Be Carefull!This repair was done on my '89 320i Touring with a M20 engine. Other's may differ a little, but the fix works for all E30's, and maybe for other models, too.

If your ventilation only works in the high (4) position, here's the fix:

bmw e30 At the engine compartment, along the firewall, on the top, there's a rubber (?) cover that goes almost from side to side, that you'll have to take off, to gain access to the blower motor (4 6 mm bolts, 2 on each side). For easier access, take of the 2 plastic nuts, on the right, that hold some electrical wires together.
With the cover out, you'll see another (!) cover, round, held by two plastic straps. Loose them, and take off the round cover. (It's a bit tricky to take it off. It's only the top part, where the straps rest. Be carefull not to brake it).

bmw e30 Having removed this second cover, you'll see the blower motor. On its middle, at the bottom, look for a small brown plastic board with two "arms", one on each side, pointing down, and three resistors, (they look like springs). This board its held in place by its electrical connector, at the back. Just pull it in your direction.
This board is probably VERY dirty - mine was! Clean it thoroughly (the contacts on its back, the thermal switch on the middle, the resistors, everything!) with electrical contact cleaner, or WD40, and test it (Be carefull with the fan!) If it doesn't work, clean it again (I had to clean it twice), especially the thermal switch, below the three resistors.

bmw e30 If the cleaning doesn't help, one - or more - resistors, or the thermal switch are faulty. Test the resistors, using an Ohmmeter. The values are:

  • 1.6 Ohm
  • .9 Ohm
  • .35 Ohm
If you find any of them faulty, replace the board.