Have an old AHM flea market GP18. (as some on this forum kindly alerted me to). Took off the shell, checked the wiper contacts on both wheel trucks- ok; put it on the test track, turned the throttle on, and voila! The headlight lit, but the motor didn’t turn! I was able to rotate the shafts on each end of the motor by hand, and I see where the light us wired to the motor, but no movement! Any suggestions- other than buying a new motor immediately? (That is my second option). Cedarwoodron
I don’t know what condition the motor/gears are in as far as damage/corosion, but try using some light oil (like Bachmann EZ- lube) on the bearings and in the gear boxes (not too much oil) and turn the shafts by hand for a few revolutions.
It worked for me when several N scale locomotives siezed up from sitting for a long time, but if the problem is still there, then I’m afraid that the motor may need to be replaced.
I would try applying some power to the motor directly, and try to turn the motor shaft while power is applied. If it takes off, you may have a bad armature winding or commutator connection. If still nothing, check the commutator brushes, andf make sure they are properly seated on the commutator…
I would blast out the commutator and brushes with contact cleaner, if you have any, and clean out the commutator gaps with the tip of an X-Acto blade. Those motors have soft brushes under too much spring pressure, and are prone to fouling the commutator with brush dust, which can mimic a burned out armature.