Can someone please advise me? My ex-husband of 31 years has always worked on our cars, so all of this is new to me. My daughter has a 2003 Saturn LS. A few weeks ago it overheated. So I had someone come out and flush the coolant system and it ran fine for a few weeks. Then a few days ago it started to overheat, but only when she turned the AC off. The mechanic came back out and said it is probably the thermostat. But as he is not sure and doesn't want to waste my limited resources he suggested I take it to a GM dealer and have a diagnostics run. I really need this car to stay running for another few years till I can get her through the first few years of college. I looked it up on YouTube on how to change the thermostat I think I may be able to do it, but do you guys think it could be something else? The mechanic could not get it to overheat when he came out the last time. And he said the fan was running. Nothing appears to be leaking in the system.

 

If the L cars are designed like the S cars were (and like most others), the aux cooling fan is programmed to automatically turn on and stay on when the AC is turned on and running.

Under normal operating conditions w NO A/C , the engine coolant temperature sensor changes with resistance (R goes down as T goes up). The PCM/ECU/ECM/whatever infers the V drop across the sensor -- which of course depends on R (V=IR) to determine the temp of the COOLANT flowing near the ECTS. When the voltage drop across the ects passes a low threshold value (= R low, Temp high), the PCM makes a FAN RELAY request, closes the fan relay circuit, and starts the aux fan running to lower the coolant temp. It runs until the coolant temp drops below a preset value ((R above a preset value) and then shuts off.

We know the fan relay and fan motor are good as the A/C uses both in its operation.

That leaves ECTS, wiring between ECTS and PCM, and radiator.
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I suspect that your ECTS or the circuit is defective. Wen this happens, the car almost always thinks the coolant is much colder than it really is. The end result is that the bogus ECTS info fed to the PCM never reaches the threshold to turn on the fan---and the car overheats.
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How does your temp gauge in the care read normally and when the car starts to overheat?

Personally I would change the ECTS and its connector with AC Delco parts to ensure an ECTS with the right calibration is used.

I doubt that a thermostat stuck closed could keep the cooling system at bay from overheating, with no coolant flow through the block, but I've been known to be wrong. The fact that it is not reproducible COULD indicate an intermittent ECTS connection OR a thermostat that sticks closed most but not all of the time.I recommend you go for the ECTS and connector replacement first. Then you will know the correct temp info is getting to the PCM

 

thank you for this email it actually enlightened me allot ple ase let me know any thing else you know on the subject please and thank you