I had to borrow a Ford Escape from a friend the other day. I put some refrigerant in it for him, because it was out. His son was there when I returned it to the owner, and son said "That’s great. I was burning up, even with the AC switch on ‘Max A/C’ "(which is right next to ‘A/C’ on temp/thermostat knob on panel. )
Dad said “Oh, son, you should never turn on the ‘Max A/C’ on those vehicles.”
Uh, what? I had sensed that the ‘Max A/C’ switch was there, I dunno, like, for a reason??? I think that Dad is full of it, but, I also realized that I don’t know jack about car A/Cs, but, I was loathe to admit same to Dad, esp. since he isn’t a mechanic. On top of that, I wasn’t sure what he meant by ‘those vehicles’. Fords? Mini SUVs? SUVs? Mini-anythings?
So, mechanics: Is there any time ‘Max A/C’ in a vehicle *shouldn’t *be turned on?
Usually the only difference between “max A/C” and regular “A/C” (or whatever they call it) is that the max A/C recirculates air from the cabin instead of pulling in fresh air. If it’s really hot out, that’ll keep the cabin cooler with it cooling already somewhat cooled air instead of really hot fresh air, but I suppose it might get gross and stale after a while.
Never? That’s silly. As GreasyJack said, the Max AC setting usually switches the air intake so that it draws air from inside the car rather than outside, so that it’s cooling recirculated air that has already been cooled instead of drawing in hot air from the outside. After a while the air might start getting a bit stale but otherwise there’s no reason not to use the Max AC setting.
The Max AC setting is also often a bit more fuel efficient for your car. Since the AC is cooling air that is already cool, that draws less heat out of the AC coils and the compressor doesn’t cycle on as much to keep the cooling coils cool. On most cars the difference in gasoline usage will be small enough that you may have a hard time measuring it, and if it’s hot enough out (like it’s hot enough that you want to use the Max AC) then the compressor may end up running almost constantly in either Max AC or regular AC anyway, and you won’t see any fuel savings at all. It’s just that the cabin will get a bit cooler with Max AC on.
What your dad may be getting confused with is that some cars had a “desert” or “desert only” setting. The desert setting tells the car’s AC system to not bother trying to regulate the AC coil temperatures and just keep the compressor running all the time. It’s useful if you are trying to cross the Arizona desert in the middle of summer. Under non-desert conditions though, you don’t ever want to use that setting. So if the “max” setting on your AC is marked “desert” or “desert only” then your dad is right, you don’t want to ever use it unless you are actually going through a desert.
Just to add on desert setting, that continuous running of the compressor will likely cause the cooling coils to form ice over them from the humidity in the air. That is why it’s OK for the desert (low humidity - so low or no ice formation), not because the desert is hot.
From another car I had which had max ac, I recall the instructions for it was use it on the initial cooldown, then switch to a/c when the desired cabin temp is reached.
I’m not disputing the folks above, but I’ve never seen a “desert” setting. Despite having lived over a decade in the desert and actually reading the owners manuals of the several cars I’ve owned there.
What model years and what sorts of cars have/had this feature?
I don’t know which other cars have had it. I was under the impression that it used to be (70s and 80s) a fairly common option for cars in states like Texas, Arizona, and New Mexico. It may be that those cars were built with no AC and had aftermarket systems installed by the dealers as an option for those who did a lot of desert driving.
I have no idea which, if any, cars still have that option today.
Although it won’t happen as often, the icing up of the evaporator can also happen on the ‘Max A/C’ setting, if its extremely humid. The same way the condenser (the thing next to the radiator) will ‘sweat’ droplets of water when the AC is running, in high humidity so can the evaporator (the thing under the dash next to the heater core) and its low temperature can cause the condensation to freeze. Even though this won’t cause any damage to the system, if the evaporator ices up it won’t be able to cool the inside of the car (you just have to leave it off for a few minutes and the ice will melt). The trick is to never use the Max A/C setting first. Run the AC on high (normal) until you lower the humidity of the inside of the car, then if it’s really hot & humid you can use Max A/C without ice forming.
