This is on a 1992 Trans Am, L98 speed density car. Car runs well, very slight searching of idle when cold started, but evens out to about 600-700 rpm after a few minutes. I don't really trash on the throttle but it seems to respond rather well. All injectors were replaced recently.

Was messing with a friend's scanner and noticed my WOT voltage reading from the TPS (with the engine not running obviously) was only about 2.6 volts. Removed the TPS and fiddled with it, but even when the sensor was unmounted and the lever was moved to it's maximum setting it only read 3.2 volts. Tested the wire; the TPS was receiving exactly 5 volts.

I hooked jumper wires to the sensor and plug to read the output from the sensor; just to make sure my ECM wasn't reading it wrong. Again, I got the exact same low voltage readings.

Bought a new AC Delco TPS expecting it to fix the problem or at least read differently. The new sensor read the exact same, on the scanner and with jumper wires. Thinking I may have bought a defective one I returned it, and have gone through several more (of various brands) clinging to hope that I was just unlucky and was getting bad sensors.

It's now obvious there is something else wrong and it is baffling me and other mechanically inclined friends. All of the sensors, when mounted, read about .54-.60 volts at idle which is correct and in spec. However, none of them read over 3 volts when at WOT. They all transitioned smoothly when the throttle was slowly opened. The strangest thing is that the wire/plug is receiving the correct 5 volts, and therefore the sensor must be to. However, even when the sensor is not mounted and the lever is manually turned to its maximum position on the sensors they all read about 3.1-3.3 volts at the absolute max. The scanner and jumper wires with a multimeter both read the same voltage.

I am stumped. The scanner, as far as I could tell, showed that the ECM was receiving information from all other sensors that were in spec or at least made sense; there didn't seem to be anything else wacky going on other than the TPS voltage. I don't have the scanner anymore but I've gotten several recommendations to get a new ECM.

Can some weird quirk in the ECM affect TPS voltage readings, even with the jumper wires directly from the connection to the TPS being read with a multimeter? I would highly appreciate any input here before I get another ECM, since I'm not sure that will, or can, fix this problem.

 

hmmm. has it ever been hard to start when it was cold??

I had the exact same problem, the ecu replacement solved my troubles.

 

It's never been hard to start that I have noticed, even when it was pretty cold out. I've had to let it crank for maybe 2-3 seconds at the max before it would fire right up.

The only thing that is sort of strange is that even after I set my IAC correctly (which is also when I found the TPS sensor problem), the idle still tends to search a little between 1000-800 rpm's for 30 seconds to a minute before it settles down to around 700-600 rpm, then it just stays there and everything seems to run like a top.

I'm still hesitant to blow more money on something, but it seems like the ECM may not be a bad idea. I'm certainly stumped as to what else could possibly be causing the sensors to give those readouts. There's no way 6 different sensors could all be defective that same way and only put out an absolute max of 3 volts. The wire is getting 5 volts, and the loom/wire itself appears to be in great shape. Any other thoughts or suggestions before I decide the ECM is the only thing left that could be causing trouble?

 

Did you try a second DVOM to verify it was reading correctly?
If it is correct, then I'd check the grounding wires closely to make sure the ground is solid before replacing the ECM.

 

Tried a second slightly older DVOM I borrowed, exact same results. I'm guessing the readout from the jumper wires is indeed correct.

Can anyone with some more wiring experience tell me exactly how many grounds there are that could possibly affect the TPS voltage? 305sbc is your wiring harness similar to mine? Where they are located would also be a huge help. I have wiring diagrams but if someone could just tell me where to look it would certainly save me some headache, and it would help to make sure I don't miss anything.

Other than a ground issue could there be anything else besides a quirk in the ECM causing this? If cleaning the grounds I can find doesn't solve this a new ECM had better, or else I may have one of those unsolvable electrical gremlins.

 

UPDATE

I went through and cleaned all of the grounds I could find, both visually and by my wire diagram. These include:

Both battery cable grounds.

Both headlight grounds.

Both of the big hard to reach grounds on the back of the engine block.

The ground on the Drivers side near the strut mount.

That was all I could see or find.

There seems to be no ground directly connected to the ECM wire loom. In fact, my diagram shows the only ground inside of the dash is for the "convienience center" on the driver's side inside of the instrument panel. I did not get to this one as it is a real pain and appears to not have any connection to my problem. Is this assumption correct? Should I tear my dash apart to get to this one? Does anyone see anything I might have missed?

Cleaning these grounds did nothing. I get the exact same readings .54 volts at idle, 2.6 volts at WOT.

 

The main grounds for TPI normally go to the back of the cylinder heads, so I'm sure you got them.
I guess it's time to try another ECM to see if that changes your results.

 

I contacted tpiparts, forget the guy name but he was VERY helpful and knew TPI systems very well. I ended up buying a used ecm for 75.00, after replacement my TPS read exactly within specs, and the car runs great.

I also ended up buying a new delco ecm from the dealer as a backup.

here the ling to the ecm, but give him a call.

http://tpiparts.net/inc/sdetail/433

 

Thanks for all of the help, really appreciate it.

I'm done fiddling with wires and grounds; it either has to be the ECM or my wires themselves are all kinds of messed up and I'm leaking voltage somehow through a low mileage untouched wire loom...electrical problems scare me to death, especially when just swapping in new parts doesn't solve them.

I'll check with the local AC Delco shop here first, they tend to have really great prices and I know the guys there. Thanks for the link though, I may use it depending on how things work out. I would like to get a new ECM but the online Delco catalog seems to only list a reman one, so I'll check around. Either way it will be a week or two before I get it I'm sure. I'll post again once I get another ECM and see what that does. I'm praying it solves this issue, I don't want to think about what it means if it doesn't.