Sears garage door opener runs 3 seconds then reverses
Have several Sears garage door openers. Old style not rolling code. Models 139.53400, 139.535006, 139.536006.
All have the 12 pin electronics module with receiver and 4 or 5 terminals going to wired controls.
The 139.53400 will start to run and after about 3 seconds it reverses, runs another 3 seconds in the reverse direction, then shuts off. Problem is NOT with the limit switches as the motor is in between the open and closed points and is not shutting down by contacting the contact on the limit switch.
Have tried 3 different PC boards and get the same results. When using the PC boards on different similar Sears openers (see models above) the boards work fine and the openers travel to the limit switches before shutting off.
Since most of the electronics is on the board except for the limit switches and actual motor windings and the capacitor do not see what is causing the motor to reverse. This unit is sitting on the garage floor and has no load or anything that could cause a bind. When the motor runs it does so at normal speed.
What could cause the motor to stop and reverse after running for only 3 seconds?
Have rebuilt these openers for years but this one has me stymied.