It’s the same as not using the recirculate setting with the heat in the winter. In that case it will cause the windows to constantly fog up, again, because of high humidity in the cabin building up because of the air being recirculated. You’ll notice that cars run the AC compressor when you set them to ‘Defrost’ even in cold weather. This is because defogging not only requires warm air, but dry air as well.
We had a '73 AMC Hornet which also had the ‘Desert Only’ setting. I’ve only seen it on old AMC cars.
Right. IIRC, the manual for my Ford Taurus says not to leave the selector on Max AC (or Off) when the car is parked for any length of time, because the car wouldn’t be able to air out. Could Dad have read/heard something like this and misunderstood or misremembered what it advised?
It isn’t the case now, but decades before A/C became standard equipment (or nearly so) the defroster sans air-conditioning cleared windshields (and still does) without a problem by the moving air over the inside of the windshield, even in pouring rain with heat and humidity that wilts concrete.
Winter depends on the definition. The A/C in any car I’ve owned won’t work in winter. My present car’s A/C won’t come on under 39° or 40°F. The A/C does come on automatically above that, though, when the defroster is turned on.
But when it’s –30° or –40°F and the engine is stone cold, even defroster air at those temperatures clears the glass.
For best results, don’t use Max AC when you first get into a really hot car in the summer. As soon as the interior temperature reaches the exterior temperature, then switch to Max AC to save fuel and get the maximum cooling benefit. Just mind the humidity, which can build up.
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We had a '73 AMC Hornet which also had the ‘Desert Only’ setting. I’ve only seen it on old AMC cars.
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I just dug out the service manual for my late father’s long gone 74 AMC Matador and it did indeed have a Desert Only setting. I used to work on his car and I didn’t remember that it had such a setting.
The service manual warns (as others have noted) not to use it in areas of high humidity due to the evap coil freezing up.
Something that hasn’t been mentioned is what it is that continues to create humidity inside a closed-windowed car. The answer is: The human body! Your sweating and breathing creates humid air, even after the car is chilled. Once you cool down (and dehumidify) the interior of a car, letting it sit parked in the sun with the AC on MAX isn’t really going to be a problem, unless it’s packed full of active kids. With the air recirculating they could cause the humidity to rise again, so it’s better to have the cabin air constantly being replaced.
Same is (even more) true for defogging in winter. I car full of people will fog up much faster on recirculate (even while driving!)
The difference is that using the system without the A/C compressor the side windows and the back window are more fogged than a teenagers at the end of a double feature at the drive in. Turn on the A/C system and all the windows will be clear in a minute or so. A very worth while safety feature.
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Winter depends on the definition. The A/C in any car I’ve owned won’t work in winter. My present car’s A/C won’t come on under 39° or 40°F. The A/C does come on automatically above that, though, when the defroster is turned on.
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This is due to the pressures in the system when the system is cold. Your car’s A/C system will only cool the evaporator to about 26F or so, and does not turn it on until the pressure in the system equates to about 45F. So start the car at 30F and you are right that the A/C won’t come on. That is until you have run the heater and the engine warms up, then the A/C system will be above 45F and the system will come on and de-humidify the air.
The windshield yes, but not the side or rear window.
At -30 humidity is not anywhere near the issue it is at say 50F-60F in the rain. Back in the day I had cars that struggled to keep the windshield clear during rain storms. As far as the other windows, forget about it. Integrated A/C defrost systems are a huge boom to automotive safety.
I don’t use Max willingly or for long periods of time. In the 1990"s we got a meter to read CO2 levels in buildings to check for sick building syndrome at the stores we maintained. While traveling on the freeway one time with the AC running on Max I turned the meter on. I don’t remember what the reading was but it was in the red level. Almost pegged the meter. Switched to regular AC and the needle began to drop. Over time it dropped back in to the white zone and a normal reading. Switched it back to Max AC and the meter began to climb to the red zone in a few minutes. From that day forward I do not use Max AC.
In most cars I’ve driven, including the Ford Escape, leaving your A/C on “Max” or “recirculate” all the time will cause a mildewy smell to build up in your car. My recommendation is to run it on “Normal” for the last few minutes of your drive to let whatever it is in there dry out.
