3800 TPS Stumps Local Repair Shop

[red88gt]

I posted this a few days ago but I didn't know how to edit it so I'm reposting. I have a 95 Transport with the 3800 that revs & surges, bucks when cruising & sometimes stalls. The TPS was open so I replaced it with a known good one from a 92 Transport. It ran the same, I replaced it with another, runs the same. Checking with a volt meter, it gets .12 volts at idle & smoothly moves up to 4.1 volts at full throttle. I couldn't find any additional problems so I gave up & took it to a local repair shop with a good reputation. They did a full scan & couldn't find anything wrong, so they drove it & got the same problems I did. While driving it with a scanner attached, the TPS voltage is extremely low (10% of value .4 volts). He diagnosedit as a bad TPS. After I told him that it was the third TPS I had on it & he agrees that the voltage is correct when in park or neutral he now thinks something is on the same 5 volt supply line that is drawing voltage when in drive, probably in the trans. He really has no clue which way to go, didn't even charge me after spending several hours on it & sent me on my way. Without paying someone an outrageous sum to chase this on an hourly rate does anyone have a clue as to which way to look first ??? Could it be in the PCM ? I have another from a 92 but I pretty sure it's way different. How can I check this ? Thanks.

[Jrs3800]

for starters .12v at idle is too low... Should be in the .36-.40 area IIRC..

Were there any codes stored in the PCM?

[red88gt]

No codes. It's pretty easy to adjust up to get that much voltage at idle & I had it higher at one time but it really seems to be the drop in voltage when driving that's the problem as it seems to run almost perfect in park.

[red88gt]

This TPS IS from a 92 Transport with the 3800 & the part # is different but it's physically the same except that the 95 version has fixed holes (no adjustment) & the 92 has elongated holes that allow for adjustment. The voltage output is the same & it's a pretty simple sensor so I really don't think that's the issue but could I be wrong ?

[Jeffrv]

The TPS is a voltage divider device, it requires 5V ref, and good griound to constant current flow and the also the signal output to the PCM. I presume you have the 5V at all times, checked from a known good ground, and with 3 TPS changed, that shouldn't be the problem, so that leaves only the ground circuit, a common problem with electical devices in vehicles this age. I don't have my manuals with me right now, they are in storage, so not sure where the TPS grounds out, but try checking the 3 wires on the ground stud hidden under the ignition module bracket. As well, I seem to remember there is another 2 wires grounded out on a bolt somewhere on the front of the engine, might be wrong there. They may be broken, making connection at idle but breaking apart as engine moves under torque ( in gear)
Jeff

[red88gt]

I've played with this a little more without success. I hooked up a voltmeter to the output side of the TPS, stuck the meter on the windshield & drove around. Supply side gives a constant 5 volt reading & the output to the PCM gives variable reading from .5 volt at idle to 4.1 volts at wide open throttle with it being pretty steady at all ranges in between. This is as close as I can come to an onboard scanner so I have to take the shops' word that they were seeing a lot less voltage than it's suppose to have. My readings directly out of the sensor are right on the money for the specs. They are taking readings out of the PCM which is telling them it's got low voltage coming in. Where's it going ? I'm going to try & find the wire that feeds to PCM from the TPS & tap into that as close to the PCM as possible to look for proper voltage there. Is this voltage output from the TPS influenced by any other sensors before the PCM ? The third wire out of the TPS is black & goes to ground somewhere, could I just tap into that & establish a good ground there for the TPS ?

[LMP]

The voltage range you are measuring is OK. The blu wire feeds that voltage to the PCM. THis blue wire goes through pin J4 at connector C111 large plug near right suspension tower and enters PCM at pin B10. THis is a low impedance source and nothing else interferes.
www.avigex.ca/xport/tps.jpg

[jojogt1]

if your TPS was bad in this model, you would lose cruise control and overdrive.