[plaztik]

Hello all,

One of my best friends swears by the rule that one should turn off the A/C before turning off the car. He's not sure exactly why, but was told that it is bad for the A/C (and car?) to leave it on.

I'm not so sure I really buy this though. I'm almost 30, have never even seen a car without A/C, and he is the only person I have ever heard this from.

Has anyone out there ever heard (or live by) this rule/myth?

If so, can you please explain?

Thanks!

- plaztik

[GordonD]

I don't think a/c on a modern car even starts up immediately when the car is started. Seems to me like there are a few seconds before the fan etc comes on. Auto a/c definately does not start immediately.

Point? can't see the harm in leaving it on

[HarryHBMCC]

I've heard it is best to run the fans with the AC off for a while so as to dry out the vents, otherwise you can get mold in the system which manifests itself as a musty smell when you turn the system on and/or off. Some cars automatically run the fans for a while after you shut off the car but the MINI doesn't do this. I've certainly had the musty smell in my cars with AC.

Harry

[plaztik]

Hmmm...

If it helps, I have Auto-A/C, but his rule applies to all cars.

The part about drying the vents is quite perplexing to me though. A/C air is always dryer than fresh air by definition, isn't it?

Thanks for the input so far guys!

[LuckyRVA]

Quote:

Originally posted by plaztik


The part about drying the vents is quite perplexing to me though. A/C air is always dryer than fresh air by definition, isn't it?

I've also heard to run the regular air after running your a/c for whiel to prevent mold from rowing inside the vents. This could be because the cold air creates condensation inside the vents and then with then cuase mold to grow and leaves a nasty smell when you do turn the air on again. But, this is just speculation.




Jason

[plaztik]

Interesting yet again.

I suppose condensation from A/C air could provide more moisture than fresh air. But, when you live in Houston and deal with 80%+ humidity quite often, I'm not so sure which is the lesser of two evils.

- plaztik

[MrDurst]

Yes, I agree with this rule. All lights, A/C, fans and other high drain electricals swtch off when you switch off the car. It's to do with strain on the electricals when the car is started(even though modern cars cut off most things when being started nowadaysanyway). I just treat it as good practice, and you don't want the A/C running without the engine anyway coz it drains the battery real quick! Also imagine if you leave it on, you stick the key in the ignition and turn to II and something distracts you, if you have left things on in the first place then by the time you come back it may not have enough power to start.

[GordonD]

Quote:

Originally posted by MrDurst
you don't want the A/C running without the engine anyway coz it drains the battery real quick!

I'll try this on my way home tonight but I don't think a/c goes on unless the engine is driving the compressor - a fan might run but unless you are going in for a couple of beers I can't see that running down your battery - and if you have had a few beers then maybe that's a good thing

[AidenL]

I was told that the musty smell is due to condensate settling in the evaporator, and that you should turn A/C off a few miles before parking in order to dislodge the condensate.....

[dinlo]

I have been taught to turn off everything before I turn off the car, and I have been doing this since 16, I don't know will it helps the car, but for sure it will not harm the car, so I don't find any reason why not to do that.

[plaztik]

I can see why people continue to do it, if it is already habit.

But, I think the question is...

Is it really necessary? (to the point that I should force the habit of turning off stuff before turning off the car)

I have the feeling like this is one of those "old ways" that people hang on to...like changing your oil every 3000-5000 miles. I think that the MINI owners manual is proof enough that this practice is outdated (for me anyway). Is the A/C thing the same?

- plaztik

[HarryHBMCC]

The musty smell is not something from memory, it happens in my cars today if I don't turn the AC off and not off 1 second before I turn the car off, but turned off at least 30 seconds or so before I pull into my driveway.

Harry

[tavis1]

Hi, I've worked as a fleet mechanic and now I install and repair heating and air condtionig equipment. first the skinny on the a/c in your car. It used to be the older vehicles if you had the a/c on when starting the car it would put a tremindus load on the starter. On top of that the electrical load from the a/c is alot(fans,the clutch on the compressor). Then with the introduction of the computer to the car they added a new thing called a fuse link. there a little fuse between the battery and the car. If the electrical load got to high it would blow. Now I have had these things blow with age and it sucks because there is no warning. The new cars some have a delay and some don't. I allways turn my off. Why risk it? Now for the question of smell from a/c. There would be no moisture in the vents. One of the jobs of a/c is to remove the moisture from the air. The minut the air leaves the evap coil it is dry. The moisture from the air collects on the evap coil trickels down to the condensation pan and out the car. It is like how you have a cold Pepsi on the table on a hot day and condensation form on the can. It is working the same as an evap coil absorbing heat into the Pepsi. Heat allways travels from a warmer substance to a cooler substance. Anyway, the smell comes from over time the build up of microbes in the condensation pan and evap coil. to combat this problem you should run the fans with out runnig the compressor to dry out the coil. Depending on how the box with the heatig coil and a/c coil are built runnig the heat a couple of minutes might not be benaficial. KEEP THE AIR FILLTER CLEAN! whether in your car or at home it is one of the most and cheapest things you can do. you would be suprised. One last tip. A couple of times for about 3-5 minuts during the winter turn on the a/c. The system has oil in it, this will keep the compressor seal from drying out and loosing you R-134a(freon)
tavis1

