Thank you for everyone that offered ideas. Here's what I figured out.
My garage door opener is a Craftsman Model 139.53992 – ½ hp screw- drive.
I disengaged the trolley latch from the screw drive and tested the door. It moved freely without binding. It would stay fully closed and opened without assistance. If I opened the door 1/2 way up and let go, it did pull back to the closed position. If I opened the door
2/3 the way up it would balance and stay stationary.
I then disassembled the housing to look at the mechanics. As a future note to others disassembling this opener, to take the housing off you: (1) Open the light lens. (2) Remove bulbs. (3) Remove the two screws holding the cover to the bracket – these are in the upper corners of the bulb area. (4) Pull the cover *forward* (toward the lens) and remove. If you pull it downwards, you’ll brake the tabs on the rear plastic piece (like I did). It just wouldn’t be a home job if I couldn’t use duct tape in the repair. Check that one off.
I was surprised at how simple the mechanism was. There are two gears on the motor. The first is a gear that engages a belt which then drives the screw. The second is a worm gear that engages the limit switch assembly.
I immediately noticed the gear on the limit switch assembly was barely engaging the worm gear on the motor shaft. I could gently apply pressure to the limit assembly gear and it would “jump” downwards and turn. Neither gear looked to be stripped or worn. However, both gears were dry as a bone.
I also noticed if I applied very gentle pressure to the bottom of the assembly (pushing upwards) the gear would fully engage. I removed the assembly and cut a slim cardboard shim (from a 9v battery package). There are three holes in the main bracket where the limit assembly “clicks” into place. I put the shim in the bottom most hole and inserted the limit assembly back into the bracket. The gear now fully engaged and I could not force it to “jump” down and turn.
I didn’t have any heavy grease, so I grabbed a tube of silicone and lubed up the friction points on the limit assembly and where the assembly engages the worm gear on the drive motor. I hope this wasn’t a mistake. Is regular silicone compound OK to use in a situation like this? The door opened this morning (8+ hours after repair).
I appreciate everyone’s help and advice. Hopefully this will save someone else a repair call in the future. If it ends up this is not the fix, I’ll post an update.
Thanks!