GDJT Allroad^2

A while ago I posed about some HVAC issues. Here's the deal.

It's summer, 80+ degrees outside. It gets hot inside my '97 A8Q. I turn the AC on by selecting "Auto" on the controls, and with the middle vent's roller slid to the middle or the blue dot, and I get a fantastic stream of ICE COLD AIR out the center console vents (all three of them.) The vents on the either side of the dash blow HOT AIR like a politician.

I broke out VAG-COM, pulled codes, and there were no HVAC codes. I tried different manual settings on the AC, like forcing the AC on and off and recirculation on and off. No fix.

I have no idea what's going on here. I'm absolutely befuddled. It's like some shunt is preventing cold air from reaching the side vents. Last summer all vents blew cold air like they're supposed to. The only thing I can think of is when I took apart the passenger side floor to drain the run-off for the condensation I disconnected or hit something, but even after I cleared it out for the remainder of last summer it still blew cold air out of all the vents. This is really odd.

Any ideas on what I should look at?

ttuling

Check fuse 7 in the middle row of fuses. (Sorry I don't know if that's 7th from the left or from the right.) Check electrical connections of the coolant valves near where the heater hoses go through the firewall. These valves when energized stop coolant from going through the left and right heater cores, it seems from reading the service manual. So no power = constant heat.

The side vents get air from the heater cores only. The center vent has dampers allowing it to get unheated air.

This fuse serves only the valves and the heater's electric coolant pump, as far as I could see on the wiring diagram.

If the valves have +12V, but the HVAC controller doesn't shut them off, then read the HVAC controller's measuring blocks as per the Bentley manual to see if it is trying to (but has bad wiring to the valves) and if not, whether it has reasonable inside and outside temperature readings.

Tom

KMU

Shows right and left mixing chamber supplying right or left side vents, so it seems one would have to have 2 bad sensors or control unit. You would think that if a door to heater core was stuck you would get no cold from center vent either.

KMU

I got all hot out the sides and cold in center. I remember thinking, "here comes on giant electronic repair headache". After about ten or fifteen seconds , it switched over and became cold slowly. Locate that valve ttuling is talking about and gently tap on it while commanding max A/C. Who knows, you may get lucky and its just stuck somehow.

GDJT Allroad^2

This has been going on for a few weeks... I'll check the fuses and such later today

GDJT Allroad^2

The fuse was missing. I must have miscounted and pulled the fuse as a spare fuse when I went to replace the fuse for the cigarette lighter. D'oh!

PaulW

Going to do the AC mod he designed for me very soon so it works like stock again.....

pw

ttuling

it took me a while -- since brun_fid's original post -- to realize the coolant valves could be open when unpowered, though one of the HVAC measuring block descriptions in the Bentley mentions that the controller grounds its wire to the valve for cooling. Apart from that, like KMU said, only unlikely double failures would explain the behavior, especially when the system was quite willing to allow cold air out the center vents (which, to my irritation, a 2001 HVAC controller is programmed not do until the cabin is very thoroughly warm).

For what it's worth, only the electric coolant valves control side vent and footwell vent air temperature. There are no air bending dampers for temperature control apart from the center vent which can, subject to controller programming, get air as cold as outside and as hot as a mix of the left and right side/footwell air.

And my wiring for your 2000 controller w/o CAN-Bus communication from the instrument cluster is less than stock in that it doesn't shut off the compressor when it's cold out or if you push the ECON button.

Tom

JimR

having a heating valve fail open makes a lot of sense!