Peculiar PCM/Calibration Behavior

[vipmiller803]

The buick has been doing some interesting things when it is wet outside. I noticed this last November, but the car sat for a good chunk of winter, and the problem has not self resolved    .

Sorry for the long post, but here it goes:

 

With the brakes not applied, it seems to surge. This happens when the car is cold and has sat overnight in high humidity. Understandably, the surge is even worse when the idle is high right after a cold start (it is enough to shift into second). When the brakes are applied, it behaves normally.

 

My first thought was the brake booster is leaking a little vacuum when not applied. I suspected that that is not the issue because there was no air sound and it was very intermittent. I have a switch that turns off the tcc by grounding the brake switch input at the pcm, so I used that to test my suspicions, and sure enough, the brake application itself is not the issue, it is just the pcm thinking the brakes are on that makes it behave.

 

Like I said, the car got to sit out the winter months, so... fast forward to today:

 

I was finally able to catch it in the act on tunerpro. I tried multiple times before but sometimes it would do it only for a few braking cycles, so by the time the laptop was on, it was behaving, or at least so it seemed  .

 

So here is the link to the xdls from my trip to work today:

https://drive.google.com/folderview?id=0B6AKqXKswy6VMVRkQzljNUZCMG8&usp=sharing

Two files, one on my street just showing the behavior at idle only, the other is the rest of my 22 mile trip.

 

The tps signal is what I can see causing the surge. This tps is about a year and a half old. I replaced the connector at that time too. The fact that the brake switch signal changes the tps makes me think that the issue is not in the tps itself, but rather at the pcm side.

The closed tps voltage is 0.57. It jumps up to 0.63 or 0.61, sometimes alternating, as soon as the brake is not applied. This happens with either the brakes actually being used or me using the switch to ground the brake switch signal.

 

Two things I noticed:

-The behavior slowly got better as the trip went on (towards the end the voltage only jumped to 0.59, and not at all at the very end).

-The pcm tried to learn the 0.61 as the 0% throttle position (but pretty sure engine still surged a little), but I would burst its bubble every time I hit the brakes and dropped the voltage back to 0.57.

 

I am not super well versed in tunerpro and calibration, so I could be missing something important.

 

I am suspecting condensation in the pcm or the wiring next to it, because the engine heat could be burning it off. Is that an idea to chase? I have thought about making it waterproof somehow to try and eliminate that. 

 

[RobertISaar]

i don't have time to look at the log at the moment, but it's almost like your brake light circuit is inducing current into the TPS signal circuit.

 

the PCM should be more or less waterproofed already via two methods: the rather complete sealing of the case itself(mine actually retain vacuum/pressure if the ambient temps change significantly compared to the last time it was open) and the conformal coating that covers almost the entirety of the PCM's PCB. if the problem were at the connectors, I would think other circuits would be effected as well since there are a few critical ones between the C30(brake switch) and C22(TPS signal) pins including O2 and IAT. it wouldn't be a terrible idea to pack some silicone grease into the connectors should you have to disconnect them for any reason, they tend to seal really well as is, but it should give another barrier to any kind of moisture intrusion.

[vipmiller803]

it's almost like your brake light circuit is inducing current into the TPS signal circuit.

That is what is throwing me off. The tps is a very analog signal but the brake light circuit is just on-off, so I thought any kind of connection there would send the tps past its 5v high threshold in a hurry and throw a code. I am going to have to look further into that plug... I guess the fact that they are on the same pcm plug does narrow it down a little.

[RobertISaar]

it's possible that there is current leakage at the board level, but that isn't so simple of a thing to test for. could rule out harness vs board by disconnecting the brake switch circuit from the PCM and seeing if it still effects TPS readings?

[vipmiller803]

Thank you Robert.

I think I will look closely at the C30 and C22 wires to be sure they are not nicked and close to each other. Usually a long shot, but not completely out of the question since I have messed with the C30 before for my switch. I can remove the plug too just to make sure no water has gotten in there.

could rule out harness vs board by disconnecting the brake switch circuit from the PCM and seeing if it still effects TPS readings?

