I have been wrestling with the hot/warm air coming from the defrost vents on my 1988 735i for a couple of months and finally was able to fix it. The hot air would come from the defroster when the A/C was off. When it was on, there was no hot air, though by putting my hand on the vent, it could be felt lurking there. Sometimes, when the car was first started and driven in the morning, there would be no hot air until the system called for some when I intentionally rotated the driver's control dial to above ambient temp. After that the heat would continue from the vent(s) even when the dial was rotated back to full cold.

I explored electrical faults by checking to see that there was 12v to the connector on the heater valve when the control dial was set below ambient temp. The valves are powered closed by 12v coming from the control unit. Since the unit was getting power, I removed the working innerds of the valve and layed it on a table. I then applied 12v to the center pin and each of the side pins to test the impellers. It appeared that the impellers were somewhat slow and did not want to extend fully so I ordered a new valve ($123 at Zygmunt) and checked it with 12v. The impellers on the new unit moved quicker and extended more fully.

I replaced the complete valve innerds retained by the six screws with the new unit's. It did not seem necessary to replace the plastic housing since it seemed sound. Now there is no more heat from the defrost vents! There have been many posts related to this and the fix has usually been to replace the valve or rebuild it. I chose to replace it because of my observations of the action of the impellers.

Sorry for the long post. I wanted to share this experience but tend to be long-winded.

Mike Roemhildt
1988 735i 118k