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So the heater fan in my ‘88 535is is not working. I replaced the hvac control unit under the advice of my garage, but still no luck. The AC blower works. The AC light in the switch works. The sunroof works. Most everything works except the window switch under the dash blows when I try to roll UP the driver’s side rear window with the driver’s control switch. Any ideas what the problem might be? AC / heater fuse seems to be intact.

Have you checked to see if the heater blower fan motor is seized? They're fairly easy to reach through the access panel on the firewall inside the engine compartment. They can sometimes be revived by cleaning and lubing the motor bushings, but that's usually only a temporary solution. There's also an off chance the fan blades are jammed up with leaves or mouse nests. Note there were two brands used (Sofica and Behr) and the two aren't directly interchangeable; this has been covered in the past, you should be able to find old discussions with the search function.

Thanks. I did check the motor itself. It spins. The cages look brand new. I used a voltage meter and there it no voltage at the pond on the motor at all even with the motor at full tilt. And of course, AC is off when I test this. I don’t think it’s the relief relay bc the AC works. But who knows.

I would hook up 12V to the motor just cause. To eliminate it as the problem.

Assuming it's good, it should have power going to it, 12V to ground, not from one side of the motor to the other, whenever the A/C switch is off, being fed from fuse 14. They control the speed thru the ground side going first to the resistor and or then thru the speed switch. Speeds 1 and 2 go thru the resistor, speed 3 goes straight to the switch and then to ground.

Easy peasy says the guy behind the keyboard a million miles away.