gonzo1028

Has anyone seen a TPS on a 91 formula 350 read 4.56 volts closed and when going to WOT the voltage drops? Same thing w/ the resistance goes from 5.6 ohms to 2 ohms when opening the throttle. It seems to me this is backwards? I don't think it is adjustable. But it seems lie it should not run at all with this type of problem.
I am stumped!

Vader

Try checking the voltage using the other two leads. My guess is that you are reading between the source voltage and signal voltage, not the signal voltage and ground.

1989GTATransAm

iTrader: (3)

Right. You are using the wrong reference point. Allen

gonzo1028

Real nice I am using alldata. Their test shows hooking the dvom to b & c. I tried a & b and i got .574 goes well beyond 6 volts tested good.
Thanks guys.

Red Devil

I've read conflicting reports. Most say that on the '90+ TPI's the computer zeros the reading from the TPS on start up, thus making adjustment unnecessary and technically impossible.
The part numbers are different as well with these years coming with no adjustment slot in the body of the TPS... most of the time. True, false, unknown?

Vader

That is the same information I have. Speed density TPI system ECMs with the fixed TPS will sample the voltage at system power-on. If the signal is below 0.75 VDC, the ECM uses that reference as the presumed "zero" point, and interprets any signal above that as an opening throttle.

I'm not sure if the ECM has trouble with an initial signal above 0.75 V, but I've also read/heard that 1.0 VDC is an acceptable starting point for GM fixed systems. (I know the EEC systems use 1.0V as a start, but this isn't a Ford). I would presume that anything above 1.0 V would be a problem, and if this occurs on a stock installation, it is likely due to a failing TPS or throttle minimum air position that has been toyed with.

GM Powertrain dropped the ball on this one. Their engineers should have done everyone a big service and routed a ninth spark plug wire to the minimum air screw, with a standby power source that kept it at about 20KV at all times, and several big, red, yellow, and black illuminated and animated warning labels reading "This is NOT an idle adjustment!"