Two years ago a 3-speed 19” (50 cm) 120vac window “box” fan stopped turning. I guessed the cap died so sourced a replacement and installed it. This didn’t fix the problem.
Now I’m getting back to diagnosing it. With the blade removed I now see that the motor hums and if I spin it, it will run fast (can’t say if this is the rated speed). This is when powering the motor’s high-speed leads, bypassing the 3-speed switch.
It’s not a mechanical issue—with power off the armature spins freely.
I don’t remember the original cap markings but this one is marked “PMS405J” and measures 4 uF. I’m usually good at looking up markings and matching replacement components but there’s always the possibility I couldn’t find the exact one and tried a different value.
Should I sub 2, 1.5, 1.2 uF caps and just see what works? How will I know when the value is correct? I presume torque and speed are max at one value and reduced at others?
I’ve watched countless videos of repairs of 120v fans and they used caps from 7 to 1.2 uF, so no joy looking for statistical help.
EDIT: I realize I’m going on the premise that the original cause IS a failed cap. Other ideas welcome.