Do I really have to shut off the ac in the car?
September 20, 2008 4:17 AM   Subscribe

My girlfriend swears that you have to shut off the AC and then the fan and then the car. Otherwise, you kill the battery AND hurt the AC. Is any of this true?

I'm currently living in India, driving a car that has a non-digital A/C unit. According to my girlfriend (and her father, and her mechanic (supposedly)), you have to shut off the A/C first, and then you turn off the fan BEFORE turning off the car. If you don't, the theory goes, you kill the battery and the A/C compressor (or some other A/C component).

Now, since this is an analog A/C unit (meaning you can turn the knob and press the button even when the car is off), doesn't that really mean I can turn off the car and then turn off the fan and a/c after the fact?

I've really thought the whole point was to not put extra strain on the battery when starting the car. I didn't think it had to do with anything else. But, I promised her I'd ask metafilter to get a definitive answer.

So, here's what I need to know:

1) can it hurt the car in any way to turn off the fan, a/c, and car in the "wrong" order?

2) if so, what would it hurt, and how?

3) if it doesn't hurt anything anymore, did it ever? In other words, if it doesn't exist now, is there a historical basis to this? Or is it pure superstition?

I really appreciate any insight.
posted by smersh to Travel & Transportation (31 answers total)
1. No.
2. N/A.
3. No. The blower and the A/C are fairly independent of each other. The only reason shutting off the fan also disables the air conditioner is that it would be pretty wasteful to run it if you're not circulating the cold air it's creating.

I've really thought the whole point was to not put extra strain on the battery when starting the car.

You're basically right. It's more work to start the car with the A/C on, but still, if your battery isn't about to die, that's not really a problem, either.
posted by knave at 4:55 AM on September 20, 2008

I'm not a mechanic nor have I actually taken an engine apart, but to me it's a moot point because compressors are built so poorly and never seem to last more than 5 or 10 years. So I treat them like fine china and only engage/disengage A/C while the engine is running to help extend the compressor's life. I seriously doubt there's any bad effect upstream on the battery or engine, though conceivably the compressor might make it harder to start the car (unless the compressor has a clutch of some kind... maybe someone can shed light on this).
posted by crapmatic at 5:04 AM on September 20, 2008

What runs the AC when the fan is off? The battery! And an AC is a really heavy duty part of the car. QED.
posted by ChabonJabon at 5:09 AM on September 20, 2008

Auto AC has a relay. Compressors have clutches. When your engine is running the alternator powers the accessories.
posted by fixedgear at 5:22 AM on September 20, 2008

What runs the AC when the fan is off? The battery!

I'm straining my automotive brain cells but I believe on most garden variety cars you need the serpentine belt to be turning to run the compressor, and the only thing that will do that is the engine. The battery will run the blower motor and maybe the heater though.
posted by crapmatic at 5:23 AM on September 20, 2008

My PsychoEx (the American one) used to admonish me frequently that failing to turn off the AC/heating, the windscreen wipers and the CD player before I switched off the ignition would cause all manner of problems, as would having them switched on when I restarted the engine.

The only problem I noticed was him ranting at me. I got rid of him 3 years ago, still don't switch things off and the only difference is that I don't have him ranting at me anymore. The car never complains. YMMV.
posted by goshling at 5:31 AM on September 20, 2008 [14 favorites]

Pure superstition.

I am a mechanic, FWIW.
posted by peewinkle at 5:38 AM on September 20, 2008 [2 favorites]

Car batteries suffer if heavily discharged, damaging the cells as well as failing to start the car. So, if the charge warning light comes on, or if the battery seems very sluggish on starting, get the system checked as soon as possible. via

So - if you take your car on repeated short trips, over time it *may* discharge your battery and ultimately lead to its early demise.

Having the AC on during startup (and the fan - the AC shouldn't actually activate without the fan being on), will cause more load on the battery - because it needs to crank the engine and AC compressor (which is quite a load). This will speed up the discharge.

Theoretically (under certain circumstances) your girlfriend is correct.
posted by strawberryviagra at 5:58 AM on September 20, 2008

Beating a dead horse, perhaps, but not only does it not matter what order you turn them off, it doesn't matter whether you turn off the AC at all. The AC compressor is powered directly by the engine, so when the engine isn't running the AC isn't either; unless the engine is running, the AC switch doesn't do anything at all. Neither does the fan keep running after the key is withdrawn, in any car I've ever owned.
posted by jon1270 at 5:58 AM on September 20, 2008

I've heard it wasn't about shutdown, but rather about startup. Sort of like the idea where after a power outage you're supposed to turn everything off, because when it comes back on everyone has everything turned on and it pops the power right back out again.

