Chevyguy358

iTrader: (4)

I recently replaced the head gaskets on my '94 Camaro. When I got everything back together I noticed that the TPS voltage was quite low, around .12, when datalogging in both DataMaster and Freescan. I went ahead and followed the directions on this webpage http://www.s-series.org/htm/how2/tpsadjust.htm, to dial in my TPS and IAC parameters. After shorting the A and B pins on the OBDII diagnostic port and setting the idle to the ideal range I went ahead and connected my laptop once more to find a voltage of 0. I figured there must be an issue with the sensor so I disconnected it and measured at the plug on the grey wire and also at the PCM at B28, once more found 0 at both. So I then thought that perhaps in this process somehow a parameter was wiped from the PCM and went ahead and reprogrammed it with an older tune via CATS. Reconnected the PCM, once more voltage at 0. I'm thinking either the PCM has an internal issue or perhaps a fuse somewhere is blown but for the life of me I can't think of any that would effect this. Any thoughts?

shbox

I don't know that I would be using that info for setting idle speed. That seems to predate our cars. There is a GM factory manual procedure for resetting the IAC on my website. The throttle stop screw should never need to be touched in any stock application and carefully in a modified application or different TB.

The 5v reference is generated by the PCM, so if it is not coming out of there, it sounds like a PCM issue.

Chevyguy358

iTrader: (4)

Thanks Shbox, I actually used your website for the pcm pinout, guess there isn't much of a reason to look elsewhere. I guess I'll be buying another PCM, again.

Ed Wright

iTrader: (1)

That procedure you there is bogus, and not for these cars. The TPS is not adjustable, and there is no reason to screw with it. The PCM looks at the TPS when you turn the key on and accepts that as zero throttle angle. There is no magic number. Does need to be under a volt.

Chevyguy358

iTrader: (4)

Yea, my car is pretty heavily modified and I used that procedure once before to correct a split BLM issue. I guess I just happened to luck out the first time around.

RamAir95TA

iTrader: (2)

The PCM will recognize anything between .23v and .9v. A broad enough range where you shouldn't have any problem zeroing in on. Most people typically target the middle - about .6v.

Chevyguy358

iTrader: (4)

So here's where it gets weird. I purchased a new PCM uploaded my old tune to it and checked the voltage both at the plug and within Datamaster. Once again the voltage is 0, everything else to be working and it fires up ok except it threw a low MAP code along side of the low TPS one. Looking over the schematics seems the TPS 5v reference is shared with AC Temp and pressure sensors but nothing else. Not quite sure if that leads me anywhere but just trying to figure this out. If there isn't any 5v reference signal coming out of the PCM at pin B28 what can cause this?

RamAir95TA

iTrader: (2)

That's a tough one. I'm wondering if you could source the 5v reference signal from somewhere else. Not sure if the lack of 5v reference is something that can be changed via programming or not.

You mentioned you have a MAP code too. Do you have a 5v reference off of B29?

Chevyguy358

iTrader: (4)

Nope nothing, actually with the B high density connector removed I didn't pick up voltage or ground on any of the pins on the PCM... Very strange. I'm not running AC and the harness has been modified from Auto to a T56 conversion but this has worked fine without any issues for several years. Perhaps a bad ground would knock these out? I can try plugging in the B connector and back probe each pin, not sure if this would provide me with anything worthwhile.

Chevyguy358

iTrader: (4)

Ok Back probing them I end up with the following values:

Pin# Vdc
8 8
13 11
14 11
15 11
19 5

There were a few pins where I ended up with a few values in the range of .01-.05 didn't think these would have much relevance however.

It looks like B19 goes to the MAF sensor, can I split this 5v reference between more than one sensor?

Chevyguy358

iTrader: (4)

Any ideas?

banshe5

iTrader: (1)

I had a very similar problem months ago with a customers car. The issue it had was that the MAP sensor connector was replaced. When he put on the new connector he had switched A and C (A-B-C connector). All I had to do was repin that harness and it was good to go. Unplug the MAP and see if it then shows the voltage you are looking for. You could unplug the AC connectors that are on the same circuit one by one and see if the voltage comes back. It's possible that a connector or sensor/switch is grounding out that circuit. Hope this helps.
Jay
Zeus Performance

Chevyguy358

iTrader: (4)

Thanks for the response. Just as a test I jumpered a 5 volt reference signal from the AC compressor connector and split this between the MAP and the TPS. Everything seems good to go, granted I still have an issue somewhere in the circuit. Besides this I now have a huge split in my idle BLMs and my voltage at idle is .61 as opposed to .53 where it used to be. I remember when dialing this in the first time, several years ago, I had to do several adjustments to the TPS, IAC count adjustments, and boring out the IAC passage a bit to get the split BLM issue resolved. I guess I'll just have to play around with it until it falls into range. Until then I'm looking at 108/160 splits in my idle cells.