Link between coolant temp and a/c compressor?

I had an incident today where my a/c stopped working. I turned it off thinking I must have a leak, but then realized my coolant temp was running almost at the top of the normal range. After shutting down for a while, I cranked it back up and everything seemed to work fine (including the a/c), after the temp came down a bit. It still ran a little hot the rest of the way home, which I think will require a coolant flush. Is there a thermal link between the coolant temp and the a/c compressor that will shut off the compressor if the coolant temp gets too high, preventing the engine from working harder than it has to? It’s a 98 Explorer V6.

Well, the condensor coils for the A/C are mounted in front of the radiator. These coils are to cool the high pressure A/C charge from the compressor so that it will cool when run through the evap coils under the dash. And it will get quite warm. If the engine ECM senses the engine beginning to overheat, it may shut down the A/C in an attempt to improve the cooling action of the engine coolant in the radiator.

I have a 2000 For Explorer, but have not experienced this scenario before. So I cannot say if this is a programmed feature of the ECM. But, the ECM does handle some functions of the engine when the A/C is being use, like bumping up the idle and running an auxillary cooling fan. So, some self-preservation programming would not be unheard of.

There are high and low pressure freon switches that will shut off the compressor if the pressure gets to high or too low to cause damage. You need more than a flush though to take care of the overheating. At least a thermostat and maybe a radiator. Tackle that first since it’ll ruin your engine.

Most more modern cars have a direct link between the A/C compressor and the ECU (Engine Computer.) When you hit the on switch on manual A/C systems, that switch goes straight to the ECU, telling it that you want the A/C on. On Automatic Climate Control systems, when you select a temperature and turn the system on, if the Body Computer determines the need for A/C, it sends a request signal to the ECU (virtually the same as if you just pressed ON, but there’s a middle man now.) The ECU will, in most cases, immediately turn on the A/C compressor. But it has full veto power and can turn it off for any reason. Most ECU’s are programmed to suspend A/C operation under hard throttle, or somtimes just Wide Open Throttle. And things like that.

The ECU, which monitors the coolant temp, can deny the A/C request and shut the system off during an overheat, or an imminent overheat. The condenser for the A/C is, as mentioned, right in front of the radiator for the engine. It hogs all the cool airflow, and the fans work to serve its whims as well. Ideal condensing pressures result in temperatures of between 130-160?F. So if you were doing alot of stop and go driving, or towing/hauling, you could see higher temps on the engine temp gauge with the A/C running. And there’s your link between compressor and coolant. =)

One other thing though. As the engine begins to overheat, especially when at idle, the heat from the radiator begins to transfer to the condenser. In extreme overheating cases, the condenser actually will absorb massive quantities of heat from the engine radiator. The slower the car is moving, the more heat can transfer over. This results in rising condensing pressures, which raises the system pressure all over the A/C. I once, while testing my Buick’s system and not properly cooling the engine and condenser, saw the gauge hit 7/8ths. When I got back to my A/C gauge, I was looking at 450psi on the high side of the system. Normal is 150-180. 450 (and rising) is decidedly dangerous, as 17 year old o-rings (which remain on two high side fittings that I could not seperate to replace the O-rings) and fittings could rupture and burst under those pressures. To prevent situations like that, A/C systems nowadays have high-pressure monitors, that work like low-pressure switches, and interrupt current to the compressor and turn it off. If your engine was overheating, it could have also been this switch that turned off the A/C.

So the A/C system is, in all likelyhood, operating normally in your car. You have to determine why you overheated. Were you driving in stop&go, hauling something, other kind of “severe” operating?