[FIXED] 2008 Buick LaCrosse stalling while driving

Hello everyone, I have a 2008 Buick LaCrosse with a 3.8 liter v6 that I'm working in for my parents. It has a problem with stalling while you are driving it. If I remember correctly it had code p2135 which is Accelerator pedal position sensor a/b voltage correlation. It also had a code for APP sensor 1 voltage low but I don't remember the exact code number for that one.
I have a snap on solus and under TAC data it always shows smooth voltage climb on both sensors but I assume the tool is too slow to pick up what my oscilloscope picked up.
I will attach the waveform from my oscilloscope so you can help me confirm that the TAC module is bad
The yellow trace is the 5 volt reference circuit for sensor 2 and is always perfectly smooth and right around 5 volts
The blue trace is APP sensor 1 signal from the pedal and shows a lot of hash that coincides with the 5 volt reference for that sensor
The purple trace is APP sensor signal 2 and climbs very smooth while pushing the pedal down in the vehicle
The green trace is the 5 volt reference circuit for Sensor 1 and has a ton of noise in it for some reason

In looking at my Mitchell wiring diagram and several diagrams I found online it appears the TAC module does not get the 5 volt reference from the ECM and that it makes it on its own but I'm not positive on that. I can upload my wiring diagrams if you need to see them I just don't want to condemn this module if there is any chance it is not bad.
The parts and having the module programmed is going to cost around 300 dollars and I want to make sure it's right since it is my parents car.



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For our reference, what time scale was your scope set to on that capture? Thanks for including it, BTW.

Strange as it may sound, that kind of pulsing on one of the APP 5V sources is normal on this system. :lol: It's part of the TAC circuit integrity test. It'll pulse the 5V on one of the two circuits (APP1, for example), which obviously makes the signal pulse the same way. But, if it sees APP2 pulsing at the same time, then it knows the two APP circuits are electrically shorted to each other. The TPS circuits usually do the same thing.

How often does it stall? I ask because, depending on it's frequency, capturing the fault may be difficult. Keep your eyes open for pin fitment and pin fretting issues, as well. Sometimes, just cycling the connector a few times can clean up a fretted pin.

It occurs almost every time it is driven now. It used to be very intermittent. I did see a TSB for this showing moisture in a connector and I did find moisture in the connector and dried it out but I'm not really sure how to prevent this from happening again. Should I use dialectic grease on the female side of the connector?

The diagram I have doesn't show any connectors between the TAC module and the pedal but I am sure there are multiple that could have issues. Does anyone have a factory diagram they could upload showing the connections so I can check them?

I know there is another TSB from gm for all passenger cars that says a connector inside the kick panel could get wet from a windshield leak but in not sure where that connector would be.

I also am not sure what the time base was set to for this capture. I believe it was pretty darn long though.



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i would reccommend dieleectric grease on all connectors it helps seal water out the components as well

I read the TSB again and apparently the first time I read it I only saw the first page. I followed the directions in the TSB to remove the weather seal on the back side where the wires go in and there was moisture in there as well. I blew it out with compressed air and used dialectic grease in both sides of the connector as well as in the connector. I reset all codes so we will drive it for a few weeks and hopefully everything will be fixed.


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Nice! Your parents are gonna be thrilled when they hear you (hopefully) fixed it with no parts changing. Which connector did you find moisture in, for our reference?

It's a little late now, but here's the GM diagram for this same system. I love Mitchell diagrams, but they do leave crucial details out...



Looks like X101 is the suspect connector?