HVAC controller

[928JimL]

I've been going through the HVAC system part by part to fix my cooling problem (running heater all the time, only cool on lowest temp setting) and eventually this led me to removing the controller. i found one of the black wire on the bottom side not connected to anything...it looks like its supposed to feed to the inside of the temp control knob (which is loose)...can anyone help me with this? whats its function, just for a light, or might it be my temperature problem?

[928JimL]

[AirtekHVAC]

That's just a light. Have you checked the heater valve under the hood? It is at the rear of your intake, near the firewall. You have to remove the air filter housing to access it. They are a known high failure part, and cause heat to enter the cabin constantly, causing the AC to not work. Not too hard to replace, most people just strap it shut.

[928JimL]

Heater valve ok, vacuum line and actuators and solenoids all been painstakenly checked one by one. my problem is that there is infinity resistance at the setting motor plug between terminal position 4 and 12, as i understand means either the outside or inside sensor faulty, or the controller. however, my outside and inside sensor have a few thousand ohms resistance, so unfortunately i guess that means my controller is bad.
i cant seem to find any actual tests to verify a faulty controller...

[AirtekHVAC]

Hmmm, have not had to work on one beyond pulling out and putting back in....someone else will chime in shortly....they will want to know what year and model 928 you have....

[928JimL]

its a 1982 928. I just spent the spring converting the A/C to R-134. A/C turns on with defrost and auto/bilevel settings, but its a mixture of hot and cold air. only vents that will open is the defrost vent.
i wont feel bad about buying a new/rebuilt controller, if i can confirm that is the problem.
all of my actuators work if i pull vacuum on them manually.
every solenoid removed and tested too.

[Sapientoni]

Part of the pain with those actuator valves are they have little foam filters that deteriorate n particles of foam foul the vacuum seal n then they can't work because there's 5-6 of them being fed by one vacuum line of the same size as on each valve

[928JimL]

each of the actuator holds a vacuum and the flaps open when i manually apply vacuum (i was actually surprised). i spoke to Dave at 928 Specialists and he gave me some tips on double checking the heater valve for proper function and also the setting motor, so i'm gonna try that and see how that turns out. he felt the control head was working based on what i told him. will let you all know, thanks for tips.

[tisilrad]

at some point in the future im going to completely re-engineer the hvac system and throw away most of the existing components...to me the hvac system on these cars in one of the only weaknesses
Im thinking about going to a basic cable operated system like on my old toyota corolla..which has 400000+ miles on it and has never failed..easy transplant.(and will also be lighter weight!)

[928JimL]

i agree...definitely this system was "over-engineered"...typical of alot of German vehicles.

[928JimL]

discovered that the brown ground wire for the bus of the solenoids apparently not working. when i bypassed it to ground, everything is working! i was just about ready to put out $300 for a new controller! whew! cold air now!

[tlister67]

Cheap fixes are satisfying!