[sjbartnik]

There is no reason in a modern car to turn the a/c off before turning the engine off. Meaning fine if you want to, but you won't hurt anything by leaving it on.

If the a/c is turned on when you start the car, it does not start immediately. On modern cars there is a delay from when the car is started to when the a/c compressor clutch engages and begins to work. Additionally, there is an electrical cut-out when you turn the key to start so that electrical accessories are shut off during starter operation so as to allow the full capacity of the battery to be used for starting the engine.

The a/c cannot possibly operate with the engine off. The a/c compressor is driven via a belt by the engine. If the engine is off, the a/c is off.

tavis1's comments about condensation and such are right on.

The bottom line is that the old reasons for turning off the a/c before shutting off the engine are no longer valid. But if it makes you feel good, go ahead

[plaztik]

Actually, the original point was whether turning the A/C off before turning off the car is an antiquated rule or not. It seems to me that it is.

As I do not care/worry about the A/C smell (since I haven't found this to reach an annoying level yet), my biggest concern was for preserving the A/C system and starter or other hardware.

I didn't mean to imply anything about anyone's age when I mentioned the "old ways". After all, I'm almost 30, and I'm half-dead. Not exactly a young one.

- plaztik

[sjbartnik]

Just an FYI techie post

The basics of the a/c system in your car are pretty simple. It works a lot like a home refrigerator, with the inside of your car being analogous to the inside of the fridge.

You start with the compressor. In a fridge the comressor is an electric motor, but in a car the compressor is mechanical and driven via a belt from the engine. The compressor is provided with an electrical clutch on its pulley so that the compressor can be activated when needed and disengaged from the engine when not needed, thus providing increased fuel efficiency and less drag on the engine (turning the compressor requires engine power. Thankfully, modern compressors are a lot less power-hungry than old piston-type compressors).

The compressor has to compress something and we need a medium to remove heat from the air, so that's where the refrigerant comes in. It used to be Freon (R-12) but now the most common refrigerant is R-134a which is not as efficient as Freon was, but as long as the entire system is designed with R-134a in mind, that's not a problem. For a refrigerant, you want something that "boils" at a relatively low temperature, and something that has good heat transfer properties. You also want something that's non-toxic or non-flammable in case of leaks. Propane makes a great refrigerant but not so great if you have a refrigerant leak. So our a/c system lines and hoses are filled with R-134a refrigerant and a light oil which lubricates the compressor.

The refrigerant coming into the compressor and leaving the compressor is in a gas form. If liquid refrigerant ever reaches the compressor, it's bye-bye compressor since liquids are not compressible.

So coming into the compressor we have cool refrigerant gas. The compressor compresses this gas and as we all know from basic physics when you compress a gas, its temperature rises. So leaving the compressor we now have hot refrigerant gas.

We want to cool this gas down and make it a liquid, and you'll see why farther down. To cool the gas down, we have a piece of equipment called the condenser. The condenser in your car is mounted just in front of the radiator where it can get a good supply of fresh relatively cool air passing through it. It looks a lot like the radiator because that's pretty much what it is. As the refrigerant passes through the condenser, it is cooled down and condenses into a liquid.
The condenser will also have some sort of thermostatic switch on or near it which will activate the electric cooling fan behind the radiator if it reaches a certain temperature. This often happens if the a/c is on and the car is sitting still. There would be insufficient airflow through the condenser to cool the refrigerant, so the fan is turned on to pull cool air through the condenser.

After the condenser the liquid refrigerant is going to run through a device called a receiver/dryer. This device has a dessicant inside and its purpose is largely to absorb any water or moisture that may be in the system. The receiver/dryer usually has a sight glass on it. If you ever look through the sight glass and see bubbles as the refrigerant is flowing through, you are getting air into the system somewhere, probably through a hose leak.

