Overheating when we turn ac off.
 

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.

It is very difficult to suggest to some one who has limited mechanical ability and approach to take on a repair and have it be validated or correct.
But for the moment let us try to think out way through this.
Assumption, there is no problem with the car as long as the A/C is on and working.
This must be true in order to take a guess at what might be wrong.
The heat gauge in the car shows normal temperature operation, nothing spooky or weird. When the car starts the heat gauge reads cold. As the engine warms up the heat gauge seems to show that happening. If it gets too warm, turning on the A/C causes the gauge to show a decrease in temperature and driving the car around seems to indicate normal temperatures.
If and the operative word is IF this is more or less true then the basic parts that help cool the car would seem to be operational.
There is water or coolant in the radiator.
The over flow tank shows water or coolant between the low and high points on the tank.
The fan does in deed turn on (and if there is more than one fan they both turn on) when the A/C is turned on.
And something you can not see, the thermostat is working otherwise it would be causing a problem.
I do not know if there are two electric fans on this car or not but let us assume for the moment there are two fans)
When the A/C is tuned on, it would add to the cooling the 2nd fan, so if when the engine is warm and the A/C is turned on you would see only one fan working.
Now if the engine is warm and getting warmer and the A/C is not turned on and no fan is running then the problem would be with the fan the A/C does not turn on. It should be on and isn't.
The do fail, and it probably needs to be replaced.
If there is only one fan and it is on when the A/C is on but not when the A/C is off then there is a problem with the sensor that controls the one fan. It does not see a hot engine.
Now I hope this makes some sense to you because I am going to make another suggestion. If your MECHANIC has not gone through this methodology of thinking, there is a probability you need a better mechanic.

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

Your first post is correct, the L series cars have 2 fans. And your second post is correct, it could be the coolant sensor and connector. As a bit of history, when I worked on police cars the fan motor failed at about 100,000 miles. Of course police cars have a lot of hours idling, more idle time than a regular car.