Jumpy temp gauge

[Atypical_decorum]

Hello all, this isn't my first post, but I guess you get ghosted after 2 years of inactivity!

My B7500 has been running like a champ for a long time, but about a year ago the thermostat gauge spiked and went down and that was it. So I finally got around to installing a new thermostat and now it's acting all kinds of funky.

In idle without the PTO engaged, it gives a fairly high reading, close to the Danger Zone. With the PTO engaged, the temp lowers to just about the cool mark on to bottom then creeps up slowly to about 1/4 way. When increase throttle to anything over idle, the temp gauge goes bananas. It goes up to the high, then down to mid low then middle then down to cool.

You're probably thinking what I was thinking. There's no airpockets in the lines. The coolant is running and is topped off. The radiator itself is around 130F to 140F.

These aren't exactly electrically complex, but I still don't know where to look. Got advice?

 

[Tinhack]

I'd be checking the wiring from the gauge to the sender. Perhaps mice/rats chewed on some wiring and it's shoring out or has a bad connection. Typically, voltage is applied to one side of the gauge. The other side of the gauge is wired to a temperature sensor.

Look for the temperature sensor near the thermostat or hose going to the radiator (top).

 

[Roadworthy]

In addition to the above check grounds at the instrument panel. Your sending unit could be defective. Make a resistance measurement cold and another when your radiator is up to operating temperature. A temperature of 130 to 140 seems on the low side but I don't know where you're measuring.

 

[JWR]

Yep, all of the above but I add that it sounds like your guage is being driven not by the steady source of voltage to a sender and then the proportional voltage coming from the sender. Sounds more like it is being driven by the voltage directly out of your alternator (rising with rpm.) Probably need to re-check all related wiring. I'm not sure if a ground left floating could cause this or not ...

 

[KubotaSam]

Check for corrosion where the wiring harness plugs into the back of the instrument panel. I bought a B7500 a few years back and the temp gauge acted kind of like what you are describing. Turned out to be corrosion as I described.