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Running a/c with top down

Yes, another a/c question. I've been e-mailing someone about the new refrigerant and he said that his mechanic told him to only ever use the a/c when the top is up. His reason is that if you left a refrigerator door open for hours it would overheat and break down. I don't agree with that, though; here's what I replied to him:

*** Hmmm, I'm not sure I agree with this, nor his explanation. I've often driven with the top down and the a/c on and it's certainly worked, i.e. the air that came out was cold and cooled me down. As for his explanation, I think there are differences between a fridge and a car a/c:

A fridge has a thermostat which checks the temperature within the fridge. Once it's down to a certain temperature it turns off the cooling mechanism; then the fridge slowly goes warmer again, and once it's gone up by a few degrees the fridge turns on the cooling mechanism again to cool it down, and so on. This means that the cooling mechanism is never on for an extended period of time, and probably isn't designed to - i.e. if you leave the door open and the cooling mechanism works for hours, it may well overheat and possibly get damaged, because it's not designed to be on for extended periods of time.

A car a/c (at least those in the MX5) doesn't have a thermostat, and is designed to stay on as long as you have it turned on - hours, perhaps. If you're driving with the top up and the a/c on, there is no thermostat that measures the temperature in the cockpit and turns off the a/c when it's reached a certain temperature. Instead, it just keeps going and going until you turn it off. It will work just as hard with the top down as with the top up, and for as long as you leave it on. The a/c therefore must be designed to work for hours, and it won't work any harder with the top down, so I don't think I agree with his explanation.

Obviously you will get more benefit from the a/c if you put the top up since the cold air will get trapped - if you've got the top down the cold air will escape, but you will still have some benefit (at least I certainly do).

***

What do people here think?

Eric

Reply to
Eric Baber
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I probably agree with you. The important thing with car aircon is the temperature of the pipe returning to the compressor as this is what provides cooling for the compressor. A previous car had low gas presure in the system and was operating at around 65 degrees. At this temperature or higher the risk of compressor damage is increasing. After it was re pressurised the return temperature was around 48, not sure how this compares to a new car though. The quick test is to run the heater on full while running the aircon. If the return pipe is warm the compressor will be too hot and you're on borrowed time.

Reply to
J Brockley

His mechanic doesn't know what he's talking about.

If it does, then it's a defective refrigerator.

Me neither. I've had four convertibles. I used the A/C on all of them with the top down. Never had a single problem with the A/C on any of them (unless you count the rock that punched a hole in a condensor/radiator one time and let all of the freon out).

Sure it does. The compressor cycles on/off all the time. You can feel it kicking in/out. The fan runs all the time (unlike the refrigerator), but the A/C cycles on and off. I _assume_ it measure the temperature of the evaperator coils or something like that, so it would run at a higher duty cycle with the top down on a hot day.

Reply to
Grant Edwards

I can't - in my car it feels like when the a/c is on, it's on all the time; I don't feel anything turning itself on or off. This is with the top down (when you might expect it to have to be on more, or even permanently) or with the top up; I never get the feeling that the compressor has stopped running. If yours does there must be a thermostat of some kind somewhere, so maybe I'd better have that checked out. Or perhaps the a/cs on different models work differently?

Eric

Reply to
Eric Baber

On my '94, the compressor switches on and off if the cockpit gets cold enough (top-up only, of course, and unlikely on truly hot days). I suspect the 'stat is in the blower box, and its purpose is to prevent icing, not to maintain cockpit temperature. The non-adjustable switching temp is too cold for comfort, BTW, so I end up adding some heat; when the compressor cycles off, the air comes out hot. I usually just switch the damned thing manually before it gets too cold.

Reply to
Lanny Chambers

I've run the AC with the top down every so often over 6 or 7 yrs with the miata. Never had any problem whatsoever. I figure the benefit of AC thru the dash vents goes away at highway speed but when it's super hot out I've occasionally kicked my shoes off and enjoyed full AC on the low vents, this can be quite refreshing!

Chris

99BBB
Reply to
Chris D'Agnolo

Not sure if they all do this, on my other car I have two aircon buttons, with the one labelled ECO acting like this.

Reply to
J Brockley

Well, I run my AC and can only say I have no problem (except with engine overheating on RT 66, but that is a different issue.) However, if you check old issues of Miata Magazine, you will find a letter column where a national head mechanic of Mazda said it was perfectly fine to run the A/C with top down.

So, what are the qualifications of your friend's mechanic?

Leon

Reply to
Leon van Dommelen

For what it's worth, I've been doing it for 10 years in the hot Dallas summers without a problem. That fact ought to be worth a lot, come to think of it.

Reply to
Frank Berger

snip...

FWIW in the new MX-5s Mazda even gives us special "top down" settings for the AC outlets which change the high-mid-low balance of the air outlets.

Reply to
John McGaw

Well, I haven't heard anyone state the obvious: running the A/C with the top down is no different *to the air conditioner* than running it with the top up, but with the "fresh air" (i.e., non-re-circulate) setting selected.

-- Larry

Reply to
pltrgyst

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