Next in line is the expansion valve. The job of the expansion valve is to reduce the pressure of the liquid refrigerant and to allow the refrigerant to flow into the evaporator at a certain rate. The expansion valve is usually mounted inside the car near the evaporator. There are different types of expansion valves, but a common design forces the refrigerant to pass through a small hole, thus restricting the flow rate and resulting in a pressure reduction when it comes out the other side.

Next stop is the evaporator. The evaporator looks a lot like the condensor (because it too is a heat exchanger) but it works in reverse. The evaporator is mounted inside the car, inside the bowels of the climate control system. As the liquid refrigerant is released into the evaporator, air is passed over the evaporator by the climate control system's blower fan. This warm air passing over the evaporator causes the refrigerant to boil, thus removing heat from the air. The air then flows out the dashboard vents inside the car and this is what makes the inside of the car cold. As a side effect, the evaporator becomes quite cold when the a/c is on, and any moisture in the air passing over it condenses on the evaporator. So a/c also has the effect of dehumidifying the air (which is why a/c dramatically aids in defogging windows). This condensate has to go somewhere or else it would back up inside the car and spill into the front footwells. So there is a drain pan below the evaporator, and a hose which carries the condensate out of the car and onto the ground.

During use of the a/c it is possible for the evaporator to get so cold that the condensate on it freezes, at which point the evaporator's efficiency drops dramatically. So the evaporator has a temperature probe on it so that the car's climate control system can deactivate the compressor (by disengaging the electric clutch) to prevent evaporator freeze-up. This can be done by a simple mechanical thermostatic switch for a manual system or by a temperature sensor for a computer controlled system. Most modern systems also will disengage the a/c clutch if the outside temperature drops much below 40 deg F or so again to prevent evaporator freeze-up.

You no doubt have noticed that you can turn the a/c on and still get heated air out of the climate control. This is because the heater core is situated in the system after the evaporator. So if you desire heated air and you engage the air conditioning, the air is first cooled and dehumidified by passing over the evaporator, then heated by passing over the heater core.

As the refrigerant leaves the evaporator it is back to being a relatively cool gas at low pressure. It then flows back to the compressor for the whole cycle to begin again.

[sjbartnik]

Quote:

Originally posted by obehave
So the old reason for turning it off IS still valid. Not just a "feel good" thing. You said so your self.
"tavis1's comments about condensation and such are right on."

I would agree that it's beneficial to run the blower without the a/c on every once in a while, but it's not necessary to do so every time you use the a/c. Just normal use of the climate control system in the spring, fall and winter (i.e. when I'm not using the a/c) more than covers that in my experience. I've never had a musty smell develop in any car I've owned with a/c.

So I would say that it's not necessary to turn off the a/c before arriving home and then run the fan for a few minutes every time you use the a/c, or indeed any time you use the a/c. But then anytime I'm in the car the climate control system is turned on and the fan is running, even if the a/c is not. Since I've never had a car develop a musty smell, I have concluded that just normal use of the ventilation system will cover it.

[HarryHBMCC]

Thanks for the technical updates but all I know is if I always turn off the AC with the rest of the car and the fans, it makes a stink. Turning it off before reduces the stink. When the stink gets too bad you replace the pollen filter in the system and it does away (for a while). If you want to replace that filter often, don't turn the AC off before you turn the fans off. To replace it less frequently, run the fans for a while to try to dry things out. That's what works on my cars, including the MINI, so I will continue to do so. Feel free to ignore my advice or take it. YMMV.

Harry

[Dan Harmon]

Quote:

Originally posted by plaztik
Hello all,

One of my best friends swears by the rule that one should turn off the A/C before turning off the car. He's not sure exactly why, but was told that it is bad for the A/C (and car?) to leave it on.

I'm not so sure I really buy this though. I'm almost 30, have never even seen a car without A/C, and he is the only person I have ever heard this from.

Has anyone out there ever heard (or live by) this rule/myth?

If so, can you please explain?

Thanks!

- plaztik

:D

[Dan Harmon]

Quote:

Originally posted by plaztik
Hello all,

One of my best friends swears by the rule that one should turn off the A/C before turning off the car. He's not sure exactly why, but was told that it is bad for the A/C (and car?) to leave it on.

I'm not so sure I really buy this though. I'm almost 30, have never even seen a car without A/C, and he is the only person I have ever heard this from.

Has anyone out there ever heard (or live by) this rule/myth?

If so, can you please explain?

Thanks! Hello,Once being a mechanic, I was told that its a good thing to turn off the air, Because it takes alot of POWER to run the thing. it save's wear and tear onthe product.
- plaztik