The MD-3 (unlike the MD-1 and MD-2) doesn't have a rewind feature (it was the "budget" motor drive for the F2), so you don't need to remove the door opening cap.
These drives have nylon drive gears that are known to break. Try contacting Sover Wong (http://www.geocities.com/sover_wong/...r_Service.html) to see if that might be the problem.
Here is an online instruction manual.
I hope this is of some assistance.
Have a nice day, and good luck to you,