pumpedTSI

Just picked up an 89 S4 which needs going over. Today was oil/filter change along with Tranny fluid/filter change which really smoothed out the tranny function and shifting.
Anyways, the AC works since compressor runs and the lines are frosting up. Now whether its 100% functional/needs a charge/low on gas I dont know yet since I'm getting what seems to be "mixed" air in the cabin......it seems to be a mix of AC and heat.....is there a mixing valve or something thats bleeding heat into the vents?

GlenL

I'm going to suggest a vacuum leak.

The heater valve defaults to open. With a vacuum leak there'll be hot water in the heater core and the mixing doors won't move well if at all. The electronics know the cabin is hot so the AC comes on, too.

87landshark

By the firewall on the passenger side of the engine there is a valve that opens/closes with vacuum control. It responds to the temperature chosen by the climate control. That's where my issue was. I let mine alone in the summer (closed) and let the ac crank, and tie it off in the winter (open-hot water flowing through the heater core) for heat in the winter. Take off the intake tubes and the air filter and you will find it below that. It worked for me.

Randy V

Do a search - 'heater valve'

safulop

Yes it could be the main heater valve, which would be a very easy fix. It could also be a much more subtle vacuum leak in one of the 100 lines inside the dashboard. My shop finally found mine by injecting smoke into the system and watching where it came out from the spaghetti of vac lines. I had crap AC for 5 years until this leak got fixed.