Should Car A/C Be Turned Off Before Engine Turned Off?

Well if the condensate drain is properly positioned it should drip directly on the ground...if it's not and is dripping on some steel undercarriage parts, then I would correct that....The evaporator coil and pan are not a problem as they are aluminum, copper and plastic or fiberglass....mildew can become a problem in a car that's left sealed up for months at a time and has moisture inside...

All the A/C evaporators that I have ever seen are aluminum, so rusting is probably not my first concern. Mold growth might be but if it's very humid and not very hot, the evaporator probably won't dry much anyways. However engine compartment heat and environmental heat will probably 'soak in' and dry it before any significant mold gets to grow.

The most important thing is to make sure the little rubber fitting or tube that drains the condensate is kept free and clear to prevent water retention in the plenum. They tend to accumulate a black sludgy kind of mud in them. I would typically "milk" them from time to time when the A/C is operating to keep them from backing up.

because condensation can cause rust if not evaporated off.

First, let's establish what you mean be turning off the A/C. Does that mean you still have the blower running with the compressor not engaged, thus blowing air across the evaporator? Or not having the blower running as well as the AC disengaged?

If it's the latter, then I don't see how that's any different than just turning off the car with the A/C still in the 'ON' mode.
Let's assume it's the latter...then:

• How long will you need to run the car with the AC off before it's safe to turn off the car?
• If you turn off the AC, then immediately turn off the car, has the temperature of the evaporator really changed enough to matter?
• If you turn off the AC and leave the car running for several minutes....has that reduced condensation enough to make a difference?

The design of the AC system is meant to handle condensation so I don't quite understand how the problem would be significant enough to really make a difference.

You know, sometimes it's better to just let it go. ESPECIALLY when it's your wife. You know the old saying, "Happy wife, happy life."

You have that right. And then there's "if momma isn't happy, no one is happy".

This reminds me of a situation a few years ago. My wife had gotten into dabbling in the stock market and was doing fairly well. I was quite okay with her doing so...as a matter of fact, she got into it at my prompting. We were having a discussion and I wanted to express my views on investing philosophy regarding individual stocks (which is start with a given amount, when you make a certain amount, shift of those earnings into a safer and possibly lower return investment).

Well in explaining myself I used a phrase something like "putting money in individual stocks is taking a gamble", which to me was a way of saying there's a risk. My wife then got really offended and accused me of saying she was gambling with our money - which isn't the message I was trying to convey at all.

No matter how much I tried to explain what I meant, I had no success. At one point, it was like I was in the third person watching us argue and telling my arguing self to shut the hell up. But the guy in the argument couldn't. At one point, I reached over and grabbed the dictionary to show her the meaning of the word gamble and which meaning I was using in my sentence. Talk about a mistake. That didn't help at all.

So now I just try to say "Yes, dear".

'Yes, dear.' almost always works but you still have to be careful with the intonation. Flowers definitely help with the vocabulary.

good idea. Maybe I'll stop by a florist on the way home.

It's been a while for me too. Good idea. Best when applied for no apparent reason. Unless she thinks you are trying to distract her. Ouch.

I agree whole heartedly.

I know if I were starting any earlier model car, I would turn off all unnecessary electrical loads, make sure it was in neutral, adjust the spark advance and crank it vigorously.

LOL & LAT (Loverly And True)!!

Unless you plan to drive the car for 5 miles, on the freeway, with the AC off and the vents wide open to evaporate all the accumulated moisture in the ductwork, don't waste your time.

It won't help.

My Cavalier would go: chukka-chukka-chukka-wheeze-putang! when I shut of the ignition, unless I kept the air conditioning on. Something always smelled like molten aluminum also.

To answer the question above I meant turning the compressor off but leaving the fan on.

Anyway, I'm getting my oil changed as I type this. I asked the mechanic what he thought about this subject. He mentioned how a lot of of older "theories" have been debunked, not just in auto mechanics, but other fields, too (nutrition was mentioned as a good example).

Bottom line: he said he always leaves his running full blast! So, with that, and the replies here, which I appreciate and value, I believe we can "put a wrap on it." Thanks!

Only time I need to do that is if my evaporator has iced up. I like driving with the aircon running and the window open sometimes. This is a an on the run operation, not a shutdown event. If it's iced up when it's time to stop I just let it melt...

Your issue with shutting off the Cavalier stems from some one not setting the engine speed controls properly. There are engine speed controls for four different functions.

