I'm stumped. Help me fix my Trooper.

Wayslow wrote: I misread the chart above. I stand corrected the first number really is in 250000 ohms. I'd bench test the resistance of the sensor at several different temperatures. Set the sensor in a pot on your stove and raise the temp. See what happens to the resistance. Compare it to the chart that you have above. If that works out then it's likely the wiring. It looks to me that you have an issue in your wiring harness. Adding the second sensor in parallel reduced the resistance and gave you a better reading.

I did the stove test. If the jumper wire doesn't fix things (I expect it will), then I'll try the stove test again.

Thanks for all of your help, by the way. This forum is way, way better at this than even the Trooper forum!

After looking at your posted pics of the manual, yeah, broken wire. Those things can break inside the insulation and you'll never see it.

Yup. A bad or corroded connection would be my other guess.

You don't have a temp controlled water pot and a dozen good multimeters?

My bet is high resistance in the wire. Knowing that it's a 4WD Trooper and what the back woods are like around here, I wouldn't be surprised if the harness is corroded.

Some vehicles have two temp sensors.

One for the ECU and another for the temperature gauge.

The sender for the gauge has nothing to do with the ECU.

Just a suggestion.

While there is a ton of good info in here I think you guys are missing the obvious:

Step one: Remove V6 and install in LeMons Miata. Step 1a: Megasquirt! Step two: Install LS1 in Trooper Step three: ????? Step four: PROFIT!!!

Electrical gremlins are the worst. I found out that my fuel gauge and fuel pump failures in my Miata were both caused by wires touching in the back of the map light harness by the glove box. When I removed the leads for the "upgraded" rear view mirror light (installed by a previous owner) that were shoved into the back of the harness everything went back to normal.

Is anything else malfunctioning on the vehicle? Maybe there's a grounding or shorting issue that could be causing seemingly-unrelated issues. In my Miata's case, the interior lighting would randomly come on and the go back to normal. I never connected the two in my head.

There seems to be a lot of assuming. Is the engine really running cold ?

Of course it's -40F in Florida. It's February. Duh.

Call me crazy but the "5VDC Return" should actually be a ground. And if that's true then the "Coolant temp signal" terminal should actually be a voltage supply line, and it wouldn't 'receive' anything back, it would measure either the voltage or the current on that line. There should be ~0v on the 5vdc return line, and it should have low resistance to ground.

Somebody let me know if i'm wrong.

Dr. Hess wrote: Of course it's -40F in Florida. It's February. Duh.

Of course.

In reply to Vigo:

I think the text you wrote makes sense but I can't see the pictures you posted and they seem to point to some corner of Facebook that I can't reach. Can you rehost or something?

Sorry, I haven't had time to read this thread again, but here's an update:

I had a spare 20 minutes tonight, and hooked a sensor up directly to the ecu, both hot and cold. It read perfectly accurate temps. Looks like I need to run a new wire.

New question: it looks like something else T's into one of the coolant sensor wires. Am I reading that correctly? I guess that means I need to leave the old circuit hooked up in addition to the new wires.

In reply to Tom Suddard:

I think you'd be nuts to leave old, broken wire in place to service an unknown T. I suspect that's going to come back to haunt you...

Seems like the best thing would be to replace the old ECU wires, and move whatever's T'd over to your new wires.

I don't know what the T'd item is (do you?), but relying on the remains of some already-broken wires to service an unknown part, to continue to service that part, and to not have an effect on the new circuit, is asking a big favor from Murphy.

Check the resistance of the applicable wiring, should be less than 3 ohms from the harness connector to the ECU connector on all the pins and from the ground pin to the chassis.

It's the "return" from the TPS. You need that. I would GUESS that the wire from C10 on the ECU to the sensor is either broken or has a bad connection on one end or the other. Fix that and you're done. Fairly easy to diagnose. Put one end of an ohm meter on C10, the other on the end unplugged from the sensor. Like 0 ohms? Not the problem. Not ~0 ohms? Problem.

I think the text you wrote makes sense but I can't see the pictures you posted and they seem to point to some corner of Facebook that I can't reach. Can you rehost or something?

It's just the same wiring diagram Tom posted on the first page.

New question: it looks like something else T's into one of the coolant sensor wires. Am I reading that correctly?

Uhh, yes, that is plainly obvious in the wiring diagram. Here's a pro-tip: If your tps always reads correctly on the scanner, then the wiring problem is between the coolant temp signal terminal, and that splice. If the tps reads correctly, everything between that splice and the '5v return' terminal should be just fine.