I am showing P1125, P1276, P1281, and P1286. The car will start and idle. It will rev up with no load, but as soon as you release the clutch pedal (even slightly) the engine will stall. Had this problem a year ago but it went away after just a few attempts. Now it is more insistent. I have not looked a connections yet so have no idea if there is a dirty connection issue or not. Assuming connections are good is it simply to replace the APP sensor?

Thanks,

EZ

 

The error codes you get are not always entirely accurate. The system has several components:

APP sensor attached to the pedal itself; the TAC module; the TPS sensor on the TB; the throttle actuator motor; and the wiring in between them all.

The APP sensor is a 3-circuit potentiometer. The TPS sensor is a 2-circuit potentiometer.

You can remove both of them and actually measure the resistance as it moves. For the pedal, I bolted it to scrap wood. For the TPS sensor, it was even easier. Just rotate the thing that rotates and make sure every circuit works properly. You can apply power and ground in order to see a voltage rather than measuring resistance (it's a different way to measure the same thing - the system runs by measuring voltage, but it's usually easier to measure resistance if you have a proper multimeter, and voltage if you only have a voltmeter.)

You should look for discontinuities, jumps, fully closed / fully open connections. If it sweeps the range as described in the shop manual and on forums (not the full 0 to 5 volts, but kinda close to it) then it works. If you see issues, replace it.

Note that OEM APP pedal/sensor combos are unobtanium and you basically end up going through used ones if you go that route but a forum vendor now sells an aftermarket replacement for just the sensor itself. Ones from other GM cars will not work. TPS on the other hand you can get a stock unit from AC Delco (OEM) for like $70. TAC modules very rarely go bad but they are also no longer made, so you'd need to buy used. And wiring, well, you can always modify wiring, if that ends up being the problem.

FWIW I went through this whole thing ... I had a marginal APP sensor I replaced with a different, used pedal+sensor assembly, before an aftermarket solution existed. It helped but didn't solve the issue, and I threw both a new TPS sensor and a whole new throttle body assembly at it and neither helped. Even replaced the TAC and it didn't help. Turns out the previous owner modified the wiring, poorly, and it was diagnosed and repaired (thanks, Abel chevy.)

Oh, and on that note, a whole used throttle body assembly is usually cheaper than a new OEM TPS sensor and will usually come with a fully working TPS sensor, just a used one.

TL;DR: Start by diagnosing the TPS because it's trivial. Then pull out and diagnose the APP sensor. If neither give you a clear solution, inspect the wiring, clean the connectors, etc. After that, well, if you're me you can throw parts at it, or you can find a proper good shop that can diagnose it and not throw parts at it, though sometimes the former is cheaper given the price of used components versus labor rates.

 

Thanks so much for your help. One quick question... TAC = ?

I do have a Multimeter and a forecast of snow for this weekend, so I will pull it out of the trailer and run through the diagnostics. Hopefully now that the problem is constant (vs. Intermittent) we can find it and eliminate.

 

Throttle Actuator Control module, probably.

Have you found references for expected voltages, if you plan to apply voltage?

Let us know what you find when you diag this!