If it was jammed, I would expect it to pop the big breaker. The
voltage across the motor suggests worn out brushes or some other
electrical open circuit. The manual seems to indicate the motor is a
sealed unit, however, so the brushes are probably not field
serviceable. There is a chance that the wiring is open or intermittent
between the solenoid and the motor. Try gently wiggling it with the
switch on.