Recently, I was in Australia for a seminar presentation. During my time there, one of the shops told me about an intermittent problem they had with a 2012 Toyota Prado 150 series with a 3.0L diesel engine and an A750F 5-speed transmission.
For us here in the US, this vehicle would be akin to the Toyota 4runner. The complaint was that the transmission would drop out of 5th gear intermittently. It would still upshift and downshift 1st through 4th gears with no problem.
Let’s start with scanning the system. The only code present revealed a memory fault for the passenger front window. Realizing that obscure codes can sometimes cause strange problems, the code needed to be addressed. The window motor learn process was performed and the code was cleared. A test drive showed that the issue was still there, so further testing was necessary.
While performing the test drive, when the fault occurred, the cruise control “ready” light would go out (figure 1). Depressing the cruise control master switch did not cause the cruise control “ready” light to come back on. The vehicle had to be turned off for at least 10 seconds and then restarted to allow the cruise control master switch to function again.
Looking at the system wiring, the cruise control input is the APP (Accelerator Pedal Position Sensor). The vehicle was driving a little sluggish at times, but it would accelerate and shift into every other gear. So, it would seem there isn’t an issue there…it would seem.
Knowing this concern is intermittent, the best way to address the concern is data stream or recording the test drive. The shop didn’t have access to this type of scan tool so they contacted one of their guys at the local Toyota dealership. One of their technicians used the Techstream factory scan tool during the test drive. As with all intermittent concerns, it either happens or it doesn’t. Unfortunately, during this test drive it didn’t act up.
Reluctantly, the vehicle was delivered back into the hands of the owner. The customer drove the car for the next couple of weeks. He reported that some days it would drive well and others it would continually fault. Finally the fault started to happen more often and they were able to capture it on movie!
The drive was about 30 kilometers with lots of medium speed swooping bends. The vehicle would always fault on a left-hand bend. With the Techstream scan tool connected, the vehicle was driven, the fault was duplicated, and a movie was recorded within a 10-kilometer test drive.
After retrieving the movie they were able to place markers on the movie when the fault occurred. After a close analysis of the data, they were able to see through the graphing function of the scan tool that the APP (Accelerator Pedal Position Sensor) data was not correct (figure 2).
Like most drive-by-wire throttle systems, Toyota uses two, redundant accelerator pedal sensors. The signal data output from the sensors should mirror each other. If they disagree, the system is designed to respond quickly, since the wrong reading could cause unintended acceleration. In this case, the movie of the fault revealed that most of the time, the two sensor signals did mirror each other. However, on occasion the signals would separate (figure 3). The times where the signals separated coincide with the vehicle falling out of 5th gear.
To verify this without committing to a new, dealer part, we used a test accelerator pedal position sensor from another similar vehicle. After a 40-kilometer road test, it was confirmed that the symptom was completely gone. It was also observed that the vehicle felt more responsive and the transmission shifts felt sharper.
After about 4 months of chasing this issue, one final test needed to be performed to completely convince everyone that the problem was, indeed, the accelerator pedal sensor. The original part was re-installed, and the vehicle was test driven. The complaint returned!
A new accelerator pedal sensor assembly was installed (figure 4). The vehicle was driven a final time to verify the repair and the vehicle was delivered to the customer. The patience of the customer and the tenacity of the shop to find and fix the root-cause problem paid off.
This case is proof positive that we no longer diagnose only transmissions, but now we diagnose powertrain systems in an effort to find root-cause issues. Ask the vehicle “why” by using as many tools as necessary to accurately get answers from the vehicle. By questioning input and output data in its raw state, we can find the most difficult issues!
A special thanks to Ivan Reck of Kennedy Automatics & Power Steering, Mackay, Queensland, Australia.