Temperature Gauge Puzzle

[Paul39]

When I got the White Rocket, 89 Festiva, temp gauge would not move off bottom. I shorted out the sensor wire and got full hot on gauge, replaced sensor with a used one form Black Rocket's old engine (around 200,000 mi.) and it indicated about where it does on the black rocket.

Drove for several days and needle kept going higher, checked coolant, good, put thermometer in top of radiator, around 200. Needle stayed about 2/3 up inside the bracket on gauge. It usually ran 1/4 to 1/3 up.

It ran fine, did not steam, did not put more coolant into reserve tank than usual.

Filled up with gas, drove away from station, needle went to 1/4 up in bracket, next day a bit lower, next day a bit lower yet, today did not move off cold position.

I suspect the 200K + mi., 25 year old sensor became disinterested in further toil. Before I put a new one in I want to be sure I haven't missed something that would kill the new one.

Any thoughts?

Paul

[Spike]

Dumb question. Replaced the thermostat itself before all this?

[Paul39]

Dumb question. Replaced the thermostat itself before all this?

Not a dumb question, I didn't mention that.

No, bought one with gasket when the indicated temp went up. Sitting on the dash. I have driven it 15 miles at 60 - 70 mph in 80 degree temp. and needle does not come off the bottom. I have driven other cars with no thermostat and they usually come up close to normal in that time.

I did think when the indicated temp. went back to "normal" after I filled up the gas, that maybe there was a ground disconnected to the cluster and the change in the gas gauge was providing more ground to the temp. gauge.

I wanted to get some opinions before I pull the steering wheel and the cluster.

Paul

[Damkid]

The temp gauge works off resistance, if you ground out the sensor wire, the gauge goes to hot and won't hurt anything, it's actually a good way to diagnose issues with the gauge

Basically the warmer the sensor gets, the less grounding resistance it has

[Paul39]

Today I put in a new sensor, $19.95 from Advance Auto Parts, Sorenson, made in USA. Drove out of town to dinner. Temp gauge worked normally and came to the accustomed 1/4 up in the operating range.

Apparently all the weird behavior was the 25 year old sender going through its death agony.