Strange noise with air on on

[dieselderv]

Noticed when I had the tracking sort of done..the car was running ..1997 tdi auto v70 it was a warm day week or so ago (only just remembered about it)
It was getting warm and the fan kept cutting in there for blasting nice hot air at the mechanic adjusting the tracking so I lent in and put the heater on full at the hottest setting to take some heat from the engine therefore stopping the fan cutting in, this stopped the fan but every 5 seconds or so there would be a loud click the engine revs would rise slightly then another click and the revs would drop off ..I noticed that the aircon was on too so turned this off and it stopped the clicking and revving..so the aircon button was on but the heating button was on its hottest setting..is this clicking normal as it doesn't know what to do ie engaging the clutch to produce aircon but then realise that I've asked it for hot air or is this the noise that something is about to fail..if someone could try it for me on their car and tell me the results I'd be really greatful unless you know the reason many thanks

[luggsey]

Sounds normal to me, the ac was coming in and out while ac was on and stopped when you turned it off?
Maybe the ac just stays on and charged when the switch is on but no cold air is demanded?

[Luxobarge]

As above, completely normal. The aircon needs to maintain a certain pressure in the HP part of it and does this by cycling the compressor on and off periodically - that's the clicking you heard.

Aircon is active all the time irrespective of heater temmperature control position, so it dries the air before blowing it into the car - thus reducing humidity which is nice whatever temperature you're demanding, and in the winter is super useful as it blows warm dry air onto the screen to demist it instead of warm damp air. For this reason my view is that the aircon should be switched on all year round, not just when it's hot weather.

[dieselderv]

Brilliant thanks for the replys phew

[cheshired5]

Originally Posted by Luxobarge

my view is that the aircon should be switched on all year round, not just when it's hot weather.

Agreed.
Also, the compressor is a machine and machines like to work regularly.
Switching a machine off for months at a time is a bad idea whatever it is.

[Ninjabob81]

By leaving the air con switched off over winter the gas will also separate, so you'll end up with liquid and gas, as we all know a liquid can not be compressed and so puts a massive strain on the compressor first time you fire it up in the summer. Suspended in the gas is also a certain amount of oil (at least in part of the system) which lubricates the compressor but also keeps all the seals supple, not using your air con allows these deals to dry out and leak.