Fans turn off when AC is running.

[faul713]

I have a fan problem that has me stumped, 2005 New Beetle, BEW and 6sp DSG.

Key on, Engine off, AC/interior fan on...the fans work fine. Turn the engine on and they shut off. Then the compressor will cycle with the high pressure shut off sensor. If the ambient temps are around 80 degrees F or so all works fine. Once ambient temps start climbing over 85 or so the fans will no longer work with the AC. I can also turn the fans on with VAGCOM.

All engine and air temp sensors seem to be working fine and agree with current weather conditions, which is hot. I am in Phoenix, AZ.

The fans will also come on and work properly when the engine temps warm up.

The car has had the problem for quite a while. I did replace the fan control unit a while back but it did not fix the problem. I cut the old one open and it looked ok, that is nothing was smoked and the relays looked good.

 

[jokila]

Check this post and go through the comprehensive list and see where the problem may be.

Maybe it's the temp gauge for the AC not the main one for the gauge on your cluster.

 

[BobnOH]

Just did some quick research on this as I have a similar malfunction (fans cycle on and off while AC is on).
So here is word from the man-

Both fans should run at the same time, at the same speed. Always.

They are usually driven by the pressure reading from the High pressure sensor on your AC system (rarely needed for engine temperature reduction).

If the AC pressure is low enough, they shut off. If pressure rises, they turn on. Normally when you need to run the AC, the pressure will reach fan turn on in a few seconds - and never (or very rarely) reach fan turn off pressure. 74 F and rain may remove enough heat to drop pressure low enough to turn off the fan, but not 74 and dry.

It sounds to me as if your high pressure sensor may be reporting lower than actual pressure. This is easily checked on the 2009 with VCDS and a pressure gauge.

And if only one fan is running at a time, that is a problem.

 

[faul713]

Turned out to be the Fan-Control-Relay board, a fried relay. I've now replaced 3 of them and keep a repaired spare in the garage for troubleshooting.