HELP!! Coolant/electrical issue 97 12 valve!

[Flake1820]

So I have a 97 12 Valve, and my coolant temp gauge has been fluctuating bad. Idling it’ll sit at 180-190 and then when you get on the throttle it shoots to 230+ and let off the throttle it goes back to 180-190. I’ve replaced the thermostat, thinking maybe it’s the coolant temp sensor? How often do those go bad? Thinking about just running a new gauge to get a more accurate reading and by passing the factory wiring. If I was to do so could I install the probe into the sensor hole on the back of the block?? Or could I use the plug on the top of the thermostat housing??

[That Guy]

I think it takes a 1" socket.... maybe bigger, that will be your best bet, hard to get a swing on a wrench. It will need to be a deep well, and keep in mind that the plastic on it is fragile. It is very simple to test with a multimeter. Hook to it and put it in a pot of water, preferably with the plastic out of the water so the water isn't being measured. Set it on high and you should see a fairly linear change in resistance from cold water to boiling. I dont think thats the problem though. It isn't something that typically fails, particularly a factory cummins unit. Engine temp also doesn't spike that quickly. The gauge can respond pretty quick (140-190 in maybe 2 seconds), but if it is jumping then that's not the probe.

 

You could plumb a in another gauge in either location, but the back of the head is typically chosen because that is where the hottest temps are.

 

It seems like you may have a ground issue/wiring issue. 

Is it purely RPM dependant? What does the gauge do when in neutral while idling/free revving? Do any other gauges seem affected when you rev the engine (speedo/fuel/voltage)? What is the resistance from the negative battery post to other grounded metal things (engine/body/metal part of dash/frame)? Shouldn't be higher than 1 ohm once scratch around on clean metal.

Edited July 29, 2020 by That Guy

[Flake1820]

Doesn’t fluctuate at idle, none of the other gauges have an issue besides fuel doesn’t work at all because of the sending unit. But I just unplugged and plugged back in the single wire going into the sensor and it seems to work now? Lol weird 

[That Guy]

Sounds like a bit of corrosion on the connector. As you throttle up, the top of the engine torques over to the passenger side, that could cause the wire to move a bit and affect continuity of the connector. By unplugging and plugging it back in, you made better contact in the connector.