If I do that, it is going to involve cutting some wires... which I would like to think is not my next step. If it comes to that... then with the brake switch disconnected, if there is still an effect on the tps reading then that would mean it is the harness right? I know it seems trivial... but is it really?

[Padgett]

You need to monitor both the power and the ground side of the TPS, I have seen a bad ground act exactly like that.

 

Brake lights are a pretty big load and if there is a bad battery to chassis ground the whole car could float a bit. In a TPS, a quarter volt can make it think you are on the gas.

[vipmiller803]

You need to monitor both the power and the ground side of the TPS, I have seen a bad ground act exactly like that.

 

Brake lights are a pretty big load and if there is a bad battery to chassis ground the whole car could float a bit. In a TPS, a quarter volt can make it think you are on the gas.

That is a good point. I am not sure where the tps gets its ground from, but I have an extra engine strap to the chassis and an extra battery strap to the chassis. Of course that means nothing when the ground is coming from elsewhere. Also, my lights do not illuminate when my tcc switch is engaged, but the tps signal still changes. Makes me think it is not the draw.

[RobertISaar]

If I do that, it is going to involve cutting some wires... which I would like to think is not my next step. If it comes to that... then with the brake switch disconnected, if there is still an effect on the tps reading then that would mean it is the harness right? I know it seems trivial... but is it really?

 

the connectors are actually really easy to de-pin, have to remove the end clip via 2 tabs on the side, lift the appropriate white retention tab and push/pull the terminal out.

 

if even without the brake light circuit being directly monitored by the PCM it shows up, there would have to be a harness issue of some kind.

[55trucker]

That is a good point. I am not sure where the tps gets its ground from, but I have an extra engine strap to the chassis and an extra battery strap to the chassis. Of course that means nothing when the ground is coming from elsewhere. Also, my lights do not illuminate when my tcc switch is engaged, but the tps signal still changes. Makes me think it is not the draw.

TPS doesn't have a ground, there is the reference voltage from the PCM, the TPS signal back to the PCM, the ground circuit is in the PCM.

 

Normal voltage operation range is between .5 volts to approx 4.8 @wide-open-throttle. This can be checked with a scanner connected ...ign on/engine off.

 

If one does not have a scanner you can *back probe* the harness connector with a meter to read the voltage change as you move the TPS.

[vipmiller803]

the connectors are actually really easy to de-pin, have to remove the end clip via 2 tabs on the side, lift the appropriate white retention tab and push/pull the terminal out.

 

if even without the brake light circuit being directly monitored by the PCM it shows up, there would have to be a harness issue of some kind.

I have actually never unplugged a pcm before 

No time like the present I guess. If depinning is at all possible then that would make life a lot better. Will be difficult to do that when the conditions are right. Maybe I should put my car in the shower so we can replicate wet heavy air conditions? 

[vipmiller803]

Well I unplugged that C connector yesterday and there was not gobs of corrosion in there, but it seemed like the case surface corrosion has gone further than the seal should have let it? The seal was intact, but there was just that white aluminum corrosion sprinkled on the removed connector. Anyway, I cleaned and put some dielectric grease in there as well as on the seal.

 

I was worried my switch connection was actually causing this, but my outer electrical tape is still nice and right around the whole connector harness, which means the individual one I messed with inside should be good.

 

Just need to wait for a good rainy night and day to test out that seal fix.

[Padgett]

Once upon a time I had a Fiero that kept stalling when coming to a stop. Turned out it was a bad wire from the TPS to the PCM that was going to zero when braking. Ran a new wire and it stopped stalling.

 

TPS on a GM car is typically .38-.42v at idle and "over 4.0v" at WOT. More importantly at a 45 mph cruise the TPS is typically under 1v a .25v different is a big change for a TPS.

 

True both A and C (B is the wiper) go to the PCM but one is the 5v reference and the other is the sensor ground. If the chassis ground floats, so does the sensor ground and the TPS will read high.