Question about automobile A/C

I have always been told that if you turn off the car with the A/C running you will harm the vechicle. I could see how starting the car with it running could hurt, but I don’t see how its any different from switching it off normally.

So, does it actually hurt the vechicle?

I’ve never heard anything to that effect.

Frankly, it’s amazing how much misinformation there is out there about cars.

Neither shutting off the engine nor starting the engine with the A/C turned on will hurt anything. That’s probably how the majority of folks have been doing it for years. I share Absolute’s amazement.

How could it? It’s simply a belt-driven compressor. When the engine stops the belt stops. The AC compressor is completely independent of the motor.

I’ve heard this before too. People seem to think that something is still trying to work when the car is turned off but that’s not really what happens. People also think that if you leave the AC turned on and stop the car, that it’s going to cause harm when you restart the car because it will be trying to turn the compressor while the car is starting, which is supposedly hard on the engine/starter. Every car I’m aware of disengages the compressor while starting, though, so it’s not really an issue. Effectively, it’s like the car automatically turns the AC off while starting. The car will turn off the radio and a lot of other accessories while starting too, for the same reason.

I always turn my AC off before shutting down my truck. Not because I think it will hurt it, but because I am set in my ways. When I start my truck I have a set order I do everything in. I get in, and press the clutch, and brake. Verify that the transmition is in neutral. Put the key in, and start the truck. Then I turn on the lights, radio, AC/Heater. Release the parking brake, put it in gear, and drive off. When I stop I alwasy take it out of gear, put the parking brake on, turn off lights, AC/Heater, and the radio. Then I turn off the truck, release the brake, then the clutch.

Nothing to do with it damaging it, just the way I do it.


Well I have heard that you should switch off the a/c before switching off the engine, but for a different reason.

I heard that you should switch it off about 5 minutes before you reach your destination, but leave the fan running, so that the air-con unit has a chance to dry out. This is meant to prevent mustiness from mould forming in the condenser.

May be BS (and I very rarely remember to follow this advice) but that’s what I heard.

A few snippets from Google searching… mostly from auto forums…

There is some sense to this. Of course, this isn’t what would be described as harm to the vehicle, and of course the overwhelming majority of folks aren’t going to do it because it’s inconvenient.

Exactly, except that it would be the **evaporator **that needs to be thawed and/or dried. The condensor is the hot part behind the grill, so has no such issues.

Really? I don’t know air-con from black magic, but isn’t the condenser the cold coil part that condenses the water vapour out into liquid form? That’s certainly what I recall a condenser doing back in chemistry lab…

The condenser condenses the Freon back into liquid form.

Yeah, so? My car has a Freon atmosphere.

:smack: That makes sense now…

a picture is worth a thousand words

I’ve heard this for over 30 years, ever since I started driving. I’d guess the car wasn’t smart enough to turn things off back then - could this ever have been a good idea? The story back then was that keeping things on would run down your battery - even if the car was turned off.

I’ve never heard of shutting off the ac before the car. I frequently leave my ac on when I shut down the car and have never had an issue with it.


If there is no function that allows the pressure created by the compressor to release, then the compressor would start under pressure. It is harder on a compressor to start up if it already has pressure built up.

A mechanism to relieve such pressure exists:

All mechanical refrigeration systems are a loop. When the compressor stops, liquid refrigerant flows through the expansion device (fixed orifice or TXV), evaporates, and quickly equalizes the compressor inlet and outlet pressures. That is the squirting noise heard after the engine is switched off.

Fixed orifice, compressor-cycling systems (possibly the most common type in automotive service) routinely re-start the compressor against a large head…so it doesn’t matter anyway.

Wasn’t true then either. That would only apply to unswitched circuits - things like headlights, brake lights, and horn (even these are switched on some modern cars). Radios, wipers, A/C compressors, heater/A/C blowers, turn signals, etc. have been switched on every car I know of for the last 50+ years. Maybe it was different in earlier decades - old (and obsolete) information just won’t die when it comes to automotive advice.