1) When the choke is engaged. This is a stepped cam that starts off very fast, lowers considerably when you first step on the gas pedal, and then progressively lowers as the engine warms up.

2) Once the engine has achieved normal operating temperature, the engine speed is controlled by an electric solenoid, that is only on when the ignition switch is on.

3) When the engine is turned off, and the aforementioned solenoid is allowed to retract, there is a screw built into the carburetor that will allow sufficient air to the engine to just barely keep it running. This allows the engine to be able to run when started warm, and not stepping on the gas pedal. This is because the solenoid used for normal speed control was not strong enough to push the carburetor return spring, and open up the throttle.

4) Lastly most small engine cars had an additional engine speed control that would open the carburetor a small amount to allow for the additional load from the A/C compressor. Some were electric solenoids, and some were vacuum dashpots that were switched on and off with the compressor. The function of these was to maintain the same engine idle speed id drive weather the A/C was on or off.

In your case, someone probably set the engine speed with the carburetor screw. Then when the engine was turned off, there would be sufficient air going through the carburetor to draw sufficient gasoline into the airstream, and ignite from the heat of the cylinder itself. Leaving the A/C on would allow enough drag from the compressor to slow the engine down and stop. The compressors would commonly not de-energize entirely dew to the design of the charging system wiring. This was solved later on when the factory installed a resistor, or diode in parallel with the compressor coil.

Now don't you wish you had that Cavalier back?

It does not matter as far as the condensation is concerned.

The AC compressor shuts off when the ignition key is turned off.

The refrigerant will continue to circulate through the system until the high and low side pressure equalizes whether the compressor is stopped by the control switch or when the ignition is turned off and all power is removed to the auxiliaries.

There is a possibility on some vehicles that the added load to the starter from the compressor being on during initial starting of the vehicle might shorten the life of the starter and it may shorten the service life of the compressor clutch.

Most newer vehicles do not allow the auxiliary devices to be powered up until the engine is fully started and operating so I cannot see where it would make any significant difference either way.

Most newer vehicles do not allow the auxiliary devices to be powered up until the engine is fully started
I don't know for sure, but it seems like that was the case with the cars I worked on in college which was in the early 80's. I certainly don't consider those cars 'newer'.
I can understand cars prior to the mid 60's not keeping aux stuff off until the car's started. If I have time this weekend, I'll see if I can find when manufacturers started implementing that type of operation.

This is now an "Old Wives Tale". Previously it was standard shutdown/starting procedure for cars with a/c's. When most cars became available with OEM a/c's in the mid 60's (first hand information) it was necessary in order to prevent the a/c clutch being engaged when the engine was started. These were the dealers instructions when buying a new car. If not done it put a heavy load on the battery, starter and engine. These didn't recover until the engine had come to normal operating conditions and shortened their overall life span. Also early EOM a/c's were not nearly as efficient and driver friendly as they now are. The ac units of today have little resemblance to those of 50 years ago.

Prior to the mass availability of OEM a/c's some car owners would have after market systems installed in their cars. They were a nightmare to install, clumsy, took up a lot of space both under the hood and in the passenger area, noisy, placed an extremely heavy load on the engine (and the driver) and were very inefficient.

Now no rust problem now, very little steel used in condensation areas. On or off doesn't matter. Letting him do what he wants to is the best solution to his advice. Doesn't harm either way and saves "differences of opinions" discussions/arguments.

Good Luck, Old Salt

But aftermarket A/Cs had one advantage I have yet to see in an OEM unit: you could have both heat and A/C at the same time. My feet get cold easily, so I like some warm air on them, but warm air on my face makes me sleepy, so I would turn the A/C on and direct the cold air at my face, while directing the heat to my feet.

I'd pay extra for that capability if I could find it in a modern car!

What do OEM and EOM stand for?

OEM is Original Equipment Manufacturer. I suspect that EOM is a dyslexic mistake; should have been OEM.

Ah. Thanks.

EOM is ancient Greek, Epsilon Omega Mu. Modern English translation is "Spastic uncontrolled CR4'r hammering keyboard in late evening".

Good Luck, Old Salt

... as I suspected, being a similar rather mature CR4'r

I was going to suggest not using the AC, as I never do, until I saw your location. Having worked for a company in Tulsa and spending 12 glorious weeks in training there mid summer, I know that is not an option.

If the factory thought that having you stand on your head would lower their warrantee costs, it would tell you to do that in their operators manual.

