96 LHS TPS issue

Been fighting this for a while so i don't recall if i got a code or not that caused me to find it. but its been idling rough, off and on, surging, shifting funky as expected, A/C compressor not working consistently, and a few other issues. Some investigation online shows others have had this issue...4.5 volts at the SIGNAL wire, with it unplugged from the TPS and key on. So obviously the signal wire is shorted to a voltage source somewhere. the connector to the TPS was cracked and causing all kind off issues but that has now been replaced. Traced wires back to behind the block where they run into the large main harness and none are stuck together. no voltage seen prom the corresponding PIN on the PCM with power on, so i don't think its coming from there. now the fun part? my test light works fine...if there's voltage there it lights up but not on the TPS connector. But i can however find voltage with the voltmeter. one of the weird things is ALLDATA is showing a chart asking if its getting MORE than 10 volts on the signal wire.....seems like a hell of a lot of voltage since 4.5 volts is already enough to signal wide open throttle.

Tech at one of the dealers said to check all other sensors since some times they can cause the issue as well. So far haven't found anything on MAP sensor or trans control module, but haven't checked A/C pressure switch yet. i am not looking forward to having to sort thru all those wires in the harness, so I'm looking to find out if anyone else knows a known culprit that causes this more often than others, so i can possibly cut the trouble shooting time down.

Reply to
rob
Loading thread data ...

That's because a test light pulls current (which will load the relatively high impedance TPS output signal - test lights are only good for stiff source voltages), a voltmeter doesn't pull loading current to make its measurement.

Have you posted this to

formatting link
and/or wwww.lhforums.net? There are some pretty good LH car experts there (some of the same people hang out at both sites).

Reply to
Bill Putney

The PCM puts a 5v diagnostic voltage on the signal circuit. It's normal. No, it won't light your 12v test light.

Reply to
bllsht

I'm on intrepid.net from time to time. seen some interesting stuff on changing rack bushings and what not but nothing on this.....yet. i'll keep looking though.

LHforum i didnt know about so that something to look into.

Reply to
rob

"The PCM puts a 5v diagnostic voltage on the signal circuit. It's

if this is so, how long is that voltage signal supposed to be there? it appears to not go away key on, connector unplugged but my volt meter leads are too big to probe the back with the connector plugged in so i haven't checked it that way with engine running. still looking for some new leads with thinner probe tips or the plug on type.

i tested for voltage at the corresponding pin on the PCM and there was no voltage there? but that's with the PCM connector off. guess i really need those meter leads....

Reply to
rob

I'm thinking if what you're seeing is not a diagnostic voltage, there may be a bad ground connection into your TPS, or the ground side of the sensor is open (rather than it being shorted to some voltage source). The TPS is a simple variable resistor wired in potentiometer mode. If the diagnostic signal (which I didn't know about) is staying on then the question is what is keeping it on (bad PCM, or is there some condition that is keeping the PCM in that diagnostic mode - perhaps once it detects the bad sensor it keeps that diagnostic voltage on the sensor?).

I don't know that you can determine anything with the connector unplugged, other than doing resistance checks back thru the sensor itself.

Reply to
Bill Putney

yeah ALLDATA shows several checks with the TPS and PCM connector off. unfortunately some call for a DRB and i don't have access to one.

But i now that i re-read it, I see where at one point they are asking you to use the DRB to monitor voltage and asking if the voltage is ABOVE 4.5 which it isn't, but right on 4.5. Another check shows to disconnect the connector and check for voltage at the sensor signal and asks if its over 10v. seems a little high.

i need to find those leads today and get some more testing done. will report back And by the way i think i saw a floating ground yesterday on the sensor ground too so thanks for the reminder.

Reply to
rob

well my father in laws meter has leads with points on them to penetrate the insulation slightly (i have a top class meter but the leads are kinda round and blunt made to plug other connectors on them but i haven't found anyone that sells them) and the TPS and circuit seem to be working just like designed. here's what i should be getting:

THEORY/OPERATION " The Throttle Position Sensor (TPS) sends signals back to the PCM in the range of approximately 0.38 volts to 1.03 volts at minimum throttle opening (idle), to a maximum of 3.1 volts to 4.0 volts at wide open throttle (WOT), depending upon the throttle shaft angle. The PCM uses these input voltages to determine the current position of the throttle. "

I got about .45 volts at closed throttle (or something close to that), and it slowly increased as i opened the throttle, until i got about 3.8 at WOT. So it looks like its working good and since i changed the connector Saturday it seems to be doing ok. Still revs too high at start up...about 2000 rpm but settles down to 950 or so. I had cleaned the throttle bodies and EGR a while back when this all started so i need to look into the linkage and springs to make sure everything is on correctly.

Thanks for the info guys, i'll report back later.

Reply to
rob

The PCM uses the voltage to monitor the TPS circuit. If it sees 5v then it'll set a TPS volts high fault, meaning there's an open circuit, the ground has been lost, or the TPS is toast.

The circuit is monitored whenever the key is on.

Reply to
bllsht

Pretty much what I said. But what is the diagnostic voltage you were talking about previously?

The OP says it appears to be operating normally now - must have been a connection problem.

Reply to
Bill Putney

yeah still idling high at first start up, but settles down and runs just fine. TPS connector was causing engine to try and die at times especially when you wiggle the ground wire (and i had changed connector once already, so that one was bad too....watch what you buy at the salvage yard) and was effecting trans upshift since it wasn't getting a good signal to the trans control module. just like an old torqfilte it's gotta see the throttle position to know when to upshift or down shift. i cleaned the throttle bodies and adjusted idle screw setting because the measurement was off back about 2 months ago. might put that back to where it was plus i need to check one of the return springs i am suspicious of. I hear of folks cleaning the IAC motor and valve but since i don't have access to a DRB to adjust it if needed, i don't want to do anything with it just yet until i check a couple more things.

Reply to
rob

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.