I believe that there used to be an issue where failing to turn off the AC before the car would leave the AC system pressurized w/o circulation, although I realize there is a high side and a low side. I think this was a more common thought in the days when car fans ran independent of the car actually being on.
posted by TomMelee at 5:59 AM on September 20, 2008

According to a guy at a Corvette forum:

The logic is that stagnant water can contribute to mold and mildew growth. Under the right circumstances (environmental conditions such as temperature and humidity) leaving the AC on until you shut the car off in your garage will leave condensation behind in the system which will mold.

By turning off the AC a mile or two prior to reaching your destination and turning the car you will allow any moisture built up to dry out and you will reduce the likely hood that you will get mold in your vent system which will produce an oder in the future. It is a documented condition and has happened in previous generation Corvettes.
posted by strawberryviagra at 6:14 AM on September 20, 2008

Ok, reading through the comments, just need a tiny bit more clarification:

1) If A/C compressors have clutches, doesn't it matter if this clutch is engaged when the car turns off? (is "engaged" the right word?) Or am I thinking about it the wrong way?

2) If this is the kind of car that still blows cold air after the engine turns off (it does, for about five seconds if you leave the A/C and fan on), does that change the answer?

Also, the fan and the air conditioning seem to be independent of each other. When you press the a/c button on, even when the fan is off, you still feel cold air coming out.

3) As long as the AC and fan are shut off when starting the car up (thereby eliminating any possible strain on the battery on start-up), there's no other possible problem. Correct?

Thanks again for the input.
posted by smersh at 6:27 AM on September 20, 2008

Occam's Razor: If this were true, many car owners would need to replace the components far more often than they do.
posted by odinsdream at 6:34 AM on September 20, 2008


I agree with you. But, since our car has had quite a few A/C issues in the past, and because my girlfriend's mechanic mentioned it, it's worth looking into.
posted by smersh at 6:38 AM on September 20, 2008

1) Air conditioners tend to use a magnetic clutch to engage and regulate the torque of the compressor. This would eliminate any sort of jamming or wear associated with gear teeth, I guess in the case of not turning it off and turning it on again at startup

In actual fact - most recent cars don't engage the AC automatically on start up - but that may not be the case with your older (?) system.

2) There will still be cold air in the system for a short time after you turn the AC off - that's normal. I guess it's possible that your AC still engages without the fan - although that doesn't make any mechanical sense.

3) Apart from *maybe* helping dry up any latent condensation to reduce mould and ultimately corrosion in the system - no.

What type and model of car is it (and year)?
posted by strawberryviagra at 6:43 AM on September 20, 2008

An Opel Astra, 2004 or 2005, I think.
posted by smersh at 6:52 AM on September 20, 2008

I just had a gander around the webesphere and although there was one complaint (in India) about an Opel Astra and the air con - it was a '97 model.

My devil's advocacy fails at this point - I think I would defer to peewinkle and odinsdream and not be concerned about it.
posted by strawberryviagra at 7:06 AM on September 20, 2008

thanks, all.

I think we can put this to bed.
posted by smersh at 7:17 AM on September 20, 2008

I would point out that when I lived in Singapore, many years ago, most taxi's were equipped with after-market, pure-electric AC's. They aren't driven by the engine torque at all, but work more like a window-mounted AC at home. These guys would definitely kill your battery if you leave them running without the car going, and IIRC, required an upgraded alternator for best effect, even with the engine running.
posted by nomisxid at 10:10 AM on September 20, 2008

So - if you take your car on repeated short trips, over time it *may* discharge your battery and ultimately lead to its early demise.

This is entirely independent of the AC - and the trips would have to be extremely short for the alternator not to recover the lost voltage. Short as in 'to the end of the road and the car running for less than 5 minutes'. The rest of the logic in that answer is flawed as a result.

To heap on conformation, your girlfriend is utterly wrong, and unfortunately, there is sufficient additional 'wrong' available on the internet to cloud all automotive questions. There is absolutely no requirement to shut off AC before turning off the car. All the moisture is external anyway, as the AC runs a dehumidifier in the system as part of the cooling, so Corvette man is hideously wrong - the AC actually helps the issue he is complaining about (and I echo the lack of trust of judgment for corvette owners).

