Decided that with the weather being so pleasant, I could take a portion of the afternoon to work on my car. The blower motor had stopped blowing, and I knew that the final stage resistor (still original to the car for all I know) might finally have gone out. I’d located the APSX FSU that I bought years back amongst my spare parts.
Getting to the FSR was easy. By now I’ve done this more times than I care to mention. I found the FSR harness difficult to unplug from the FSR. Closer examination revealed that the wires going into the harness were discolored. Fortunately, I was able to separate the harness from the FSR. I noted that the harness had melted somewhat.
After scraping at the harness with a small flathead screwdriver, I plugged it back into the old FSR. The blower motor came back to life. Plugged into the APSX FSU, blower motor unresponsive. Conclusion: APSX FSU must be defective.
I conducted a quick search to see if anyone else had encountered a melted FSR harness, and turned up a number of interesting forum threads relating to the FSR. This one in particular piqued my interest: failed blower resistor (final stage resistor) not really dead | Bimmerfest
What I gather is that the FSR has been through a number of revisions over the years.
One obvious change is the heatsink design. Updated part numbers have a larger number of fins (prongs), increasing the surface area.
Additionally, the potting material has changed from a hard epoxy resin to a silicone.