FEGELEIN

I notice it in all of my Benzes, loaners, etc. When I push the auto temp button, the light for the a/c comes on. I can understand if the exterior temp is higher that what I chose, but in the winter it's a pain in the *** to always have to shut it off!

Live Oak

The reason for the AC light to come on is that the AC unit is actuated with heat. It dries the air. It's that way in virtually all AC'd cars of any make. Good idea. Even in the desert.

And, I might add, it's good for the AC compressor to be run regularly, rather that shut down all winter/spring/fall.

mnje350

Also helps keep the windshield clear of interior fog because of the dried air.

Live Oak

I might also add, AC compressors used to be a high maintenance item. Not any more. I haven't had a compressor go out, or had to top off refrigerant, in over 35 years. (Now, in the 60's and 70's, it was a different story. Of course, the cars weren't worth a crap, either!)

Raymond Lee

Live Oak
Very nicely said.

El Cid

Not sure you can turn the AC off even when you press the AC button and the light goes out.
I turn my system off, then use fan button to start the fan and then press AC button to turn off the compressor and have just the ventilation open. Can still use fan button and AC light stays off. That's the theory.
BUT, I'm not sure that really works. If temp is high outside, cold air comes out of the vents or warm air if cold outside. So, is compressor always on if fan or mode is turned on?
In my previous car with Auto HVAC, you could turn it off, then hit AC button for on and then hit it again for off. Compressor would remain off, but you could use fan to bring outside air in.

K-A

Do you guys have the A/C on when you use the Heater? I always thought this would be bad, as I equate A/C with cold air only?

GermanCars

In these days where fuel economy takes up so much of automotive engineers time, spinning the A/C compressor under all most all conditions is beyond my comprehension. I have the same struggle with my 2012 Jeep G. Cherokee. You really have to work at it to shut off the A/C compressor. Trust me when I tell you that A/C repairs, including compressor replacement, is still a good share of our business. Yes, operating the compressor will dehumidify the interior air. But when the relative humidity approaches 50% or less, it is not really necessary when all you really want is a little heat on your toes.
A/C compressors are much more efficient than they once were. But they still cost a couple MPG. Hey, make sense to me, run the compressor all the time and shut the engine off at stop lights

K-A

Forgot about the MPG factor. Makes it pointless for me to even consider running the A/C on when the heater is on, especially considering I live in a mostly dry climate.

mnje350

I believe that it is almost always on in auto mode. This way it dehumidifies the air. In winter time even a small amount of moisture will fog the windows.

El Cid

I believe that in most every car manufactured today, the AC comes on whether set to heat or cold. In fact, if you turn on the Defroster, it turns on the AC to remove moisture from the air.
As for turning off AC compressor, only way is to use OFF button, but then you get no airflow from the vents.
See my comment above.

Busta Riles

It makes little sense to my why anyone would be worried about 1-2MPG difference while running the AC compressor when you're talking about a $70,000 vehicle.

Is it that you're concerned about the environmental impact or the extra 40 miles you'll get out of your tank, which will save you $5?

Am i in the minority on this one?

K-A

If it's necessary and takes away 1-2 MPG then that's fine, but if it isn't necessary, then I'll take 1-2 MPG without question.

FEGELEIN

A lot of interseting points above! Personally, I could care less about the MPG, I write off the gas anyway. To me it's the annoyance of having to always shut the button and the fact that it comes on in the first place. I don't notice it in the Porsches I own or any prior owned BMWs. Although my last BMW was a new 04'. I only notice it in Benzes.

mnje350

I haven't read where money or mileage was ever a point of this thread.

GermanCars

The wear & tear and mileage issues are just side bars to the main point of this thread-that the climate control system appears to be programed to run the A/C compressor at times when it is clearly not needed and, at least for some of us, not desirable.
I have been servicing Mercedes automatic climate control systems since the 1970's and have learned that extracting the desired result from each version (maybe 6-8 variations) requires a little trial and error. It used to be that if the compressor off or economy button was pushed the compressor was OFF! We have been a 212 owner for only a few weeks, so I haven't had much time to play with it. But at mild temperatures the climate control unit at times unnecessarily defaults to compressor operation.

Busta Riles

I highlighted it for you. If you re-read my post, i was asking about whether it's an environmental issue, or monetary...The money thing wasn't an issue until i mentioned it, but it's typically a factor when you're speaking about fuel mileage.

ImInPA

The compressor itself does not run all the time. It has a clutch and is electronically activated. Even though the light for the A/C may be on, this only means that the system that controls it is on. If the outside temperature is cold and you are running the heat, the compressor is not used often if at all. For full defrost/defog, the compressor is on because of the additional drying benefits of ac systems (mentioned above somewhere). The default setting is probably ideal for 99% of the folks that buy this car. It is good that MB gave us the option to prevent the compressor from coming on for us 1 percent-ers that wish to completely disable it at times. I have run my Bluetec with it on and with it off and can tell you that there was not a measurable difference in fuel economy, with some of my highest mileage tanks occurring with the A/C light on. Of course, the massive torque of the Bluetec spins the accessories effortlessly and does not take the hit a gas engine may.

K-A

You know what, I gotta agree with PA on the MPG thing, now that I really think about it.

In any of my modern cars, namely my E's, I've never noticed a measurable drop in MPG with the A/C on. I still have the theory of it embedded in my head, though. I actually remember some of my best MPG coming while my A/C actually happened to be on, I think.

El Cid

Consumer Reports did a test on this issue a while back. Could be wrong, but if I remember correctly, MPG is better with AC on than driving with windows down. Better if driving with windows up and AC off, but not much.
Cars are aerodynamically designed for least wind resistance with windows up.

GermanCars

We are not always driving at high speed with the windows up. What about enjoying some rare moderate temperatures (high 60's-low 70's) with the windows and/or sunroof open at moderate speeds. Why can't we have a little fresh air without the compressor operating? I'm sure there is a way, just haven't had the time to figure it out as I'm only in the car occasionally. Right now it is definitely SL weather so the sedan is getting light duty
And.............the SL has an A/C off button

K-A

Just press the "A/C" button on the E so the light turns off, and voila, it's off.

mnje350

And it will stay off and won't default to on the next time you start the car unlike the c/s tranny switch that always defaults back to c mode.

El Cid

That's the theory and that's the way it worked in my Hyundai that had auto, dual AC.
However, in my Benz, even if I turn AC off with the AC button, cold air comes out if temp is about 7-10 degrees higher than the temp setting on the dispaly panel. Does the same thing with heat if temp is 7-10 degrees lower than setting on panel.
So, does the temp setting actually cause the compressor to come on? Or the heat or AC?
BTW, the AC light does not come on when it does this, but air coming out of vents is definitely cold (or warm) air.
I enjoy riding with the sunroof open and maybe a window open when temps are nice.

mnje350

Noticed the same thing on mr '10 w212. Cycle the system off and and on once and the compressor will stay off.