Is it a good idea to turn my Automobile's Refrigerant Compressor before Parking Car?

About a ½ mile before I get home I like turn off the air conditioner in my car, but I leave the blower running. The following is my logic.

#1. By turning off the compressor and leaving the blower on it reduces the condensation that forms in the vents. In turn that reduces the mold and mildew and the associated odors.

#2. The compressor’s electromagnetic clutch uses a lot of energy and by turning it off before I get home it gives the battery chance to recover more charge for the next engine start.

#3. Finally, when I park the car I turn off all accessories and then turn off the engine.

A friend thinks I am nuts to do this.

I like to get a mechanics viewpoint on electronics and an allergy sufferer’s (or even an allergist) viewpoint on the mold.

Thanks for Any Feedback

I don’t know about #1, but #2 is plain wrong. The electromagnetic clutch draws current only for the few moments it takes to get engaged, not all the time.

Besides, when the car’s engine is turned on, all the electrical energy is delivered by the alternator, not the battery.

Actually, what you said is plain wrong. The clutch draws current the entire time it’s engaged. It takes somewhat more to engage than to stay engaged, but it’s still a significant electrial consumer. Drawing zero current = disengaged. That’s how you control its operation.

I don’t know too much about automotive AC compressors. But isn’t the clutch engagement issued by an electric current, and then the engine takes over? Is current required the whole time the clutch in engaged?

Anywho, if my Alternator can handle a 1000 watt RMS amp, and a 300 watt RMS amp from the time I leave work and go home (1hr drive), which is a LOT of current, I think your car will start just fine when using the AC constantly. #1 sounds good though.

> The clutch draws current the entire time it’s engaged.

Although the alternator is supplying current, ultimately it is charging a battery that is powering the cars electrical system? Am I understanding this right?

If this is so, is there a greater voltage drop to the battery thereby not allowing it the greatest charge possible. In the end wouldn’t that shorten the batteries life span?


#1 - you may have something there, not sure. If you’re having mold problems you can spray lysol into the vents while the car is running (turn the vent fan up on high).

#2 - Your car’s alternator is sized to be able to deliver power even when everything is turned on. If your car’s battery isn’t getting fully charged even with the A/C on then you’ve got an electrical problem with your car. Your battery should be getting charged just fine without your intervention.

#3 - I can’t see any harm it in, but I’d like to know what benefit you think you are getting from doing this. Your car will automatically keep the accesseries turned off while it’s starting the engine. It will keep the A/C compressor clutch disengaged as well. If this is about draining the battery again, unless you’ve got one of those stereo systems where the speakers could probably push the car down the road as fast as the engine, you aren’t pulling enough current to make a difference.

I usually do #3 too though. I hate getting blasted in the ears at 8 am because I had the radio cranked too loud on the drive home the previous night, plus I don’t like turning the A/C on unless I need it the next morning.

Alternators put out alternating current, that’s why they are called alternators (the ones that put out DC are called generators, and those haven’t been common on cars in a long time). The current out of the alternator isn’t steady, and needs to have all the peaks and valleys filtered out so that you get a basically constant DC electrical source. Most cars that I’m familiar with rely on the battery to do the bulk of the filtering. This means that at some points in the alternator’s cycle, the alternator is supplying not only all of the electricity to power the car, but also enough electricity to charge the battery as well. Further along in its cycle, the battery has to take over and supply all of the energy that the car needs. These cycles happen very quickly, many many times per second (the exact rate depends on your alternator and the engine speed). On average, the alternator is going to charge up the battery much more than the battery is going to supply during these cycles, and your battery ends up fully charged.

There’s not enough current draw from the A/C system to significantly affect the car’s charging system. You aren’t dropping the voltage anywhere near enough to make a difference, and you are not shortening the battery’s life.

Possible weakness in your theory: the AC acts as a dehumidifier, hence the water dripping onto the pavement. So the air going through the AC ducts and out the vents into the cabin is dehumidified. After using this for awhile, you have cooled the plastic ducts to say 55°F. Suddenly you turn off the AC, and send normal humid air through the system. This air will tend to form some condensation on the cold sides of the ducts.

