Ammonia smell might be coming from overheating on the windings of the motor, loose brushes or connections... Dirt could be causing excessive arcing as well and burning the crap in there giving the smell, and on / off again .
If you seen smoke though, that's not a good thing, .... it might just be a switch getting ready to give up the ghost, and not the motor.
or could be years of gunk / grease too... it's hard to really say w/o opening it up and taking a peek.
Aaron