CanadianQtip

I suspect that my car's AC compressor is on all the time which is why I am getting bad mileage and lower than expected power.

Any easy way to tell if the compressor is on? Naturally the AC light on the HVAC controls is off.

Thanks

NWS4Guy

edit - weird I replied to the Vag-Com cable post and it went here.

Not sure if you can tell other than when it cycles on and off when the A/C button light is pushed, it should make the engine lug under load.

Sunstealer

Pop the hood with the engine running and turn the thermostat way down on the HVAC controls with the AC button off. Have someone turn the AC on and listen closely for the click of the clutch on the compressor as it turns on. Turn the AC off and you should hear it click again. The engine RPMs may also change slightly.

CanadianQtip

If I understand correctly, you mean that the A/C compressor should not activate if the A/C light is off? My understanding was that the car would automatically cycle the compressor on and off as needed depending on whether it is needed to reach the desired temperature or if there is too much condensation.

Is my understanding incorrect?

LeadToRome

Well, are you hot or cold?

CanadianQtip

Hehe. I was comfortable, but let me elaborate. The ambient temp was 3 degrees Celsius and I had set the HVAC controls to 21 degrees Celsius. It was raining at the time.

nfs1000f

Today's cars often run the compressor even when the A/C is not toggled on.For example when the defrost is on. I read this last week in an article about improving gas mileage. My '01 A6 had an "Econ" button which would ensure the compressor was off. Not owning a B8 S4 I cannot comment on whether this button is available.

NWS4Guy

Ours don't have an Econ, it has 3 main buttons:

Recirculate/Fresh toggle
A/C On/Off toggle
System On/Off toggle (this is marked OFF on the button but pushing it will toggle the system off or on to the last setting you had it at)

If you put on recirculate, it will force the A/C button on, if you manually turn the A/C off, it will NOT run the compressor. If you have it on, the compressor (should) turns on only when required to drop the temp in the car, then it idles the rest of the time.

I can confirm that if you have the A/C to off on the button it doesn't kick on the compressor, you can put the DIS on the efficiency setting where it lists things that are costing extra fuel, and the A/C will be listed but show as zero on the screen, if you hit the button for A/C you can hear the load on the engine and see the DIS reflect the extra fuel consumption.

CanadianQtip

You're correct, this is good information. Thanks.

nfs1000f

My B7 S4 had an "Econo" button as well.