Does your AC light turn on when switching to defrost

[BarrieCommuter]

In my Jetta Highline when I switch the air control from foot/defrost to full defrost, my AC light turns on automatically. Is this a default behaviour or should I look into this further? AC goes on even in frigid outdoor temps.

 

[NB_TDi]

Default.

 

[davebugs]

I believe the theory in all cars is that the AC Compressor helps remove moisture so it doesn't freeze on the inside of your windshield when on defrost.

 

[MayorDJQ]

My '09 did not do this, but the '10 does. For years, in temps below 45*F, VW A/C compressors did not engage. I don't know why they made the change in '10.

 

[NB_TDi]

I believe the theory in all cars is that the AC Compressor helps remove moisture so it doesn't freeze on the inside of your windshield when on defrost.

Nope. There is a sensor that disables AC below 5C. In the Mk4 that is.

 

[BarrieCommuter]

so on my MK6 it's OK to be running the AC on defrost mode at -30?

 

[NB_TDi]

I think there is a sensor in the mk5 and mk6's as well.

 

[itsfatboy]

Every car I've had since 99 has done this.
Its exactly as stated, it takes the condensation out of the car.

 

[mo_focus]

yes takes condensation but also lubricates the AC parts to they dont loose refrigirant. Remember the old times when you dad used to tell you to run the AC once a month to lubricate the parts, well you dont have to!

 

[itsfatboy]

My cheap dad told me not to run the AC, it costs more to run.

 

[Pelican18TQA4]

Since the '09 model year (maybe even late '08), all U.S. VWs have automatically turned on the A/C compressor when selecting the front defrost position on the HVAC knobs. Unless something's changed, the A/C compressor won't actually turn on if the ambient temp is below 40F. <-- This has been the case for VWs for many years now.

 

[MayorDJQ]

I'm getting curious about this now. I may plug in with VagCom tonight after work to see if the comp actually kicks in. It's butt cold here now and will still be butt cold this afternoon.

 

[NB_TDi]

Since the '09 model year (maybe even late '08), all U.S. VWs have automatically turned on the A/C compressor when selecting the front defrost position on the HVAC knobs. Unless something's changed, the A/C compressor won't actually turn on if the ambient temp is below 40F. <-- This has been the case for VWs for many years now.

A close friend of mine owns a 06 TDI and his does this as well.

 

[tcp_ip_dude]

Normal. A/C comes on to dehumidify the air (you can override it), many cars do this. Had a Benz, BMW and Mazda that did the same thing; as does my 2010 TDI.

 

[MonsterTDI09]

Does the a/c compreser come on?

 

[scrappytdi]

Does the a/c compreser come on?

yes. the compresser has to work.
the a/c works to de-humidify the air by lowering the temp below the ambient dew point. once below that point the moisture falls out of suspension. it is then run thru the heater core to warm it back up.

 

[tcp_ip_dude]

yes. the compresser has to work.
the a/c works to de-humidify the air by lowering the temp below the ambient dew point. once below that point the moisture falls out of suspension. it is then run thru the heater core to warm it back up.

What he said.

 

[El Dobro]

My '09 has the Climatic system and the A/C compressor valve does not work in any mode position unless the button is pressed and the light comes on.

 

[tcp_ip_dude]

My '09 has the Climatic system and the A/C compressor valve does not work in any mode position unless the button is pressed and the light comes on.

On my '10 Sedan, I can confirm the A/C compressor cycles (as observed with the hood up) when the 'defrost' setting is selected. I have not confirmed on the VW, but on my E420 Benz the compressor wouldn't engage below a certain ambient temp (to prevent evaporator icing).

 

[El Dobro]

The A/C compressor not coming on below certain temperatures has been around for years. I had a '69 427 Vette and the compressor wouldn't turn on below around 38 degrees.

I don't know about the '10 compressors, but the '05.5 thru '09 compressors don't have clutches, so they're always turning. The refrigerant cycling is controlled by a valve mounted on the back of the compressor.

 

[tcp_ip_dude]

The A/C compressor not coming on below certain temperatures has been around for years. I had a '69 427 Vette and the compressor wouldn't turn on below around 38 degrees.

I don't know about the '10 compressors, but the '05.5 thru '09 compressors don't have clutches, so they're always turning. The refrigerant cycling is controlled by a valve mounted on the back of the compressor.

That makes more sense. What I noticed wasn't abrupt clutch on/off, more a variance in the load (at idle); but there was a noticeable/discernible pattern and cold refrigerant piping.

 

[MyAvocation]

A close friend of mine owns a 06 TDI and his does this as well.

Not so on the 06 with Climatic.