Make sure you put the black cup back on the motor shaft. It is a big black circle, make sure it is pushed all the way on. Also make sure the small circuit board just to the left of it is plugged in. It will have a four prong connector. Either one of these will produce the problem you are describing.

I have "clicked" the black cup back onto the motor shaft. I assume there is no special way to position it. Also, the 4-pronged power cord is plugged into the small circuit board. Is this board the RPM sensor? Perhaps the small board is bad. Sears parts does not sell the RPM sensor separately - only with the motor assembly ($109).

I have no clue what to do next.

Ok, lets start from scratch here.

Did this problem occur right after you changed out the drive and worm gear? I assume that your lights on the opener are not flashing at all when this happens??

The initial problem was that the door would open about 2 feet and then shut back down to closed. I then suspected it was the gears and when I took the opener case off, I noticed the main gear was stripped, so I changed out that gear and the worm gear. Then I clicked the button on the garage wall and the door raised all the way. Then I clicked the button on the garage wall again to shut the garage and this "semi-new" problem occurred where the trolley goes down the track 2 feet and returns. I do not see any lights flashing.

Ok, if you disconnect your door from the opener will the opener close or do the samething?

If I disconnect the door from the trolley by pulling the rope, I have the same problem.

Ok, double check your work around the RPM sensor. You may have the motor shaft slid to far forward not allowing the Interuptor cup to fit properly on the shaft. There has got to be something throwing off the Rpm sensor. I can't imagine it would just go bad all of the sudden it is possible I just don't think it is probable. You might try loosening the coupler on the back of the motor shaft and make sure you have all the bushings in the correct spots.
You should have a flat washer, a washer that looks bent and a ring that slides over both of those and then the collar. If it is all in place push on the front of the motor shaft opposite the side of the collar. Push it back as far as it will go and hold it there while you tighten the collar back up. Put the interuptor cup back on leave the cover off and watch everthing while you hit your transmitter, see if anything looks out of whack, if not, I would have to say you have a bad Rpm Sensor.

I pushed the motor back and reattached the coupler and black cap and I'm happy to say that the problem is fixed. Thanks doorguy06!!!

Great Job!! Have a Merry Christmas.

hey door guy 06 you helped burnt orange I went through the same steps and I am still unable to get the door to work. I have two openers that are the same and I took the 4 prong chip and swapped openers and the door that worked still works and the door that dosen't still dosen't is it possible for the black cap to be bad

Here is a link that might be useful: garage door opener

Black cap may be bad

I have a 1\2hp Craftsmans GDO. It's probably 25yrs old. Has worked great until it stuck in the open position. The wall switch and remote activate the internal GDO switch but the motor will not start. When I rotate the large pully inside the GDO manually - lifting white plastic ring off of the close limit switch - the garage door coupling on the rail moves about 5 inches away from the GDO - the motor starts and moves the GDO coupling on the rail back toward the GDO closing the door again. Weird huh? The door moves freely when disconnected from the GDO. I think there is something inside the GDO between the switch that gets the signal and the motor that is malfunctioning. If there is a $10 fix to this I hate to spend $150 on a new GDO. I know GDO's aren't made to last forever.

If you say your garage door opener is 25 years old, it does not pay to repair it, that is if parts are available.
Do not waste your time in repairing it. You would be wise in purchasing a new garage door opener.

Have a chamberlin opener about 6 years old and the door won't stay closed. The only way to get it to stay closed was to adjust so there is a gap under the door. Did try and unplug to see if this would reset things, but still didn't help.

door opens using door control and remotes.
door will only close keeping door control depressed until closed. Sensors are aligned. flashes about 10 times.

Anytime the main light flashes 10 times when the door reverses it is a door sensor problem. Make sure the small light on each one is lit and not flickering. Tweak them a bit. If you cannot get both lights to come on you have bad sensors.