[DWA0550]

I changed the drive gears in a sears 1/2 hp opener, press the remote and it moved as it should for about 12" and reversed to its starting point. Followed the instructions for adjusting travel, same result. I disengaged the mechanism that controls travel, same result. the same results whether the door door was attached or not.. any body have a clue?

 

[NJMarine]

Unplug the unit from line power and then try to reprogram the remote and reset. It may be the internal memory of the unit is corrupted

 

[a7ecorsair]

Openers have three conditions that cause it to stop or reverse. 1) It has reached its limit of travel, or 2) There is a bind in the travel, or 3) There is interference in the door path blocking a sensor.
A lot of Sears openers are Chamberlins. The ones I've worked with have two screws with actuators. When the upper or lower limit is reached the actuator trips a switch and stops the door.
These openers also have a bind detection that causes the door to reverse if it can't move properly. For instance, if the lower limit is incorrectly set and the door reaches the floor before the lower limit switch activates, the bind mechanism will reverse the door.
Since you had the gears out, I think you have something out of time between the limit switches and the door travel.

 

[iamrfixit]

Make sure you reinstalled the segmented wheel on the motor shaft correctly, position does not matter but it must be fully seated on the shaft so the sensor portion reads the segments. The segmented wheel is part of the RPM sensor, it confirms rotation of the motor and without it the opener thinks it is jammed, it will travel a few inches, reverse, travel a few inches and stop.

Also check any wiring inside that you may have taken loose, it is easy to forget to plug things back in, also many of the plugs just fit over pins and can easily be installed one pin off leaving one pin outside the connector.