I'm having issues with my TPS. I replaced it with two aftermarket ones (one from Autozone and then one from ACDelco that was the same PN as GM stock). And neither one would read less than .75V no matter the position of the blade on my NW 102 TB. I did this, because my stock TPS started flipping out an reading 2% and throwing the P0123 code.

So I went back to stock, and I can't get the car to idle anymore. It reads 2% no matter what I set the voltage to (.63V was were it was set when I put it together - and I've set it from .61 to .67) and it idles really low - like 650RPM even though it's set for 900. I figure it wasn't getting enough airflow. But it's also throwing code P0123 with the stock GM sensor and the aftermarket ones. Mind you I had the car running well on the current tune for about 4 months before this popped up.

What's the issue? Is it wiring? Is there something in the tune I need to adjust because it's reading 2% at an acceptable voltage? Is it the TPS sensor (all three of them)?

Car is pretty undrivable. The whole issue popped up in August and I've more or less parked it as I've traveled for work. But now I wanted to get it back on the road and this is pissing me off. It was high idling then so I bought the new TPS sensors and played with the TB, not knowing if the NW was actually moving of me or if the TPS was going out. But the new ones won't read low enough...

Btw, it also throws the code with the new TPS as well. But that one won't ever read less than .75V... so even if I close the blade all the way, it reads that. So I lose resolution for throttle position. I set it with the old TPS and put the new one on... but it reads 1-2% no matter what. I don't want to clock them, because I didn't have an issue before...

I do the TPS relearn each time I make an adjustment or swap TPS.

Any ideas as to why it wants to read high? I have drilled a small hole in the blade previously, but then everything was working fine for months... and now this.

 

Key ON, ignition off, I see 5.94V over the wiring harness. I believe it's supposed to be 5V... so now I get to track down some oddities in the wiring?

 

So if I understand this correctly, even with the stock TPS you were seeing idle voltage readings of 0.63? That's still too high, it should be ~0.50v with the blade in the stock idle position. But then it's an aftermarket TB and you mentioned drilling a small hole so I assume there was no hole to begin with? Perhaps the hole you drilled isn't the same size as stock and therefore it needed to be set at 0.63v rather than 0.5 initially. I think the stock TB blade hole was 5/32", or whatever the metric equivalent is for such.

If the hole is smaller than that, perhaps you can enlarge it (and correspondingly reduce set screw position) to get the voltage into the 0.5 range and percentage into the 0.0-0.4% range (anything up to about 0.4-0.6% should be OK at idle, but I wouldn't want to see it above 1%.)

Odd part is that you said everything was fine for quite some time, then suddenly there were issues with no deliberate changes....that would suggest some sort of failure/damage/etc. Do you have any old logs from a time when everything was running well that show TPS voltage and throttle percentage at idle? Would be nice to see as reference.

 

Sounds like a ground issue in the wiring?

 

Here's an article that shows how to test the TPS and confirm if its good, or not: www.wellsve.com/sft503/counterp_v2_i1_1998.pdf

 

Consulting my GM Factory Service Hymnal under the Book of Engine Controls, Verse P0123:

- The reference voltage should be 5V
- The reading at idle should be .6V
- The WOT reading should be above 4.0V, but when it goes above 4.75V, the DTC gets set.

So, that voltage above 5V is probably an indicator of whatever the problem is - it is most certainly the cause of the code. (It sounds like your sensor is probably not the problem.)

One thing to check: The PCM shares the same internal reference for the TPS, EGR, and MAP. All have a grey wire that provides that 5V reference. If your EGR and MAP both show 5V, that would be an indication that the PCM is good and you have a crossed wire somewhere in the system.

When you test the voltage at the sensor, what are you using for a ground? The sensor ground or the chassis? Have you tried using different grounds to see if you are getting different readings?

 

In my experience, this reading would be considered on the high side. Looking back at various stock logs of my own LS1s, 0.50-0.55v is the norm for idle TPS voltage. Perhaps the manual was indicating that up to 0.6v was considered 'acceptable', but it's not the norm as far as I've seen.

I know it's a small difference, but considering the idle issues and the fact that 0.1v represents a 20% difference, I wouldn't necessarily feel OK dismissing that .63v reading that Jake observed above as normal/OK.

So IMO, both the reference voltage and idle reading are suspect, though the idle reading may be more a factor of the aftermarket TB and a different idle setting for the blade. I'd still like to know if previous logs showed the same TPS voltage at idle (.63v) when there were no apparent idle issues.

 

I'll check it tonight. Horrible travel!!!

I used the internal wiring. I'll check ground to the chassis and battery as well.

Ryan, I have a NW 102. Car wanted voltage set at .69V to get my IACs into the 60s. So I drilled a 1/8" hole and got it to .63V... this was in the Spring. I had a FAST 102 on there and it liked .48V with no hole to get IACs into the 60s. But the IAC passage was smaller there... but there was more room around the blade. When the NW shut, you couldn't see light around the blade. You could with the FAST. I may go back to the FAST since I don't have cold start issues with the smaller IAC on the FAST.

But first things first, is I need to get the TPS settled out. It just sort of happened sitting at a light. Idle went to **** and when I read the voltage, it was .8V with the stock sensor reading 2%. So I adjusted the screw down, and the car wouldnt start. So I replaced the sensor and haven't had any luck. Which is why I started looking at the wiring harness now that the TB itself and the TPS sensor have been ruled out as problems. But 2% TPS means no idle routines and my car doesn't idle very well without adaptive idle unless I lean it out even more to the point where it is non-responsive off-idle.

 

Yea, the TPS is just a potentiometer, so if the input voltage is high, the lower end of the reading would be high and the sensor may never read that one is off the gas.

 

Well, I checked the signal from the MAP (since I could get to it - need to find where the hell I stuffed the EGT)... and it was 5.04V. Great. Used the body ground I have for the battery.

Double-checked the TPS. 5.94V through the body ground. Not great. I have something shorting somewhere.

 

So I'm going to try to work on this some tonight. If the voltage is higher just on the TPS, I can assume the PCM is probably ok and the voltage regulator for this isn't toast. But why would one wire in the chain be reading more voltage than others? Where should I start troubleshooting this?

 

I assume that's the case, yes. The Factory Service Manual implies (through it's troubleshooting procedure) that this is the case.

I would think that each circuit is protected by a diode, within the PCM, which would prevent any back-flow of power in to the PCM, which might fry it or contaminate other circuits with electrical noise/voltage.


This is the tough one... My inner-electrician tells me to unplug the PCM connector, see if the TPS connector then reads 1V, 6V, etc. and/or trace the wire from the PCM connector to the TPS module.

However - I know that removing that connector is a PITA and requires a new seal each time you remove the damn thing.

Have you done a visual of the wire loom from the TPS back to the PCM? Anything look off?

6V is such a weird number. I'm trying to think what in the engine bay would run off that voltage, but nothing is coming to mind. I wonder if you migtht have an inductive bleed to the wire?

Have you tried measuring the voltage with the key on ACCY and the engine off? (I'm not sure if the sensor would even be powered under that condition.)

 

I'll spend some time tomorrow with it. I've been transitioning out of my job and need to spend a few hours uninterrupted with it. Blah.