My temp guage was reading erratically so I bought a new sending unit. Then it just stayed down on 100 all the time. So, I thought the new unit was bad and I got another one. Same thing. When I start the car the guage pegs out then drops down to 100. If I ground out the wire going to it then turn on the key it pegs out. Why won't it read temp??

 

are you using the temperature sensor that came with the guage or are you just plugging the gauge in to the original sensor? Some temperature sensors have differnt resistance so if you are not using the sensor that is coming with the gauge I would recommend putting that one in and trying it.

 

I had the same problem that you are describing in my 86 TA.

It turns out that the wire harness had been cooked by my exhaust manifold and the wires to the temp unit would sometimes ground out and peg the guage. Other times it would just bounce around.

Moral of the story is.. check your wiring, and dont trust a visual inspect, use a volt meter.

 

It is the factory guage and the correct sending unit. The guage was working before I replaced the sending unit but it would sometimes bounce up to about 230 then come back down to normal. That's why I replaced the sending unit. Could the guage be bad? Is there a way to check it?

 

Im having the exact same problem. I checked the temp wire and its fried along with my o2 sensor. I bet yours is cooked as well.