EUREKA !!!!! Problem solved and How. (archive)

First, thanks to all for providing tips and Info on the Fan speed control system. I finally took the bull by the horns and went HVAC diving to fix the fan speed problem. I got it to work as designed and learned a few things which I will share.

The biggest problem for me was getting the fan cover off and back on so I could get to the electronics. There are two control module boards,
one on each fan (there are two squirrel cage fans, one on each end of the blower moter.). The control module is located on the top half of the shell that comprises the fan enclosure. the two halves are bound together by a metal band that is fastened by a latch. the latch is fairly easy to unhook.

The challenge is to remove the top half of the fan enclosure as there is not enough room to just simply pull it out.

Before removal, the module connector must be unplugged and the relay that is attached to the top half must be removed. This should cause no problems. A sensor is attached to the passenger side fan and that should also be popped off.

There are two removable pieces on each side of each fan enclosure that must be removed first before the top half can be taken out (One piece is kind of a screen the other appears to be a holding piece)

Even with the two end pieces out of the way, the top half has to be jiggled and somewhat deformed to get it out. I found that forcing the bottom
of the top half downward and rotating the shell allowed enough space to bend the other end of the half so that it would clear the top of the firewall.

Once I had the top half out, removal of the module was easy as it slipped out of the space made to hold it. The culprit is usually the top transistor that is held in by two small bolts secured by nuts on the circuit board side. It is a
2N3771, NPN transistor, which can be purchased thru the web from any number of sources at a cost of no more than $2.50. I used a Radioshack 2N3055 as a replacement but will eventially replace it with the original. The 2N3055 doesn't have enough power dissipation (115 Watts as opposed to 150 watts for the 2N3771).

Removal of the transistor requires soldering skills as BMW didn't use a socket to secure the part to the board thus requiring the use of a soldering iron and a desoldering tool (Solder Sucker or Copper mesh) to remove the old solder and free the transistor pins from the board. Once the transistor pins are desoldered, removal of the bolts on each end of the transistor will free the part for replacement.

Remember there are two of these modules to work on and two half enclosures to deal with. Why BMW couldn't put the whole thing on one board only their engineers can justify, I sure can't. I know that the way the modules are mounted inside the fan enclosure provides a easy way to dissipate the heat from the transistor. I am sure there was a better way than to split the electronics in
half and force future owners to pay handsomely to have it fixed.

I should note that the problem I found was not related to a blown transistor. What I found was that the nut on the underside of the module that is used to make an electrical connection between the transistor and the module board had over the
years corroded. That caused overheating because there is a too much power flowing thru that part. The excess heat melted the thin copper on the board and broke the connection between the power transistor and the circuit board. I fixed the problem and now have full range control of fan speed.

Follow Ups:

Transistor better for variable speed Mike W. 00:08 9/03/2001 (0)
Re: EUREKA !!!!! Problem solved and How. Charles V. Stancampiano 22:01 9/02/2001 (0)

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