmission operation. The BPP sensor (Figure 1) has been implemented on almost all GM models as the years have gone by. The BPP is used on both front and rear-wheel drive applications in everything from the 6-speeds to the 9- and 10-speeds. Issues with the sensors or their circuits have made many a technician lose sleep at night.

The brake pedal position sensor (BPP) is a potentiometer similar to an old-time throttle position sensor (TPS). The BPP utilizes two potentiometers mounted in one assembly. One potentiometer provides a signal to the ECM, while the other sensor provides a signal voltage to the BCM. The BPP position data is then sent over the communication bus to the TCM and the EBCM (Figure 2).

Like a typical TPS, each BPP sensor is supplied a 5-volt reference, a ground, and provides a variable output from a signal circuit to the appropriate controller. If you suspect that the BPP is causing issues, you will need to scan both the ECM and the BCM for DTCs and the data they are receiving from the sensor.

A problem with BPP can cause any of the following symptoms:

  • Unwanted grade braking.
  • Shift hunt, upshifts/downshifts, even when the transmission should not be shifting.
  • The ECM can command reduced engine power due to a brake pedal override software feature.
  • No DTCs or DIC messages or possible DTCs such as P057B, P057C, P057D, or P057E may be set.
  • Any of the symptoms in any combination may occur, and the symptoms may be intermittent.

If the ECM reads that the brake pedal is applied even though it is not, it will signal the TCM, which can cause any or all of the transmission symptoms mentioned above.

Your scan tool typically displays the BPP signal voltage, and the BPP sensor learned value. The signal voltage with the brake released should be approximately 1 volt. In addition, if you have a brake pedal % displayed, it should always register 0% when your foot is off the brake. The learned value represents the sensor’s lowest voltage during that key cycle. The learned value is updated when the sensor signal voltage drops to a lower value than the current learned value. This means a wiring or sensor problem can cause the learned value to update.

So here is the confusing part, let’s say we have an intermittent wiring or sensor issue that causes the sensor voltage to drop. The controllers will learn the lower value. So even when your foot is off the brake, the learned value will now be lower than it previously was. When the circuit starts working correctly again, the controllers think that your foot is on the brake, and thus the vehicle develops the symptoms you are experiencing.

Keep in mind, if you replace a BPP, replace the ECM or BCM, or update the calibration, you will need to perform a BPP relearn with your scan tool. The scan tool relearn will need to be performed for both the ECM and the BCM.

As I previously stated, understanding system operation is critical to a quick diagnosis. Until next time remember, “You know you’re getting old when you stop to tie your shoelaces and wonder what else you could do while you’re down there .”