Engine coolant temperature gauge?

The driver insist the car overheats. I have not yet seen it overheat. When I drive the car the temperature gauge generally stays at the VERY low end of normal operating temperature. Occasionally with hard acceleration, it'll hit the middle of normal operating temperature, the fans will kick on, and it'll drop back to the very low end of normal operating temperature. It's kind of cool out, so thats probably keeping it cool, but when it hits mid temp and the fans come on, it drops fairly quick (like maybe a minute) so I wouldn't think the heat outside would cause such problems?
I don't think the radiator is the original/correct radiator for the car. Is any of this normal? I've never seen a car run that cold (still puts out heat if you turn the heater on)

Yeah, I was kinda thinking it was a thermostat stuck open (or missing), but wanted to check if there were any other possible reasons. I don't think the radiator is the original, because the stop cock on the radiator drain doesn't fit in side the bottom 'tray' (the support under the bottom.)

No warning lights come on, but sometimes the gauge goes pretty high. I am suspicious that my mechanic hasn't been checking my fluids when I get my oil changed. I wonder if it's just that my coolant is low. How do I check that and top it off? Thanks!

Other Category problem
1996 Chevy Lumina V6 Wheel Drive Type unknown Automatic 150000 miles

We recently bought this 96 Lumina APV minivan. It immediately showed that it needed a new water pump then it ran fine (no more running hot).

Two weeks later I took it for an oil change and got 6 blocks down the road and the temp gauge heads for the hot side. Our mechanic discovered that the radiator fan isn't coming on right and says there is nothing they could have done to cause this when performing the oil change... Do you agree?

He has ruled out all the common suspicions (thermostat, etc) and tells me he thinks it's the car computer. He knows we can't afford a new computer and is suggesting that he can make a mechanical fix to bypass the computer.

Do you have a recommendation?

Also, can you tell me why the temp gauge rests in the 3/4 (toward hot) position while the engine is cold? I suspect that the temp gauge is not accurate - can you tell me how to test that?

I know I have asked alot of questions but maybe you can help me to help my mechanic - he has spent several hours already and I'm wondering if I should find another mechanic.

Thanks,
Karen

Temp gauge doesn't work. Why?

My temp gauge fluctuates from normal temp to hot all the time or it just says its hot all the time even when I just turn on the car. I changed the temp sensor in the engine twice. Then I looked at the pigtail. The wires were burned so I got a new pigtail and soldered it in. It read the right temp for a couple minuates then it shot to overheating again. I'm completely lost at this point. What else could be doing this? Also my primary fan won't turn on but my a/c fan does. I tested the primary fan motor and it works. But it won't turn on when it needs to.

The temperature gauge has stopped functioning. Could this be because of a bad thermostat or a heat sensor?

If heat sensor, where is this located and how hard is it to replace?

Thanks

updated 7-27-09
We finally found the heat sensor. When it was removed one of the prongs broke off. We thought it has broken off inside the wiring connector because there was a silver wire inside one of the connection holes - we tried to remove this wire and determined it was not the broken piece. However, now the wiring harness connector is now damaged. How difficult is it to replace the wiring harness or connector?