Oh, and a 2004 car is not, by any means, an 'old car' in terms of air conditioning. You'd need to go back to the 1960's to find anything mych different. There is, incidentally, no such thing as a 'digital' or 'analogue' AC system. They are all electrical systems of similar style, the only difference is the display for the temperature that makes you think they are different. The computer control in later models just controls when to turn on the AC and how hard to blow the fan. The presence of this control is not necessarily reflected by having a digital display.
posted by Brockles at 10:55 AM on September 20, 2008

Heat exchanges never rot out, do they - ever owned a SAAB?. There are external parts to consider - and I believe this is what the Corvette thread refers to, I'd never heard of doing this prior to finding that post, FWIW.
posted by strawberryviagra at 3:55 PM on September 20, 2008

I would again look to what kind of car it is, and how high the AC is set. Some of my less fortunate friends who live in warm climes have cars that will die, actually stall out then and there, if they AC is turned up to the max. God only knows what the strain would be if their weary vehicles were started with AC on full blast.

If your girlfriend has cars that are smallish, and were purchased somewhere mild, and the moved to blistering heat, she may have a point.
posted by Lesser Shrew at 5:50 PM on September 20, 2008

Hold on here, gang. There is definitely truth to this but as the OP mentions, it's related to *starting* the car with the A/C on versus shutting it down (to which knave and TomMelee alluded). On cars with manually operated A/C systems, if the A/C is on when the car is in the process of starting, there will be more strain on the starter motor since the belt turning the A/C compressor will need to turn it with the compressor clutch engaged. Given that the alternator is not running when you engage the starter motor, the starter must draw direct power from the battery (which is not getting recharged by the alternator), and hence if there is more strain on the starter, it will need to draw more power from the battery which over time could shorten its life.

A couple things can help us sort of "anthropologize" the advice she's getting from her mechanic. If you think about older cars (esp. those with carburetors) it takes relatively longer for the engine to start combustion. If you're old enough to remember cars with carburetors, sometimes it took a few seconds or even longer -- even on a well-tuned and well-running car -- for the engine to start up. If in this scenario one had an A/C compressor being powered by its belt, which is in turn powered by the starter, which is in turn powered by the battery -- there is absolutely a material effect on the life of the battery (though it's admittedly small) given that it will have to pump out more power to turn the A/C compressor. Does that make sense?

Now if her mechanic is older and worked on older cars back in the day it would have been *totally* sound advice to tell people to start their cars with the A/C turned off, especially in a country like India where car parts are relatively more expensive (compared to wages, lets say). In a way, it's not unlike the situation you'll find in many African countries (and maybe other places) where people drive with their lights off at night in order to lengthen the life of the bulbs. They then turn them on just just before encountering another car. (From personal experience I can tell you that it's pants-shittingly scary sometimes if as the driver of the "other" car you experience bright lights pop on out of seemingly nowhere as you're speeding down the road.) So why would the advice be to turn off the A/C when you shut down the car? Simple psychology. It's when you're turning off the car, stopping things from running. One is far more likely to remember to turn off the A/C when turning off the car than one would be to turn off the A/C when turning on / starting the car.

Indeed for this very reason, on many modern cars with climate control systems there is actually a delay of 10-15 seconds from when you engage the ignition to when the compressor clutch engages. Is it going to make a gigantic difference in the life of the battery? Probably not but if you have a bum battery that's barely got enough juice to get the car going (like I imagine many cars in India might have) it could make a positive difference in how long the battery lasts.

To conclude: if you have an older car -- or maybe better stated, one that takes a bit of time to start up -- it's not a bad idea to have everything not related to starting the car turned off when you're starting it. Lights, wipers, radio, A/C, etc. It really is better for the life of the battery. You'll also be more likely to remember to do it when you're turning the car off.
posted by lazywhinerkid at 6:10 PM on September 20, 2008

Just wanted to add something I neglected to address. Given my scenario of "this will affect older cars more than new ones." Well I actually sort of misspoke, it will affect older-style batteries, not modern batteries (i.e., "flooded cell" ones). Older lead-acid car batteries require water as a component of the electrolysis process. The water gets used up during the electrolysis process, hence the need to top up the battery cells with more water periodically. The more charging the battery requires, the longer it takes to charge, and the more water it will use, and hence you need to top it up more frequently. If you are a typical car owner (50 years ago or now, doesn't really matter) you rarely consider maintenance on your car. If you're not checking the water level on an older style battery and it's going down more rapidly due to requiring longer charging time, it's more likely that you will ruin your battery quicker because it will run out of water quicker. (My explanation here is abhorrent, but I hope it makes sense.)

