My 89 Corolla started overheating when I turn on the AC. If I don’t use the AC, the temp. is fine. 2-3 min. after turning on AC, temp. gauge rises and I must turn off AC. My husband, a devout CT listener has some mechanical skills & ck’d it out. He replaced the fan relay, but that didn’t fix it. History: the 1st time it happened, the AC was on and blowing cool air, suddenly, the air wasn’t cool and I noticed the temp. rising and turned off the AC. I stopped and checked the radiator fluid and it was low, so filled the overflow with coolant. Drove home (30 min.) and it was fine. A week later, same thing - I’m on the freeway, air is blowing cool, then not and heats up. Ck’d radiator - fluid level is fine. Problem was somewhat intermittent as it wasn’t happening ‘all the time’. Yesterday, same thing again on the way to see the dentist 40 min. out in mid-afternoon sweltering heat. Had to drive there and back with no AC - quite miserable. Hubby is at ‘huh’ level w/no ideas about what to ck next. He replaced the thermostat not too long ago and the radiator also had to be replaced year before last. I have health issues and really can’t handle the heat and fumes and must have AC in this climate. We can’t afford to take it to a mechanic. If this car goes, them I’m car-less. It’s a great car and keeps on going - 240,000+ miles. Thanks for any replies and suggestions.
Make sure the electric fan is switching on when the AC is turned on. That’s the only thing that comes to mind at this point. Others may have a better answer so please check back.
Sounds like two separate problems, though maybe related.
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AC blows cold for a while, then suddenly the air isn’t cold any more. This could be the AC compressor automatically disengaging because the compressor itself gets too hot, and a temp sensor in the compressor disengages the compressor in order to prevent damage. So, the AC comoressor may be getting too hot for some reason. When was the last time you had a refrigerant charge? I’m just guessing that maybe your refrigerant level is low and maybe the reason the compressor is overheating. I’m not an AC expert, so someone else may have a better guess as to why the compressor is disengaging.
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You notice that the coolant temp gauge is rising. The fact that it happens even at highway speeds suggests that it’s not a fan problem, because plenty of air is blowing through the radiator at highway speed, even without a fan. Also, you said you had a new radiator and thermostat recently. All that’s left is…the water pump. A water pump’s vanes erode over time and reduce the flow of coolant through the engine. If you’ve never had the water pump replaced, that could well be the culprit.
Another potential cause for overheating is a leaking head gasket that allows coolant into the engine cylinders. This is noticeable as a steady loss of coolant over time. If your coolant level is not dropping slowly but steadily, then you can rule out the head gasket.
To see if those fans want to turn, tell him to pull that fan relay back out. If he has some electrical skills, see if he can figure out which is the coil side and which is the contact side.
Once he knows which is which, short the contact side to see if the fans turn on.
If they do not turn on with a shorted contact, it could be that the connection from the relay’s contact to power/ground* is broken, from the relay to the fan is broken or from the fan to power/ground* is broken.
If they do turn on with a shorted contact, whatever drives that relay’s coil is broken.
*“power/ground” because it depends on how they switched the relay and the fan. They could source the 12V through the relay or ground. It all depends on the car. If he’s handy with a multimeter, he should be able to figure that out.
PS…I’ve heard it’s also possible for a radiator hose that looks OK from outside to have a collapsed inner wall that restricts coolant flow, so that’s another possible cause of overheating.
I don’t think it is coolant related. A car will overheat if the AC is on and the radiator cooling fans are not running.
Remco, but the thing is, it sounds like her AC compressor is disengaging. Twice she said that the AC suddenly stops blowing cold air. So I’m dubious that the compressor is even running while the car is overheating.
If the compressor is actually running when the car is overheating, why is the AC then blowing hot air?
First thing to check is are the fans running when the ac is turned on. If you have two fans on the radiator, both must run anytime the AC is on. If they are not, then that problem has to be addressed first. The fans are needed even at highway speed when the AC is on. If one fan is not running, it is probably bad. Use remco’s advice for checking the fan motors.
I hear you but why would the AC disengage when the car overheats?
It also seems to only overheat after the AC is on.
You may be right in that it could be a chicken/egg thing, with AC being the effect not the cause but I don’t quite understand the mechanism that would make the AC turn off when it overheats - unless the car decides to protect itself somehow. If I were to design cars, I’d definitely put hat in there somewhere but the cars I’ve had are more than happy to overheat with the AC on.
Then again, I also don’t quite understand the mechanism that makes the AC suddenly push warm air so touché. 
We’ll have to see what she comes back with.
As for the AC shutting down when the car overheats, that maybe be a protection circuit and not a defect. If both fans are working, my next suggestion would be to replace the radiator cap. The AC puts an additional load on the engine and a marginal cap can cause the engine temp to rise under an increased load. If the new radiator came with a cap, most do not, it might not be the right cap, it might not meet the pressure requirements for the car.
Keith, I do believe most/all modern auto AC’s have built-in thermal protection circuit that disengages the compressor if it gets too hot, so yes, it’s a design feature and not a flaw. But the question remains, if that’s what’s happening, why is the compressor overheating?
That’s a separate question from why the engine is overheating.
jesmed, the compressor may not be overheating. Some cars also shut down the AC when the ECT (engine coolant temperature) gets too high. That is a separate protection circuit.
Thanks Keith, did not know that.
Wow! Thank you all for your responses and suggestions. I will have my husband read them and check out the possibilities. I’ll let you know what he finds. You are awesome!