My wife's 1999 Accord EX has a temp gauge that is erratic. It will move to hot and back for no apparent reason. When in the "normal" range it will twitch a small amout. I took it to a mechanic and he found that if you turn the AC on the temp gauge jumps to HOT immediately. Clearly a gauge/electrical problem.

Any ideas to start finding the problem here?

The car warms up to normal in about 3 -4 miles and runs fine. Its just the erratic temp gauge!

 

The problem can be the temperature sending unit, the wire, or the gauge inside the car.

You should put your car's info in your signature, because I don't know if this is a 4 or 6 cylinder accord.

This is advice for the 4 cylinder. Under the distributor there are several sensors bolted into the cylinder head. There is only one sensor that has just one wire going to it. That is the temp. sending unit. Unplug the sensor and make sure that the wire does not touch anything. Turn on the key and the temperature gauge should be stuck at cold. Turn off the key, touch the wire to ground. Turn on key (make sure to turn off key when the needle is close to H, or it can get damaged).

Let us know how the gauge responds to these two tests.

 

Thanks! I will check that out.
I still don't know why turning the AC on would make the temperture gauge jump up though. Thoughts?
This is an EX with the V6 engine

 

For V6, coolant temperature gauge sending unit is located as shown on attached pic.

 

Ok! I unhooked the sensor as shown in the photo and turned on the ignition. The temp gauge did not budge off the bottom. Next I ground the sensor and then turned the ignition on. The temp indicator rose rapidly towards hot. I turned it off before reaching the top of the gauge. I repeated a couple of times with the same result.
Next test?

 

Since the gauge moved, the gauge and wiring are probably okay.

The next step would be to test the coolant temperature sending unit. With a multimeter, you would test resistance of the unit. You would disconnect the wire from the sending unit. Measure the change in resistance between the unit's positive terminal and engine ground with the engine cold and with the engine at operating temperature.

At low engine temperature, it should be at about 137 ohms. At high temperature (radiator fan running), it should between 30-46 ohms.

If the readings are substantially different than the specifications, replace the sending unit.

 

Swinging to HOT means that wire is shorting to ground. Follow the wire as best you can, look for anyplace where the insulation might be rubbed off or cut.

Is there any pattern to when the gauge jumps hot? Like driving over a bump? Steering a certain way?

 

JimBlake misread your post. You grounded the wire, then it moved to H on the gauge.

You can do the multimeter test on the actual sensor, or just replace the sensor. When you go to replace, have a catch pan below the sensor, because you will lose coolant. Replace the sensor as quickly as possible.

If the coolant hasn't been changed in a while you can drain the coolant, replace the sensor, then add new coolant.

 

I can see where Jim was going.

If the wire has a "cut on the insulation" somewhere and it makes contact with metal....than that can act as a ground, sending the temp gauge up....then the wire moves off that metal and goes back down.

Unplug the wire from the sender, start the car. Turn the ac on, what happens to the temp gauge? Drive the car for a bit, does the temp gauge move at any point......again it should not as it is not connected......however if it does than that tells you the wire is getting "ground" from "somewhere". That "somewhere" is going to be fun to find

 

A short was my first suspicion. Just not sure why turning the fan on or off makes the gauge move at the exact same time. Seems like a change in resistance therefore a move on the gauge!