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I have an unusual air distribution problem on my 24V 164, ie the car *without* the stepper gear problems.

I've just completed a conversion from manual to automatic climate control, I've described the conversion under the 164 model forum. Everything works as it should with one exception:

After the car has been driving for perhaps 5-10 minutes something weird happens to the air distribution - it's as if the stepper motor is "off" by a 1/4 turn - if I select the windscreen it blows on my feet, likewise the other settings direct air in incorrect directions. If I power down and start the car again, so long as I set the "windscreen" when I turn the ignition off everything resets to where it should be and the air distribution works fine - each setting directs air exactly where it should. It stays this way for about 10 minutes then it happens again!

I don't think it can be a gear problem or it wouldn't come right after an ignition off-on cycle. Likewise, if the spine on the stepper gearbox that connects to the distribution drum was broken it wouldn't come right after an ignition cycle.

I doubt very much I've mis-wired something in my conversion - the wire colors for the manual and automatic
HVAC units are identical for the steppers and I studied the wiring diagrams and pin-out on the connectors very carefully before I did my conversion. Also, it works fine for the first 5-10 minutes with accurate and correct air distribution so I think the actual connections to the stepper motor are just fine.

Is it possible the stepper motor itself is failing, and somehow it gets slightly out of alignment during operation, then corrects after powering down?

I understand the actual motors are common to BMW and not a difficult part to source. I did a stepper motor repair 17 years ago on my 12V 164 and I can recall at least some of the challenges!

Any advice appreciated.
 
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