Now, it may be that the cold air without the AC on isn’t much warmer than with the AC, so there won’t be much condensation, even with the humidity. But if the ducts remain cold, there is the potential for the moist air to condense after you turn off the car.

Perhaps it would be better to leave the AC on, but turn the temperature control from cold to hot. In this way, hot, dry air is gusting through the AC ducts. No condensation will be formed at that time, and since the ducts will be warm, and most of the air in them dry, no condensation will form after turning off the car.
Downside: who wants heat coming out their vents when it’s 95° out?

Jim, as to #1, have you actually got a problem with mould in ducts etc?

I don’t know where you live but I live in the sub-tropics, in a state that extends well into the tropics. Humidity, moisture etc is a constant.

I suffer from some allergies and usually keep my ear to the ground on possible causes. I also have an active interest in cars, mechanics, maintenance etc.

And I have never heard of this being a problem. Not once. Ever. Until your post.

And as to #2, according to this page, Chryslers use about 2.4 amps to hold the electromagnetic clutch in.

Which amounts, in relative terms, as us Aussies would say, to about 3/5ths of bugger all. Pretty much nada, nuttin’, zip.

Er… won’t it come right back out?

I rather suspect ecg means the outside ie intake vents.

I don’t know what you mean by “the engine takes over,” but it certainly doesn’t take over holding the clutch engaged. Clutch engagement is initiated and maintained by electric current. It’s an electromagnet. No electricity = no magnetic effect = no engagement of the clutch.

Absolutely yes.

Actually I believe mold in the vents is a fairly common problem. It’s usually not just due to condensation though. More often there is some other cause, like a coolant leak or a clogged up drain in the ductwork. Cars that have been underwater (due to a flood or really bad driving) usually end up with mold in the ducts, along with a lot of other problems. From what I hear, once you get mold in the air system it’s very difficult to get it all out. I’ve also heard that it can be very rough on folks with allergies.

Yes I did. Sorry for the confusion.

Actually I have never experienced mold in any car that I have owned. Nor have I had any issues with premature battery failure. And not, “apparently” because I have been neurotic about things

Thanks to all the dopers for their input. From now on the AC stays on until I park the car. :stuck_out_tongue:


Condensation in the evaporator is an ongoing problem for automakers. You have an area that is dark, moist and warm (when the car is parked) perfect for the growth of mold. This problem has gotten so bad that many car makers are coating their evaporators with an anti mold coating, and have installed an “after blow” function that will turn on the A/C blower after the car has been parked for awhile to dry the evaporator. BTW I have never heard of A/C mold growing in the ducts, just the evaporator. After all the ducts have dry air being blown through them, no moist air. No moisture = no mold.
Spraying of Lysol is of limited use, since you are not applying the stuff directly to the mold, so getting enough on the problem area to do the job can be difficult.

The alternator (correct terminology is now generator, regardless of AC or DC per the SAE) supplies DC current to the battery. The battery is not a filter, and plays no part in the rectification of A/C voltage to DC. This is the job of the diode bridge in the voltage regulator. Straight DC is then supplied to the car and battery. The battery only has three jobs in a modern car.

  1. Supply current to start the car so that the generator can supply the current needs of the auto.
  2. Be able to handle shock (momentary) loads until the alternator can ramp up.
  3. Act as an electrical shock absorber so that voltage from the generator does not fluctuate too much.
    That’s it. Everything else in the car electrically is the generators job. In the automotive business we have a term for someone whose car’s generator is not producing the current necessary to run the auto. That term is pedestrian.

So to answer the OP’s questions

  1. Can’t hurt, may help. The exact answer would depend on the temp and relative humidity of the ambient air. In Vegas it would help a lot, in Huston Tx. not so much
  2. Assuming that the alternator is not approaching it’s max charge rate, all that is going to happen is that the alternator’s output will adjust downward to reflect the lessened load on the system. Turning off the A/C (radio, blower, lights) will not change the charge rate assuming that the alternator was not charging at or near it’s max rate. I did not notice what car the OP is driving, but my current car has a 180 Amp alternator. The extra 2.5 or so that will be freed up by turning off the A/C doesn’t amount to a hill of beans.
  3. Whatever. :rolleyes: I think you are going through a lot of work for little to no return, but if it makes you happy. It’s your car knock yourself out.