Chilling out

October 2, 2006 7:23 AM

Is turning off a car's AC before killing the engine (a) prudent, (b) baseless, or (c) once wise but now passe?

We have two cars (a Subaru and a Saab, one auto and one manual) that have non-automated climate control: you need to manually elect whether to turn on the AC, increase or decrease heat/cooling, etc. I "learned" as a youth that it was best to turn off the AC before turning off the car, that it prolonged the life of the compressor . . . even if, for example, you expected the weather to require it the next time you traveled. But it appears that I am in the minority nowadays. Is this advisable, was it once advisable, or is it like the rule about waiting 30 minutes before swimming?

posted by Clyde Mnestra to travel & transportation (11 comments total)

The compressor doesn't know the difference between a manual shut off and just killing the engine. On old cars without rpm feedback you might have had problems with the engine stalling at startup because of the high load the compressor puts on the engine. Not a problem for practically any car with an engine computer.
posted by Mitheral at 7:30 AM on October 2, 2006

My step-father is a mechanic and always told me to turn off not just the AC/heater, but the radio, lights, and anything else for a minute or more before shutting off the engine. This was suppposed to help a whole slew of systems, from preventing battery draw just as the engine stops acting as a generator, to somehow reducing engine wear by "stepping down" instead of going from full to stop.

This was for an old 1990 Ford Escort in Marquette, Mich., however.
posted by JeremiahBritt at 7:37 AM on October 2, 2006

Put it this way: it cannot hurt, but it may help. I always do it, but my wife refuses to. I start her car and the A/C is blasting fetid air, the radio is cranked, the wipers may be going. Enough to give you a heart attack sometimes.
posted by unixrat at 7:47 AM on October 2, 2006

What I can think of is that you will get moisture in the ac system when you turn your car off and outside air makes its way in and condenses on all the cold bits. If you turn off the ac a few minutes before you turn the car off, outside air will enter the system, condense, but then dry up again as there is still air flowing over the moist parts, while heating up the cold bits. You will then lower the chance of growing mold etc. in your cooling system.
posted by defcom1 at 7:54 AM on October 2, 2006

The only downside to leaving the A/C on, (IMO) is the next time you go to start your car, the compressor will put extra load on the starter and require more current draw from your battery. If your battery is on the unlikely borderline of being able to start your car (in a semi-discharged state, or advanced age), the compressor load may be the straw that keeps your car from starting.
posted by knave at 8:21 AM on October 2, 2006

knave writes "the compressor will put extra load on the starter and require more current draw from your battery."

A valid concern but the A/C clutch (along with things like the radio, fan motors, power window motors) is not engaged during crank/start.
posted by Mitheral at 8:32 AM on October 2, 2006

There are situations where this is necessary - my car, for instance, has a lot of electronics in it that draw a great deal of power (they're necessary to operate the car). If I leave the AC and radio and whatnot on, I increase the risk that the power draw of the necessary electronics plus that of the 'amenity electronics' will be too great for the car to start. Once the engine is running, of course, there is enough power to run both sets of equipment. To give you an idea, this car has two batteries, and still has a tendency to die if it's not left plugged into a trickle charger.

In other words: if you have a system that draws a lot of power, or a really crappy battery, then this might be a problem. Otherwise, don't sweat (hah!) it.
posted by spaceman_spiff at 8:35 AM on October 2, 2006

Spaceman_spiff, what do you drive, a supercomputer?
posted by craven_morhead at 8:52 AM on October 2, 2006

Car Talk
posted by TedW at 8:56 AM on October 2, 2006 [1 favorite]

I can't see any possible advantage when shutting the car off, unless your car is designed to run the fan for a few minutes after running the A/C. That can actually be (slightly) beneficial in that it will reduce the musty smell coming from the vents when you turn it on again. Eventually that can turn into deadly mold and kill you and your family. (/sarcasm)

The only potential drawback would be when you go to start the car up again. Though, most car makers quickly figured out the "don't let the a/c engage until after the car's started" technology. If you're driving a '40 Packard, you should remember to turn off the a/c before starting up again. Other than that, no difference.
posted by Civil_Disobedient at 9:28 AM on October 2, 2006

Spaceman_spiff, what do you drive, a supercomputer?

An adapted Toyota. An electrically operated ramp for my wheelchair, hand controls (steering, gas/break), and all the major functions - gearshift, lights, wipers, even the radio and AC - are routed through a touch screen so they're within reach. Two computers control the setup, for redundancy; they don't draw as much power as a desktop, but along with the controls, they draw a lot.

The first year I had it, I jumped it about once a month. Then I wised up and bought a trickle charger.
posted by spaceman_spiff at 9:53 AM on October 2, 2006 [1 favorite]