What does "Max A/C" do?

[brianl703]

Originally Posted By: ekpolk
In "traditional" systems, the MAX setting would cause the compressor to run all the time (or cycle on more than it would have)


Not on any system I have ever looked at. MAX turns on recirc and turns on the AC compressor, the cycling of which continues to be governed by the low-pressure cutoff switch which keeps the evaporator coil from freezing up.

If anything, using MAX will cause the compressor to cycle more and run less because the air passing by the evaporator is cooler. Using a lower fan speed will also cause the compressor to run less often because the slower speed of the air adds less heat to the evaporator, keeping it closer to the cut-off temperature.

Quote:
Note well that you can use the RECIRC setting in the winter to keep warm air in and various noxious odors/fumes out.


You may well end up with fogged windows if you try this.

 

[ekpolk]

Originally Posted By: Hokiefyd
In my Camry, MAX and RECIRC do essentially the same thing. MAX is at the full cold side of the temperature dial. If I move it down to MAX, the RECIRC LED illuminates, to indicate that the system is in RECIRC mode. If I move the temperature dial back "up" one notch to simply full cold, the RECIRC LED goes out, indicating that I'm getting outside air.

I can also request RECIRC mode by pressing the RECIRC button. And, if my A/C is also on and at the coldest setting, this would be the same as me turning the temperature dial down to MAX.


My Camry has no MAX setting at all. But again, it has the auto climate control system, not the manual. That said, I have owned two Camrys (previous generation) that did have the manual system.

Yes, when you select the MAX setting, the car automatically goes to the RECIRC mode. BUT you can stay in the MAX mode, AND turn RECIRC off too if you choose that option. Similarly, with the auto climate control, if you turn the car on on a hot day, the climate control will select RECIRC all by itself, but you can deselect it manually if you want.

RECIRC and MAX really are not the same thing. In an of itself, all RECIRC does is determine whether the car will draw in air from the outside, or whether it will not. Of course, the Big Brother computer ties the two together in some situations, most notably during defroster operation.

 

[ekpolk]

Originally Posted By: brianl703
Originally Posted By: ekpolk
In "traditional" systems, the MAX setting would cause the compressor to run all the time (or cycle on more than it would have)


Not on any system I have ever looked at.
Well, look again.
Originally Posted By: brianl703
MAX turns on recirc and turns on the AC compressor, the cycling of which continues to be governed by the low-pressure cutoff switch which keeps the evaporator coil from freezing up.
I think you just said what I was saying. Yes, on most cars (the ones that I've seen), selecting MAX A/C does cause an automatic turn on of RECIRC, but RECIRC is a separate function, simply where the "input" air is coming from, the atmosphere or the interior.

Originally Posted By: brianl703
If anything, using MAX will cause the compressor to cycle more and run less because the air passing by the evaporator is cooler. Using a lower fan speed will also cause the compressor to run less often because the slower speed of the air adds less heat to the evaporator, keeping it closer to the cut-off temperature.
I would surmise that the exact answer to this question depends upon how the AC system is programmed, whether it is a manual or automatic system.

Originally Posted By: ekpolk
Note well that you can use the RECIRC setting in the winter to keep warm air in and various noxious odors/fumes out.


Quote:
You may well end up with fogged windows if you try this.

See my last post. The only "mode" in which I've noticed actual computer "intervention" in this situation is with the auto systems in the Prius, Camry, and Avalon. In those cars, the system simply WILL NOT permit you to remain in WINDSHIELD DEFROST with the AC compressor OFF. The computer WILL permit you to select RECIRC with the defroster ON, so long as you do not select the AC compressor OFF. I presume that this mode is permitted to exist so that drivers can defrost AND avoid noxious fumes or odors.

This said, you are correct that if you try hard, you can set up recipe for fogged windows (again, if you try) and you have a car that won't override your choices. However, so long as you have not ordered the AC compressor OFF (which will dry the air, whether you're in FRESH or RECIRC modes) selecting RECIRC on a cold winter day (as in when you're trying to warm the car, and so, will not have MAX A/C selected) should not cause any problems.

 

[Rick in PA]

My favorite was the name on the control for max AC that the old AMC cars used, it was labeled "Desert Only".

 

[Eddie]

RtexasF. A rude comment for sure. The two vehicles I am familar with that have an AC MAX option, set the compressor to run full time or more than usual and in the REC mode. In add ition, the fan speed runs at full output also.

 

[Cutehumor]

I have a/c button and recirculate button. pontiac vibe says to push both buttons for "max a/c". my wife hates to be cold and only uses the a/c button. I always press both for max a/c.

 

[sciphi]

At least in my Buick, it looks like it kicks the compressor on for longer. It doesn't recirculate. That's another button. It gets cold pretty quickly when on MAX and recirculate.

Strangely, I use the AC more in the winter than the summer. Wet boots along with warm air and cold windows means condensation. Lots of condensation. I'll have the temperature on full hot and the AC on MAX. It clears the windows very well.

 

[daman]

Originally Posted By: sciphi
At least in my Buick, it looks like it kicks the compressor on for longer.

A Buick lesabre has a v-5 variable pump compressor,,,they DO NOT cycle instead the control valve "destrokes" the pump to prevent freeze up's and over cooling of the system.

if a v5 cycles it's low on charge and should NOT be ran.

