olbigred

Hi,

I just had my AC compressor replaced and it works great on the highway except when I'm at a stop light it will blow hot air. I was told something isn't working when the car is stopped and the pressure get high. Any guesses what is wrong with it?

Thank you,

swamper8

I would have your A/C system pressure checked by someone other than the shop that replaced your compressor. Or buy yourself a set of AC manifold gauges from Harbor Freight. I honestly suspect that the system was not charged properly. This is always a basic first step with AC diagnostics. Can't assume the shop did it right. Once your pressures have been confirmed to be correct, I would then check the health of all your cooling fans. It's possible that one (or both) of the electric fans are not pulling enough heat off your condenser when you're sitting at a stop light. You always have the viscous fan though, so if you're truly blowing hot air, it's likely that both your electric fans have failed or the cooling fan control module has failed.
If you have VCDS, you can also go in and connect to the HVAC head and check the compressor shut down code. If there's anything funky going on with the compressor it will report something there.

olbigred

Thank you for your reply.

The two electric fans aren't moving so either both are bad or the module went out or all of it is bad.

swamper8

If you search you'll find that lots of people have issues with the electric fans and they can be tricky to diagnose. Outside of the fans and/or FCM being bad, the problem can also be as simple as a thermo-switch or fuse. Also, the electric fans don't turn on all the time. So just because they're not on doesn't mean they're malfunctioning. The high pressure switch on the condenser has a switch in it used for signaling the FCM to kick the fans on high. There's a thermo switch in the lower radiator hose which signals the FCM as well. There's a good document at www.allroadfaq.com which describes the function of the circuit but right now that website is down.
You should find yourself a local Audi shop/guy who's very familiar with the system to help you. VCDS is invaluable to have so if you haven't purchased that yet, you should get that. (or some equivalent)

olbigred

Shoot, I was thinking it could be a bunch of issues.

Thanks for the help, I'll try to take the fan out and see if I can make it kick on with a battery.

dustboy

My AC fan crapped out, like you said it was easy to test with voltage from the battery, also check if you have volts from the harness connector. It either works or it doesn't. AC worked great when it was moving but in traffic or at stop lights it blew warm air.

It's a pain to replace, the bottom screw is way down and is blocked by a hard oil cooler (tx cooler?) pipe. Other than pulling the bumper off the only way to get at it is with a disco and a torx bit. And some scraped knuckles.

olbigred

It's been to dang hot to mess with. Once it cools off outside I can mess with it more. Not sure if I need to replace both fans or just the one in front. Might order one soon and try to mess with it at night maybe.
​ Even if I need two fans, replacing one should help.

olbigred

I'm going to unplug the fan and hook it up to the battery before I buy a new fan. Does it matter which wire goes on which side of the battery? I figure it'll just decide which way the fan turns.

olbigred

I unpluged the fan and ran two wires to the battery and the fan came on.

so I'm thinking the control unit is bad. Where is the fuse just in case that's it?

olbigred

I wired the fan to the battery so it was on and I started the engine with the ac on. It seems to blow cold and then the engine stumbles and it blows hot for 30 secs and then the engine stumbles and it blows cool again.