Incorrect temp indication?

[tcstig]

I have a 99 Grand Prix GT with approx. 120K miles. Recently I had issues with coolant loss and some temp flucuations in the process, obviously normal due to low coolant. I fixed the leak issue, refilled with coolant, bled the system and all is back to "normal" and consistant concerning cooling except that I believe the indication is approx 30 degrees high in reading. I have a 195 thermostat installed (new) and when the motor warms up to the point that the thermostat opens the temp indication is approx 225 which then drops and stabilizes at 210. Before the leak issues the indication showed about 195-200 before dropping and stabilizing at about 180. I changed the coolant temp sensor with no change. Also the fans work fine with AC and they don't come on when the indication reaches as high as 230, which I figure is ok since I think the indicator is the issue. My question, if what I think is correct, is it possible to recalibrate or change the temp indication somehow back to the correct range instead of possibly having to change the whole cluster to fix this problem?

[tblake]

What brand thermostat did you use? Its possibly you have a bad one.

[3100]

gauge needle is controlled by stepper motor which I don't think is your problem. The best is to verify with scanner capable of reading live engine data. If scanner reads 200 deg F. and your needle shows 200 then everything is fine. Tomorrow at your place at 2pm should be about 50 deg F use DVOM to test ohms at your ECT you should get close to 5760 ohms. You can always verify temperature of water with analogue thermometer. Or if your needle is showing 195 deg ECT should read about 240 ohms

[tcstig]

I have actually tried 2 different types, the first was a fail safe type which was a more expensive one than the one in the car now. The original one was doing the exact same thing as the 2 new ones I've tried. The change in indication happened around the time that the car was slightly overtemping due to coolant loss. I don't know exactly when it changed but it was around that time.

[tcstig]

3100, I'll check the ohms reading tomorrow and post the results. If the ECT sensor reads ok, what would you suspect is the possible problem.

[3100]

Or if your have time to play with it, you can install potentiometer type resistor inline with the ect sensor signal wire, and then you can fine tune added resistance to where you like your needle to be. Remember when you add resistance needle will go down, so if you could find potentiometer that will add resistance in increments of 2 ohm from 0-100 ohms that would be perfect.

Maybe you exidentaly misred the needle. Your gauge is showing scale with 5 deg F increments. So one line below 210 will be 205 etc.

[tcstig]

3100, The resistor idea sounds like something that I would probably do only if the sensor was the problem, but I did replace it because it's not too expensive. If I added resistance and the sensor wasn't the issue I would be afraid that the fans wouldn't come on at the correct temperature in the event of an overtemp condition? Also I did figure that each mark on the guage is a little less than 5 degrees and that the thermostat opens at around 225 (verified by feeling the hose and watching the temp drop) and then stabilize at 210, steady. This is on my cars temp indicator and is not normal, correct?

[tcstig]

Today I was able to borrow a scan tool from a coworker and observed it compared to the guage. The 210 mark on the indicator equals 185 on the scan tool. When the thermostat opens the scan tool reads approx 194 and the indicator was at approx 220. The scan info is coming from what the computer is seeing right? So does this info point to the indicator being the problem? or could the computer and/or wiring between be them be the possible problem?