 

[mechtech2]

Cars have had different versions of MAX AC, for sure.
Usually, it just forces the recirculation mode.

Some past models did indeed make the compressor engage full time, and a few made the fan go on high automatically.

Max AC is not a legal term that Mfrs. are bound to.

 

[brianl703]

Originally Posted By: ekpolk
I would surmise that the exact answer to this question depends upon how the AC system is programmed, whether it is a manual or automatic system.


Whether it's a manual or automatic system, the temperature of the evaporator coil is maintained above freezing. Whether this is done by cycling the compressor or by the use of a variable displacement compressor, it is still a requirement.

That being the case, the compressor will either cycle off or reduce it's displacement when using MAX AC, if the interior of the car is colder than the outside temperature.

Quote:
However, so long as you have not ordered the AC compressor OFF (which will dry the air, whether you're in FRESH or RECIRC modes) selecting RECIRC on a cold winter day (as in when you're trying to warm the car, and so, will not have MAX A/C selected) should not cause any problems.


The compressor may not run at all if the outdoor temperature is sufficiently cold (below freezing), or may not run long enough to adequately dehumidify the air. In that case, you will need to run in fresh air mode.

 

[brianl703]

Originally Posted By: daman

A Buick lesabre has a v-5 variable pump compressor,,,they DO NOT cycle instead the control valve "destrokes" the pump to prevent freeze up's and over cooling of the system.

if a v5 cycles it's low on charge and should NOT be ran.


The Saab 93 has a variable displacement compressor. It will cycle (or may not even run at all) if the outdoor temperature is low enough.

 

[brianl703]

Originally Posted By: ekpolk
Originally Posted By: brianl703
Originally Posted By: ekpolk
In "traditional" systems, the MAX setting would cause the compressor to run all the time (or cycle on more than it would have)


Not on any system I have ever looked at.
Well, look again.


By the way, how are we defining "traditional" here? If it's a fixed displacement compressor with manual climate controls, which is what *I* consider to be a "traditional" system, it operates exactly as I have described on the vast majority of vehicles out there, and on those vehicles MAX AC does NOT cause the compressor to run all of the time or cycle more than it would have. Quite the opposite, in fact.

 

[daman]

Originally Posted By: brianl703
Originally Posted By: daman

A Buick lesabre has a v-5 variable pump compressor,,,they DO NOT cycle instead the control valve "destrokes" the pump to prevent freeze up's and over cooling of the system.

if a v5 cycles it's low on charge and should NOT be ran.


The Saab 93 has a variable displacement compressor. It will cycle (or may not even run at all) if the outdoor temperature is low enough.

It's also a R12 system and a saab not GM, probably a different animal,but still don't believe it if it incorporates a CV and it's a VD pump...

i missed read your post to fast reading..yes your right if ambient is to cold it wont run,UNLESS defrost is chosen.

 

[brianl703]

Uhh, my Saab 93 was built in 2006 and R12 was banned long before that. It came with R134a.

If it's too cold it won't run even with defrost selected. Probably though that doesn't much matter since heating up cold air results in dry air. It's why we buy humidifiers to use in wintertime, as the indoor RH goes below 25%.

 

[daman]

Originally Posted By: brianl703
Uhh, my Saab 93 was built in 2006 and R12 was banned long before that. It came with R134a.

Oh lol i thought it was a 1993..lol sorry

 

[brianl703]

By the way, the air flow is through the evaporator first and then the heater core. Since the compressor will NOT engage if the evaporator is below freezing because there are sensors to ensure that, it won't engage if the air passing through the evaporator is cold enough, as it may be in winter.

 

[daman]

Originally Posted By: brianl703
By the way, the air flow is through the evaporator first and then the heater core. Since the compressor will NOT engage if the evaporator is below freezing because there are sensors to ensure that, it won't engage if the air passing through the evaporator is cold enough, as it may be in winter.

Right..but that wasn't the question in topic.

 

[ekpolk]

This one had descended into a "Tower of Babel" pit -- we're talking about a variety of different systems that obviously aren't programmed in the same way. Some cars don't have a "MAX" setting at all. Many others do, but from the comments, it's clear that they don't all do the same thing. Most of our cars have RECIRC positions too. Whether and how RECIRC is linked to the MAX setting is yet another open question. I'm pretty convinced at this point that the "disagreement" in this thread is not based upon anyone's fundamental lack of understanding, but rather that different cars are designed/programmed to work in different ways.

 

[daman]

Right ekpolk...good enough for me.

 

[brianl703]

With fixed-displacement compressors, when the AC is on the compressor is running as much as it needs to to keep the evaporator coil as cold as possible without it freezing up. Therefore, if the input air is warmer, the duty cycle of the compressor increases. If there's more airflow, the duty cycle of the compressor also increases.

The same is true of variable displacement compressors--these alter the displacement as needed to, again, keep the evaporator coil as cold as possible without it freezing up. Instead of the duty cycle increasing, these increase the displacement.

Running the AC on RECIRC (or MAX AC if that's what you need to select for RECIRC) with the fan set at the lowest speed that will keep you comfortable is the most efficient way to run the system, because that will decrease the duty cycle or displacement of the compressor, making it use less power from the engine.