Have 2007 Designo Launch.

Old school here; Benz since the '80's. Was told then running the AC, which meant the compressor was running, was fuel inefficient. When hot, you paid the toll. But, when AC not needed (winter) you used "econ" setting and compressor was off.

Is the old "leave the AC off when not needed, your burning fuel unnecessarily" still valid?

Why leave it run, why not run it?

Thanks.

 

My understanding is that when the A/C is "on" you're burning at least a little extra fuel.I've also read that it's wise to run the A/C a few times during the winter so as to prevent damage to certain seals and/or other parts.Don't know if it's ever been true..let alone if it's true today.

 

I've heard the same ...

 

I believe you are not able to use the A/C in winter, I sometimes feel hot during winter trips and unless I use maximum cooling I don't feel any A/C output at all. But it doesn't have any effect on consumption anyway.

 

I thought benz turns off ac compressor when not needed automagicaly.

 

When you turn on the winshield defogger the ac compressor will turn on to remove the moisture off the winshield.

 

gents the compressor is a variable design in that there is no longer a compressor clutch. The compressor runs at a variable rate to match the demand. Hence it only really runs at it lowest level below 35 degrees or when you hit the off button. Therefore, above 35, it will be running.

 

The varied answers are appreciated. But, sadly, by the word varied, are not definitive.

DB or others, is there a tech reading these posts who can say if there's a miles/gallon impact?

Arguably, we're not talking big moola here. I simply seek mechanical and ownership efficiency.

Thanks, to all, for caring enough to weigh in.

Em Bee Lover in Wisconsin. (Not yet shivering, but it's just a matter of time....j)

 

This is pretty much what I was about to say too. The AC compressor on this car runs always (no clutch) and then there is no need to switch it on at times to lubricate seals (modern seals should neither need periodic lubrication equally as older units did).

Once the AC compressor is automatically adjusted to the lowest possible volume, there is no difference on fuel consumption if AC is turned on or off (there is no difference to the HVAC operation).

However the ambient temperature where the AC compressor is tuned to the smallest volume depends on relative humidity and the HVAC version coding in addition to the ambient temperature.

The AC can be coded ("fixed" setting via diagnosis tools) either to always cool the evaporator to the +3 degree Celsius temperature or to be adjusted automatically for sufficient dehumidification. Of course at ambient temperatures below +3 Celsius (37.4 F) cooling is always at minimum (in practise "off").

If AC has been configured for humidity dependent cooling, the AC compressor would work at reduced level or at the minimum also at higher ambient temperature levels. At low ambient humidity the AC compressor would only start working once the ambient temperature approaches the set interior temperature (roughly, anyway at a significantly higher temperature than 35 F).

My understanding is that US W221 cars already have the AC setting at "humidity dependent" mode. I know W211 used to have this setting turned off on US cars while in Europe it was set to automatic.

My conclusion about a MY 2007 W221 (and any similar MB) is that at low ambient temperatures there is no fuel consumption difference if AC is switched off or not and there neither is any need to switch AC on periodically for lubrication purposes. Even at temperatures above 35 F the AC should mostly be off and the AC "switch" would not make any difference. Within a certain ambient temperature and humidity range it would be possible to reduce fuel consumption by switching AC off (this is when cabin cooling is not needed and windows do not collect moisture but the car wants to be conservative and already starts to reduce humidity).

Note that the AC compressor clutch may be there again in the very latest cars, I have not checked which ones have it. The AC compressor loads the engine marginally even at the minimum volume setting (2 to 3% if I remember right) and now engineers want to get rid of even this small unnecessary load, when possible.

Attached for reference the HVAC version coding setting menu from Star in case someone wants to have it verified for their car.

 

Again, coming through for all of us. Thanks, DB. Your time, effort and expertise are always appreciated and on target.