Re <E36> AC.

[Jim Cash <j.cash@xxxxxxxxxxxx>]

Someone raised the question:

> Another "feature" in the vent system is that sometimes
> there's no air coming from the middle ventilation (the
> one above the radio). I can fix it by pretending that 
> the car is a PC and turn off the ignition and start it 
> again. Voila, the vent works! Any ideas?


I am not sure about yor E36, but all vents that are controlled
automatically by the A/C system (as opposed to those controlled by
manual sliders) are set to a specific position by a small electric
"stepper" motor. And while not a PC, the system is controlled by a chip
and a software program.


The system "assumes" that vents stay in the position they are set at. 
There is no "feedback" signal from the vent flap about it's position. If
the flap has lost it's ability to "stay put" against the pressure of the
air flow then it may shift to full open or full closed. 

In this case, restarting the car causes the climate control system to
"reset" all the vent flaps to their "full tilt" positions, from which
they are then repositioned according to the current requirements of the
operating program. 

If yours are closing at times when they should not, then you will have
to pull the vent and motor and find out why it is not holding position. 

Some of the later cars (E39 for one) solve this by making sure that
vents that are supposed to be fully closed, or fully open, stay in that
position by pulsing the motor a few overrun steps every 20 seconds.  You
can sometimes hear a faint creak from the vents as this occurs. 


Hope that helps.

Cheers

Jim Cash
London, Ontario, Canada
BMW CCA 102929
E39  97 540iA

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