Make sure you put the black cup back on the end of the motor shaft and that it is pressed all the way on. Make sure the RPM sensor is plugged in. If neither of these are the issue then you probably did not have the motor shaft pushed all the way back when you tightened the coupler after replacing the worm gear. Loosen that coupler and from the front of the motor (opposite end in which the coupler is on) push the shaft back as far as it will go. Hold it there while you tighten the coupler back up and then try it.

don 1 you are the man, the black cup tabs were not in far enough to allow the opener to work properly. Thanks for saving me an expensive call out or replacement opener.

You are the man It fixed mine to!!!! i owe you a beer!

Can you also tell me what is the RPM sensor? Thanks, Tony.

I just recently replaced the gear on my older craftsman garage door opener and have tried everything to get it to run properly. It will run for 6-10 inches and then stop and reverse itself. Is it possble that the motor has gone bad in it? I have replaced several gears on this unit and have never encountered this problem with the motor running like this, I even took the chain off like you suggested to anther person to try and get the motor to reengage itself. Any suggestions? Tony.
PS. What is the RPM sensor and would my model 53600 have one? Thanks for any help.

The RPM sensor is a small circuit board about 2 inches square just in front of the motor.