In Reply to: New 735i owner requesting help posted by dj on December 24, 1999 at 00:58:21:
I believe the defroster comes on as the default mode if something is wrong with the brain of the A/C system. It also will stay on if there is a vacuum leak, which is more likely. Does your vacuum pump run all the time? Turn key to on and you should hear it clicking. Left front just behind the battery. Then crank engine and it should go off after a few seconds unless there is a leak. If it does go off, then turn motor off but turn ignition back on, and the pump should not run. It should hold a vacuum. If there is a leak it will come right back on. The servo motors that operate the gates under the dash that direct the air to the different vents are vacuum operated. The air hoses and diaphragms dry out and crack and then they leak and won't operate. If too many leaks occur then there is not enough vacuum to operate any of the servos. One leak and the vacuum pump runs most/all the time. If it runs too long it will burn out. Its not made to run continuously. Plug the hose at the pump so it won't leak if you can't fix the leak right away. You can trace the two vacuum lines to see where the leak is. Also, if you look down into the air vents by the wiper blades you can see the vent gates that control outside air versus recirculate. Have someone turn switches off and on while you watch. Watch your fingers.
The servos can be changed out but its a pain to take dash apart to get to them (from underneath.)
I had the dealer replace mine. I should have had all (several) replaced at once, as they all went out one by one over a couple of years, but that was about four or five years ago. I'd be surprised if yours have lasted this long, but this is Florida. Then again, it could be the water valve at the core or something else. You didn't say whether everything else worked. Other symptoms could help a diagnosis. Callan on this site is the real expert.
As to the overheating, I agree with the other writers. Mine would do that if it were even a little low in water, so keep the tank full. But its probably a fan clutch or maybe even a dirty radiator. Or partially plugged? A complete drain and flush couldn't hurt. I assume you are not running the air conditioning. But if it gets hot enough the auxiliary fan in front of the radiator should come on. It should not be necessary today. I suppose it could be the thermostat. Replace it and flush as a first step. Since the fan's expensive. It should spin fairly freely while the motor's off. Radiator flush and a thermostat are cheap and good insurance regardless.
: I just bought a 1985 735i, and there a couple of things I was hoping you experienced guys might address.
: First, the defrost vents are always open and there appears to be nothing I can do to close them. After the car has warmed up, the air is blazingly hot; and the faster I drive, the more powerful it blows.
: Second, the car seems to get a bit warm when I am idling at stoplights. Then, when I get a bit of speed, it drops back to the center position. I feel like I have to keep moving, so it feels like some strange rendition of the movie Speed.
: Other than those relatively minor items, it's a great car with a smooth ride. Thanks in advance for your help!