Listen -- this is an academic argument. For cars with modern "flooded cell" batteries, there is no effect. For cars with older style batteries (like you would be more likely to find in India) it could have an effect. It's a combination of human psychology with regards to car maintenance + older technology. To just flat out state that it's "utterly wrong" is . . . well . . . correct but kind of overstated. I'll admit that the effect is negligible to tiny on older batteries but in a "less developed" country like India, even a negligible effect could help keep a poorly-maintained battery offer more time of service. Is it misguided advice? Yeah, yeah probably. But it's understandable with some historic context, no?
posted by lazywhinerkid at 6:57 PM on September 20, 2008

The question wasn't "is this advice erroneous but understandably psychologically". It was "Do I need to do this".

The starter circuit on most cars since 1970 (or even before, in many manufacturers) cuts off all unnecessary electrical drains while cranking the engine - this is why everything stops when you turn the key to start. This is not a modern thing by any means. In other words, everything that turns on when you turn on the ignition will be turned off while you are cranking (and starting) the engine. There is no extra battery drain. Hence, there is no drain on the 'system' when the engine is cranking from the AC. It is a compressor that is not turned on until electrical power is allowed to it - ie after the engine starts and the starter is released. The AC, the blower, the lights.. nearly everything is disabled while the engine is cranking to give it the best chance of starting.

This advice is badly misguided and stems most likely from either a complete misunderstanding of car electrics (at worst) or on knowledge from 1950/60's cars (at best). Hence either way it is plain wrong for modern cars. There is no truth at all in the AC 'adding load' to a starting engine.

Manually operated AC systems? That's just made up faux-technology. They are electrically operated systems using mechanical pumps to compress and cool the air through an interchanger. There is no such thing as 'manual AC'.
posted by Brockles at 7:23 PM on September 20, 2008

Brockles: I kind of expected this sort of response from you and I just don't feel like arguing with someone who is coming across as a bit of an Internet blow hard. Sorry but you are. The OPs third question was: "If it doesn't hurt anything anymore, did it ever? In other words, if it doesn't exist now, is there a historical basis to this? Or is it pure superstition?" You say it's superstition. Fine. I've explained why it's an understandable superstition in the context of a modern car.
posted by lazywhinerkid at 7:30 PM on September 20, 2008

The question relates to a 2004 Vauxhall/Opel. In terms of AC, this is nearly as modern as things get. It's certainl nowhere near 'pre-1970's'. I only picked that date as I don't have extensive experience of electrical systems before that date - it is limited to 1950/60 Ferrari, Jaguar and Triumph, so I was picking a date before which my experience becomes less then 'typical' rather than a date of known change in electrical systems.

By Blowhard, I shall assume you mean "someone how actually understands these systems rather than someone who is arguing hypothetically about a system that may or may not exist in cars". Wild guesses, even when based on a reasonable level of knowledge such as yours, do not help the situation. Unfortunately, exactly those sort of answers seem to plague the automotive Askme's.

I've explained why it's an understandable superstition in the context of a modern car.

No, you've attempted to rationalise a myth as 'at some stage having basis' without being able to cite any evidence at all beyond your assumed logic of the systems on older cars (despite that not being relevant). I'll also put hefty money on the vast majority of pre-1970's cars on India (yes, I'm aware there are lots) also being most often without air conditioning installed. Being as (for example) the Morris Oxfords and Cambridges that are still produced there never came with AC, there is more than a fair chance the systems are more modern (if they even exist for these vehicles) than the vehicles they are fitted to.
posted by Brockles at 7:45 PM on September 20, 2008

I'm just not going to get into an argument about car batteries in India, mate. I'm also not interested in a pissing contest about who knows more about cars. All I can say is that I have spent quite a bit of time keeping shitty old cars running in less developed countries -- and have spent a lot of time around people doing the same, "laypeople" and mechanics -- and have thought a lot about from where these automotive "superstitions" come (like the headlight example) and also seemingly odd but interesting differences in cars around the world (for example, why for so many years did the French think that yellow headlights were superior to white ones?). I'm a nerd. An overanalytical, over-think-a-plate-of-beans geek. That's why AskMefi and the site overall is so great to me. The bottom line is that I read part of his question to be whether there is any historical context for the advice and well, there just is -- as misguided as it may be.

