Repairing Garage Door Opener

Yesterday the garage door opener (Craftsman 1/2 HP Model 139.53975SRT1) stopped raising the door. The motor would turn but it would not move the chain that pulls the door up. In the last few weeks the door was not going all the way up on the first attempt so I sprayed silicon on the door joints. Since the temperatures in the area were at freezing or below freezing it seemed like the joints maybe getting stiff which caused the opener to stop due too much force being used to pull the door up.

One test I did was to detach the opener trolley from the arm that is used to move the door up and down. I then opened and closed the door manually to see if it was sticking or hard to move up and down. It was not.
Once the door quit moving, I took the casing off the opener and saw the issue. The plastic drive gear's teeth were stripped off thus unable to drive the gear moving the chain.

As it turned out a number of year's ago I was fixing another garage door opener that had been damaged by lightning. It turned out that replacing the damaged part costs just about as much as a new opener. So the homeowner opted to buy a new opener instead of waiting for a part to be shipped in. After working on it, I stored the old opener will all the other parts from the new one in the loft at my father's garage. Luckily, one of those parts (part # 41C4220A) helped fix the opener.

First, the eight screws need to be removed from around the opener outer casing (use 1/4" hex socket). The casing can then be removed. Next, the chain need to be detached from the trolley threaded shaft. This action allows the chain be removed from the opener sprocket on top of the opener. After this remove gear and sprocket assembly that has three hex head screws keeping it in place (use 5/16" hex socket). It is a little tricky removing one of the screws close to the motor. I had to use a couple of tools to leverage the ratchet to  remove the screw.

There is plastic gear at the bottom of the part shaft. It is called the limit switch drive & retainer. The plastic retainer clip needs to be removed which will allow the switch drive gear to be removed. The entire assembly can then be removed.

The new assembly can be slid into place and the three screws put in place to fasten it. Reattach the chain to the  opener sprocket and then fasten the chain to the trolley threaded shaft. Make sure the chain is not twisted when being put back on. The down and up adjustments might need to be made so the door will properly open and close. Last put the opener housing back on.

When I took off the housing the first time, white plastic shavings could bee seen from damaged gear. This opener had been in use 12 years before it failed.