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Temp. gauge not working

just bought a new rig. temp gauge does not work on my 1990 bronco xlt. it has a 5.0 302 and runs and drives great. does not over heat or burn/leak oil or water. So im guessing its just the temp. gauge. is it ran through a fuse or ran directly to the thermostat? could the problem be the thermostat? how would i go about testing the temp gauge? any help would be appreciated.

What does the temp gauge actually read?

First, some clarifications. The “thermostat” is basically a metal plug that sits near one of the radiator hoses and stops the coolant from circulating into the radiator when the engine is warming up. If it sticks closed, the truck will overheat and if it sticks open it’ll take forever to warm up, if it ever does.

The way the gauge works is it’s basically an ohm-meter that’s hooked up to the temp sender which is a variable resistor. As the temperature changes, the resistance either goes up or down, which is reflected by the needle moving back and forth on the gauge. A basic diagnostic trick you can do is to take the wire off the sender and the gauge should read all the way full with it disconnected and all the way empty with it grounded to the engine. Or vice-versa-- some work the other way around so having a service manual would be very helpful. The gauge itself is likely powered on the same fuse as all the other instruments, so if they’re working it’s likely the gauge is getting power.

Also, just to be sure, you may also read about the “coolant temp sensor” which is the sensor that tells the engine computer how cold the coolant is. This is separate from the gauge sender. Usually, but not always, the sensor will have some sort of slotted plastic connector whereas the sender will just have a simple blade connector.