The blowhard comment was harsh and I apologise for it but it was speaking solely to your tone. I'm also sorry to have gotten you so seemingly riled up, really I am. I look forward to future threads -- automotive or otherwise -- where our opinions are more aligned. Oh, by the way -- I hear that I can buy a magnetic device that clips onto my fuel line. What do you think of those? :-)
posted by lazywhinerkid at 8:20 PM on September 20, 2008

I'm just not going to get into an argument about car batteries in India, mate.

See, the quality of the car battery is a total distraction. You attempted to justify the existence of an urban myth using the following logic:

AirCon units add load to an engine.
When a car starts, the load on the battery is substantial and increasing that load is a 'bad thing'.
Older style batteries (most likely still in extensive use in India, I agree) are more prone to wearing out and need more maintenance than modern batteries.
Therefore it makes some sort of sense that this urban myth may have some basis.

This is why urban myths exist, and this is precisely why they are perpetuated - by someone with seemingly good levels of knowledge (the information about relative battery types is 100% correct) directly or indirectly adding credence to the myth with faulty logic. Yes, I can see how the link has been made, and the advice created, but it is flat out wrong despite all three of the statements above being completely true.

The main stumbling block of the link between AC load and cranking causing battery drain being made entirely false is that the air con, through the cranking circuit only leaving the starter and ignition systems of the engine only live through starting, means that the car itself turns the AC off while it is starting - the position of the switch inside the car is entirely arbitrary at this point. Turning the AC off is to deny it electrical current - without that, it is just a spinning pulley with no/trace loading on it. It's as off as it can be while the engine is cranking. So any discussion on battery types is a complete distraction as it is irrelevant to the core perceived 'issue' that created the myth.

The bottom line is that I read part of his question to be whether there is any historical context for the advice and well, there just is -- as misguided as it may be.

This is why I clarified. There is way too much noise and misguided advice on automotive (and medical, from what I read in MeTa) AskMe's, and this is very seriously not helpful. Your answer, while based on a reasonable level of knowledge, has become noise. While it may be enjoyable for you to wax lyrical about why these myths come about, you have directly implied in your answer that it will have a tangible effect on battery life:

If in this scenario one had an A/C compressor being powered by its belt, which is in turn powered by the starter, which is in turn powered by the battery -- there is absolutely a material effect on the life of the battery (though it's admittedly small) given that it will have to pump out more power to turn the A/C compressor. Does that make sense?

No. It doesn't make any sense if you know how the electrical system is wired in every single car I have worked on or seen. If the AC is part of the car (rather than an external, fully electrical system, as mentioned above) then your argument is plain wrong as there is no power to the AC when cranking. Feel free to start with a 'pissing contest' as to who knows more about cars, but please do so with some sort of fact-based position for your argument, and please stop insisting that your position is correct without evidence. The idea of this site is to answer questions accurately, not have fun playing with urban myths.

(as a complete aside, when I was in Egypt, I experienced the pant-wetting moments of the "lights off unless a car is coming" thing. The explanation given to me was that it ruined your night vision if the stars/moon were out, and the driver insisted he could see more with them off. He didn't even turn his lights on when the cars approached, just put his indicator on on the side he wanted them to pass on. Scary as all hell.

And the french lights being yellow was, as I understand it, because some government bright spark decided that yellow lights dazzle oncoming traffic less. I haven't heard any research anywhere that either discounts that, or whether it was discarded over the increased safety element of better vision for the driver at the risk of dazzling).
posted by Brockles at 8:39 AM on September 21, 2008 [1 favorite]

Real apologies again for any mean-spiritedness, Brockles. Have a good one.
posted by lazywhinerkid at 6:11 PM on September 21, 2008

FWIW, if I get in my car and turn the radio, A/C, and wipers on, and then turn the ignition to start the car, everything stops while the engine is turning over. It would seem that all power from the battery is applied to turning the engine over, and nonessential items are disengaged until the car starts.

But I'm no mechanic.
posted by Chris4d at 8:44 PM on September 21, 2008

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