Temp Gauge fluctuates big time
 

Installed a new sending unit as the old one completely died. Now the crazy thing goes from normal warmup to peg and back in a matter of 15 seconds while driving. The engine doesn't seem to be running hot, I've checked it with a temp gun. It can idle for 1/2 hour and be steady, drive it a few blocks and it's all over the place. I've replaced the thermostat no change, I have a coolant reservoir and it keeps the rad full to the top. Before the sending unit died (no reading at all) it was solid reading all the time even with the AC on in 100+ days

Re: Temp Gauge fluctuates big time
 

It goes back and forth between the 'expected' reading and maximum hot?

If you pull the connector off and touch it to ground, does it read full hot? Wondering if there is a short to ground somewhere.

Re: Temp Gauge fluctuates big time
 

Exactly. Yes if I ground the wire it goes full hot. Also wired in a 100 ohm resistance to ground and it reads exactly where it should. Wiggled and pounded on everything I could find and the gauge never moved. I have also tried 2 new sending units. The only thing that makes any sense is some sort of steam pocket but I've never run across this before.

Re: Temp Gauge fluctuates big time
 

The air pocket is a good possibility since you had to open the cooling system to install the sending unit. If all is good there then a defective temp gauge may be the problem.

Re: Temp Gauge fluctuates big time
 

Pull the gauge and make sure the posts and sockets are clean. Same for the cluster connector and flex circuit board. If the gauge has a resistor across the back, clean those contacts as well. Double check for charred sender wire. Carbonized rubber can act as a conductor when it contacts metal.

Assuming a V8, for peace of mind while you troubleshoot, you can install a 2nd gauge and sender using the hole on the opposite side head.

Re: Temp Gauge fluctuates big time
 

Good idea on installing a second gauge. I did that on my 68 when I was having problems with cooling. I installed a new radiator, new radiator cap hoses and overflow container. On top of this, I redid all the grounding straps.