I think it's melfunctioning. Is it possible to replace it by yourself or do you need to pull the refrigirant out of the A/C system?

I'm talking about the F129 switch that sits under the right front light and is connected to the A/C condenser line. It's a 99.5 A4 1.8T.

 

You mean on the side of the condenser on the driver's side right? You need to evacuate the system or your gonna vent all your refrigerant, also you'll need to replace the little green o-ring that goes with the switch.

 

It's on the passanger side and the manuel says it can be replaced without venting the system.

 

Well than you probably can replace it if the manual says you can, although i've never replaced that switch before, what makes you think its malfunctioning?

 

The A/c works fine while driving but in stop and go traffic it will switch the compressor on and off (I can feel it turn on/off) and then warm humid air will blow out. After 20-30 seconds it will turn back on and the a/c works fine.
If I park the car then I have the same problem. Even if I rev it to 2000 rpm and hold it there.

 

That probably has nothing to do with the cycling switch, especially since they never go bad.
There is a couple of possiblities for the a/c not to work well when not moving
A) the condenser isn't getting a good amount of air through it (either blocked or the electric fan is inop)
B) the a/c compressor has too much PAG oil init probably from self charging using those damn kits from autozone or whatever causing the compressor to build either too much pressure and turning off or doesn't compress the refrigerant like it should because its saturated.
Either way you need to hook up an a/c manifold gauge set and check the pressures.
The fan being inop is a given so just check that, and if that works hook up the gauges and look for stable pressures on both the high and low side. If the pressures look normal but the a/c isn't cooling efficiently there is too much oil in the system more than likely. Evacuate the entire system and put it under a vaccum for at least a half hour then re-charge it with the correct amount according to the a/c tag under the hood.

 

I'll check the A/C fan tomorrow.

Regarding option B. If there was indeed a problem with too much oil in the system then wouldn't the problem be consistent?

Again, The system works fine when driving but when I stand still or go through stop and go traffic it will switch off and on. It will still do it if the car is standing and I rev the engine.

 

Option B should be more consistant, however when your stopped the a/c system does work harder to keep the car cooler.
Im leading more towards option A as being the culprit though.
I would still recommend getting the system completely evacuated and re-charged by a certified shop because this will cure so many common problems: undercharged, overcharged, oil over and under use will all be repaired due to a complete evacuation.

 

After writing the last post I realize that it is very likely that the fan is not working. It will explain why there is no problem while driving and why it doesn't help when I rev the engine while the car is still.

Accordin to the manual the switch controls the fan. To be tested tomorrow.

 

I tested things this morning and the fan doesn't work at all.

I disconnected the fan power connector and tested the voltage without a/c (was 0) and with a/c (was 14v). The fan is probably dead.

The next question is where do I get a replacement from? ECS has one listed (ES#1693) but they say it is only a replacement for Valeo fans. Not others. How do I know if mine is Valeo without removing the whole front of the car (which I will have to do when I replace it anyway)?