Computer and TPS went bad at the same time?! P0123
 

Coming home from school today, I pulled out of the parking lot and my jeep wouldn't shift out of 1st. Shortly after, I got a CEL. After limping it a quarter mile in 1st and turning it off and on again, it started shifting, although not correctly. It delays the shifts pretty far up the rpm range and the torque converter won't lock up anymore.

I got it home and had a P0123. Started doing some testing, I unplugged the TPS and had 4ish volts on the sense wire. That tells me my sense wire is shorted to my 5 volt reference. I unplugged the connector from the computer and started measuring for continuity to other circuits. Couldn't find anything, everything seemed normal. So I made the decision to cut the sense wire at the computer. And sure enough, I had 4 volts on the computer side and live data telling me my tps is pegged at 100% with the wire cut. So my computer is shorted internally right? I then measured the other end of the wire, and that was also at 5 volts. So that confused me quite a bit until I unplugged the TPS and the voltage went away. So my TPS is also shorted internally.

I made this post to ask does this seem right? Did I miss something? Both of these seemingly failed at the same time. Also how does the TPS affect shifts, does the pcm relay that info to the trans controller or how does that work? I ask that because if I can throw a new TPS in it and have it start shifting normally, that would be nice because then I could at least drive the thing.

4-volts on the wire coming from the computer is normal, and the computer is fine. You either have a bad TPS which is very pretty common, or a bad ground wire to the TPS which would explain the 5-volts on the signal wire. Ideally, you want to backprobe the connector with the TPS plugged in and look at the voltages. At the very least, unplug the TPS, and measure the ground wire on the harness side and if it's more than a few ohms, the ground is bad. The ground wire connects to the grounding stud over by the oil dipstick. Never a bad idea to clean that ground point anyway.

The TPS hates getting wet, btw. Make sure you cover it if you do any engine cleaning.

The TPS signal wire is connected directly to the trans computer. No relaying involved. I believe the wires from the TPS and the trans computer both go to the same pin inside the connector to the computer, versus being split somewhere in the harness.

Thanks for the responses. I did wash the engine bay the other day so that might be it. The ground also looked pretty suspect so I'll try cleaning up the ground and report back.

Ended up buying a new TPS and that fixed the issue. Not knowing the theory of operation for the TPS lead me astray, but that's why I asked before dropping 150 on a new computer.

Glad it was an easy fix. You wouldn't be the first to kill a TPS with a pressure washer or solvents. They just aren't sealed very well.