Ever wonder why the OEM fans seem to draw more current when they age? This has always bothered me for as long as I've owned my D. My thoughts about this were just rekindled when an owner with my fan fail unit said it is flashing an overheating warning.
Years earlier I did take an old OEM fan motor apart and cleaned the commutator and brushes thinking that would reduce the current draw. Well the current went from 13 amps prior to cleaning to 15 amps after cleaning. This just amazed me since I thought that was the cause of current change of the motor. Now bearing wear could also cause some increase current draw if the bearings started to add drag. But more thought I started to think maybe the permanent magnets in the motors may be loosing magnetism. I would think loss of magnetism would reduce motor current but I could not find info to prove that thought.
It looks like older magnets did loose magnetism more so than the newer alloys used today. Also you can demagnetize a magnet using an AC field held close to the magnet. The old TVs you would degauss the metal around the CRT. So I wonder if turning the fan power on and off many times would produce a field that could degauss the permanent magnets. My fan relay does turn the fan current on and off slower than a mechanical relay so not sure if that is better or worse for permanent magnet motors.
So if anyone has nothing to do one day, try finding some information on permanent magnet DC motors to see what effect of magnet loss would have on fan current.