DIYbanter - View Single Post - Root cause insight into the common BMW blower motor resistorfailures

Francis C.

wrote:

One key question in all this is if the interface from car to "resistor
block" is some kind of simple digital interface, ie it sends some bits
that get interpreted as "go to speed 3"
or does it send a PWM signal.

Your idea about testing it is correct, however a bad one can work fine
until it gets hot enough for the IC chip to fail. Hvac control voltage is
0-8 VDC.

The load resistor should have a metal case mounted on a heatsink to
dissipate the heat if you are going to run the test more than a few
minutes.

My original 13 spike FSU was replaced twice under warranty, so the car
had 3 FSU's. When the 3rd old style failed i replaced it with the new
design FSU. That was 8-9 years ago, it is still working. One thing not
considered in your post is how many did not use OEM units, the $75 eBay
ones are junk. The $175 units at the stealer seem to hold up a little
better.