Other Articles |  March - 2021

Tackling a Shift Busyness Issue

If you’ve been in this profession long enough, you have likely seen some pretty crazy things. Years ago, things were fairly predictable; a seal leak causing a clutch-slippage issue or a bad governor causing the vehicle to develop shift related problems. In other words, the things you saw made sense based on your understanding of how the components affected the operation of the transmission.

Then, in the early 1990s, things started to change. Electronic control systems became popular. Now, not only did you need to understand the operation of the transmission components and control systems, but you also needed to understand system operations you’d never suspect as being part of a transmission failure or complaint.

A great example of this was the 1997 4L60E transmission. I can still remember the ATRA expo in 1997 when I explained our new GM software package for engine torque modeling and EC3 converter design changes. I spent about half the class time answering questions regarding what, why, and how the system worked, and why they need to pay attention to areas such as the MAF sensor if the transmission is not working correctly. It seems like old news now, but back then, it was cutting edge and new to everyone sitting in the class.

As the years go by, we have talked about numerous changes that manufactures have made. Many of those changes have made the job of diagnosing a transmission problem much more difficult as the manufacturers, for the most part, have done a poor job in updating the industry on the changes and their effect on the operation of the transmission. Don’t feel bad. This is not just an aftermarket problem; the dealer technicians are just as much in the dark on how these changes (generally software related) affect diagnostic strategy.

Each year in the ATRA seminar, we try to talk about more than just fixes. We spend time explaining the what, why, and how the vehicle software and electronic components affect your diagnostic strategy. So why do we do spend time on things like that? The longer I am in this industry, the more I am convinced that the secret to repairing tough problems is understanding how the system is designed to operate. The best technicians and service engineers I know truly understand the what, why, and how of the system they are working on.

I was in 3 shops last week looking at drivability related transmission issues. One of the shops was a GMC/ Buick store with one of the best drivability techs I know. He said their transmission technician was struggling with a shift busyness issue, and since he did not do transmissions, he wondered if I would be willing to give them a hand.

The tech is fairly new to the transmission field and was battling a 2016 Yukon. While driving the vehicle, the truck would occasionally hunt between gears, and grade braking would engage even though we were not decelerating down a hill and his foot was off the brake. GM had no bulletins on this issue, but I knew I had seen the problem before. But as usual, I could not remember when that was or what the cause was. A few years back, we covered a software and hardware change starting in the 2012 model year relating to grade braking operation in our ATRA seminar.

GM had started using a brake pedal position sensor (figure 1), (sensor id # B22) and had updated the software to use the sensor as an input for grade braking as well as engine and transmission operation. The brake pedal position sensor (BPP) is a potentiometer very similar in operation to an old-time throttle position sensor (TPS). The BPP utilizes two potentiometers mounted in the one BPP assembly. One potentiometer provides a signal to the ECM while the other sensor provides signal voltage to the BCM. Like a typical TPS, each BPP sensor is supplied a 5-volt reference and ground. It then provides a variable output from a signal circuit to the appropriate controller.

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

  • Unwanted grade braking.
  • Shift hunt, upshifts/downshifts even when it should not be shifting.
  • The ECM can command reduced engine power due to a feature in the brake pedal override software.
  • No DTC’s or DIC messages or possible DTC’s such as P057B, P057C, or P057D 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 will 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. The learned value represents the lowest voltage the sensor registers during that key cycle. The learned value is updated any time 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 that even when your foot is off the brake, the learned value will now be lower than the signal value. This causes the controllers to think that your foot is on the brake, and thus the truck develops the symptoms you are experiencing.

In our case, we determined that the BPP sensor was faulty and needed to be replaced even though it was not setting any DTC’s. Keep in mind if you replace a BPP, you will need to perform a BPP relearn with your scan tool, so the customer does not experience any issues. 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. Remembering where you have seen the information is another story.

Until next time remember, “You know you’re getting old when you stoop to tie your shoelaces and wonder what else you could do while you’re down there.”

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