If the factory does not suggest that you shut off the A/C first, it does not matter.

Dear Mr.chemprof,

Yes. The AC should be switched off before you switch of the Engine.

Apart from condensation, rusting etc., while starting the engine, if AC switch is not opened, the engine has to started along with AC compressor, which will drain the Battery fast, and life of Battery may come down fast.

DHAYANANDHAN.S

My air conditioner has been stuck on high since last November. It has just now started to supply warmer air since May rolled in. I'm still having intermittent periods of cold, warm, wet, and wind. I said something at the dealer where I bought it, and I was told to take it to a guy in Florida who could get the cold air to shut off.

Mine had the same problem! I set my temp. limit for riding at 35 deg F after I once couldn't start it after it sat throughout the work day. That was when I was younger. Now, I'm a fair-weather guy.

Just when I thought we were ready to move on, a dissenter (dhayanandhan) showed up!

It is unheard of to have total agreement on any discussion on CR4.

Then there are some that enjoy the Cavalier questions?

I agree wholeheartedly!!!!!!!!!! Dam, used up all my exclamation point allotment.

My question was answered long ago, but I am curious about a related question: is it a good idea to turn off the a/c (i.e., compressor only, not fan), while engine is running but the car is in park? Let's say you run into a convenience store, e.g., to buy something. I turn mine off unless the car will be idling for only 1-2 minutes.

I say that if you drive a vehicle that is less than 20 years old and is in good condition, it doesn't matter.

I live in the Arizona desert and I let my AC run whenever my truck is running, when the temperature is over 90°F.

A/C is on all the time here.

The compressor clutch is crank cycle interlocked so no problems there.

Idling with A/C running all day is normal eg traffic jams or even as the onsite environmental refuge/office/nap zone.

I lived in a small country town in a hot part of Oz for a few years and leaving the car parked at the shopping strip with A/C on and unlocked was normal. Pre-dates the smart key thing by decades so the key was in there too. Nobody stole cars in rural areas.

The A/C doesn't matter, but you are inviting anyone in the vicinity to steal your car!

GA. Very true.

Um, of course I lock my car. Those new smart keys are wonderful for a number of reasons, this being one of them. The biggest reason is they won't allow you to lock your keys in the car--made for me!

I've heard rumors that those smart keys are fairly easy to hack into. Is this one of those keys that you just have to have in your pocket, and don't insert in a keyhole? My car is less than a year and a half old, but I've managed to lock my keys in it; my wife has done it several times in her somewhat older car...

In any case, I would never lock my car with the engine running, no matter how smart my key might be.

A friend helping my wife turned the headlights of my car to the "ON" position, while we always use "Auto". She got out of the car and went into the house, assuming the headlights would shut off automatically as they always do... Of course in the morning the battery was dead. With a dead battery, the electronic key won't open the door!

THAT is the primary reason I am still driving a 2001 Chevy Silverado

- Removed off topic political content.

CR4 Admin

CR4 Rules of Conduct
CR4 FAQ

The reason I drive a 2001 Ford Supercrew is because I can afford a new one.

BTW, I thought everyone in Texas drive new truck.

We must be dyslexic here. We have just the opposite problem. Heat on all the time.

That be because you live in Cavalier hell. Santa Ana: "If I owned hell and Texas, I would live in hell, and rent out Texas." What an idiot!

I leave my A/C on when I stop my car. Otherwise what is the point of having remote start? Although it's far more important for me to warm up the car in the depth of winter than to cool it down in the heat of summer.

I have "Armstrong" air conditioning. It makes no difference if the engine is running or not. The only real disadvantage is overstressing the window crank during White castle induced emergency roll down.

If your "Armstrong" air conditioning works with the engine not running, you must live in a very windy location!

I did not say it worked, only that it didn't require that the engine be running. The exchange of interior with exterior air could be considered air conditioning. Parking next to a John Deere chip slinger could be considered air conditioning or aroma therapy depending on direction of and intensity of crosswind currents.

Holy Cow, Batman!!!

I have always thought of it as shutting down gradually as opposed to shutting down abruptly. Leaving everything on when shutting down is like an emergency where individual devices are prematurely halted before they can complete their natural cycle. I think shutting down an A/C conserves items like relays and other electrical components. The same goes for headlights and all other electricity draining devices. I feel shut down should be the opposite of start up. Take flying a jumbo jet for example. There is a set procedure for start up and shut down. It just appears to me to be logical.