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Did a LONG study of the electical diagrams in Bentley. There is only ONE stepper motor for the center vent. I'm going to GUESS that the vent pulls air from both heater cores and mixes it ONCE per the center wheel setting.

The wrecking yard wants $85 for a used heater control valve w/a 3 month warranty. Bavarian wants $150 for a new one. Going to do it.

: Ya know, if you own one of these E34's long enough, sooner or later you should be able to describe every system in your sleep.

: As you may remember, in the last episode, I was having problems with the drivers side temperature control. Basically had little to no control over the temp. Shoe melting hot or ice cold. Best guess was that the heater control valve was stuck. Test procedure was to check the "pulse" of the signal to control valve via a 12 volt bulb.

: Did this test - the signal seems to pulse correctly and frequency of pulse seems to vary with different setting of the heat temp switch. Took apart heater control value, looking to see if there are any "user replaceable" part in there. Nope. Looking to get replacement control valve from local wrecking yard. Have not done so yet, but planning to replace the valve ASAP.

: OK - on a long drive home tonight, the heater system was doing it's usual freak out with some strange symptoms. I'm interested if any of you HVAC guru's might be able to help figure this out.

: - Center vent temp was either HOT (ie working correctly) or COLD. By cold I mean it was "unadjustable" and just blew cold air. When the vent blows hot, the thumbwheel adjustment works fine.

: - Turn the car OFF for a 10 seconds or so (just shut down while coasting), and the center vent now WORKS correctly (if it was "cold" before") and blows HOT air.

: - Now this is the really wierd part - IF the center vent blew COLD, sometimes the heat to the drivers side footwell/defrost was HOT. HOW CAN THIS BE??

: Specifically, I have never seen any explanation about WHERE the center vent "sources" it's heat. It seems to be able to vary the left and right side - that is if the passenger has their temp set for something hot and the drivers is something cooler, the center vent will blow relatively hotter air to the passenger vs. the driver. IF TRUE (?can anyone confirm this?), this would imply that the center vent has a left and right sourcing from each heater core. COMMENTS? How does your car work? CAN YOU VARY YOUR LEFT / RIGHT CENTER VENT TEMP WITH YOUR PRIMARY DASH TEMP CONTROLS?

: Does the center vent just source from ONE heater core (drivers?) and just allow one adjustment (the thumbwheel) for BOTH SIDES?

: Now...going back to my drive tonight..HOW COULD THE CENTER VENT BLOW ICE COLD ON THE DRIVERS SIDE AND WARM ON THE PASSENGER SIDE when THE DRIVERS SIDE FOOTWELL HEAT IS BAKING HOT DUE TO THE ASSUMED STUCK HEATER CONTROL VALVE?

: You would think that if the drivers side heat is stuck HOT there would be no way for the center vent to ONLY blow ice cold !?!

: Am I going crazy here? Is it possible that something besides my control valve might be malfunctioning? The HVAC control system is total undocumented in Bentleys so I'm need everyones help to try to put together how it works.

: There is a good writeup provided by Paul's website on the basic control system, but he doesn't (hasn't yet?) document the airflow system. Maybe we can help him by figuring this out. (see http://silverstone.fortunecity.com/ferrari/464/heating.htm)

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