98 XLT AWD Temp Gauge Stuck Past Hot

[Smuffy]

Hi! It's been a while, but I've recently had a problem pop up at the worst possible time. I start school on Tuesday, but for the past few days I've had my temp gauge stuck past the "Hot" section, with the "Check Gauge" light on. I've had a coolant leak for a while(have no idea where it is), and one day it shot up, so I pulled into a parking lot and saw it was low on coolant. I put some bottled water into the reservoir and hurried home. When I got home, I added coolant/water mix and let it cool down. Then when I started the truck up later, to try to get more through the system, the temp gauge wouldn't come down. I've tried a few things, like starting the engine to let it run, hoping coolant just needed to work it's way through the system, and after a day of that, I swapped the thermostat, thinking maybe it had become seized. After that failed to change anything, I started looking around on the forums. A couple people mentioned senders maybe relaying an incorrect signal to the gauge, or the gauge itself being bad. Someone else mentioned a blown head gasket, but the truck seems to run fine, the oil seems clean, and the exhaust has nothing to it, from what i can see. The idle is a bit lopey, but that seems common. I really have no idea where to look next. I have a multimeter, but to be honest, don't really understand it, and I'm only guessing where the coolant temp gauge is(looking at the engine, above the front drivers side section of the intake manifold, with an orange plug cap, and only 1 wire feeding into it?). If anyone has any ideas, or could help me organize some of these thoughts, I would greatly appreciate it. Like I said, my first day of school is Tuesday. I can't miss it or I lose all my free tuition money, and I'm worried that I won't know if the engine is actually running at a decent temperature or not. As someone who can't afford to have it replaced, the idea of driving without knowing is pretty frightening. Anyways, thanks for your time!

 

[FIND]

If the gauge pegs the moment you turn on your truck, replace the sending unit. If it rises after it has been running for a little while, then burp the air out of your system by squeezing your radiator hoses while it is running and not hot.

I believe you are looking at the right sending unit. You test it by checking continuity (resistance -- Ohms).

 

[Smuffy]

If the gauge pegs the moment you turn on your truck, replace the sending unit. If it rises after it has been running for a little while, then burp the air out of your system by squeezing your radiator hoses while it is running and not hot.

I believe you are looking at the right sending unit. You test it by checking continuity (resistance -- Ohms).

Thank you for the reply!
Normally, the gauge reads above "hot" with very little fluctuation, and does not return to the rest when the truck is shut off. I turned the key to on, and pulled the connector, and the gauge dropped, so this should rule out the gauge itself, correct? Forgive my lack of knowledge, but that would mean I was trying to close the circuit to test it? How would I do that? Power at the wire contact then ground on anything metal? It's only one wire, and I don't know much about this side of electronics.

 

[FIND]

You want resistance between the sending unit and ground. No need to power it. Just set your multimeter to the ohms setting and test.

From the sound of it though, you have a bad sending unit. Replacements are ~15 bucks at the parts store. When you go to pull it out, have the new unit close by so you can quickly thread it into the hole the old one comes out of. Then you won't spill much coolant.

 

[Smuffy]

Thanks for the help and suggestions. I swapped the sending units(26.99 at Autozone. I couldn't find a cheaper one in the area) and upon starting it, the gauge acted normally. I'll be running into town later today to see if the problem is fully resolved and it heats up as it should.