I already replaced the temp. gauge sender.

When I connect the green sender wire to ground, the gauge goes all the way up like it should, but the gauge stays close to 100F when the engine is on.

I took the dash apart and took out the gauge cluster. I connected the gauge straight to ground, 12V, and to the temp. sender and the gauge seems to be fine. I turned on the engine and let it run for a couple of minutes until the temperature on the gauge said 230F-240F and then I turned on the radiator fan (connected it straight to the battery) and the gauge went down to under 220F in less than 30 seconds. Then it gradually went down to 160F (it was already dark outside and kind of cool with a slight wind). When I turned off the car, it slowly went back up to 220F and then slowly went back down. So it seems the gauge is fine.

My guess is the problem is somewhere in the green wire going from the temp. sender to the gauge. I tried tracing the path of the wire, but all I know at this point is that it leads somewhere in the back of the engine. I think this is where it goes through the firewall? Where does it go after the firewall? When the ignition switch is on (right before turning on the engine) the temp. sender wire is grounded so that the gauge pegs all the way up and then comes back down, so is it safe to assume that the green wire goes up the steering column and then back down?

Im going to take the dashboard apart some more later. How difficult would it be to completely replace the green wire with a new one?

 

It goes through the bulkhead connector near the brake booster. What do you mean the gauge stays close to 100F when the engine is on? And then you say the gauge is working fine?

 

I mean that the needle in the gauge always stays near 100F when the gauge cluster is connected through the original wiring inside the car, but I already tested the gauge by connecting it directly with new wires outside the car and it works.