the unit should have an RPM sensor on the motor with a plastic "interrupter cup" mounted on the end of the motor. the RPM sensor is an optical encoder and the interrupter has blades that cross the plane of the RPM sensor to tell the logic board that the motor is runing and where the trolley is. Make sure the interrupter cup isn't loose as this will give the logic board false information and may result in erratic movement.

Another option is that there's an issue with the logic board and it needs to be replaced.

Note that after 13 years, you've gotten your money worth out of it and I wouldn't spend a lot of money fixing as opposed to just buying new one. Check compatibility of a new liftmaster and I'll bet it's a direct replacement for the old one so you just need to swap out units without installing new tracks/trolley.

Good luck

Cheryl Smith - Sunlight isn't an issue in this case but good for others to know as they work through problems.

sdello - I didn't know that existed. Thanks for bringing that to my attention. I'll take another look at it. Some further research on that seems to indicate that the trolley will reverse if the RPM sensor is malfunctioning. I have my doubts this is the issue in my case because my trolley does not reverse back to the home position - the door just stops. Worth looking at anyway. Thanks!

Does the door open and close smoothly by hand, with the opener disengaged?

Yes it does. No binding or tightness at all.

Disclaimer: I know very little about garage door openers.

I'm a tinkerer. With an intermittent problem like this, I'll usually tear into the device and have a look. At best, I'll find and fix the problem. At worst, I'll either not fix it or make it worse, and then I have a good excuse to buy a new one. :)

So I'd take the opener apart and look around for indications of an intermittent or loose connection. Such a connection might be burned or blackened, but not necessarily.

I'd disconnect the logic board and examine it under a strong light with a magnifying glass, looking for iffy ("cold") solder connections. I'd reheat any suspicious ones with a 40 watt or so soldering iron, maybe adding a touch of extra solder. The right solder for this is good old rosin (or resin) core 60/40 lead/tin electronics solder.

I'd give any fastons a squirt of contact cleaner and wiggle them.

I'd give any screw connections a dot of the same stuff and a twist or two.

Then I'd button it up and see if I'd improved things.

If all this sounds like too much trouble, you could always just buy a new opener, and give the old one to a tinkerer like me. :)

DavidR

I too like to tinker with devices - nothing like a few hours of putzing to clear ones head!

In this case it's a bit different and I'll get to that in a second.....

UPDATE: I inspected the RPM components and everything is clean and snug, and no reversing behavior after a foot of travel so....my conclusion was it must be the logic board. But there is more. A day after my initial post one of the light bulbs (one closest to the garage door) went out and while replacing it I noticed that the bottom contact was pretty fried. The spring contact inside the opener's bulb socket shows a little "toasting" too. And I got to thinking ( a more sporadic behavior these days) that I've replaced that same bulb a few times over the past two years. The other bulb is at least 5 years old. So why the issues with just this one bulb? All of the wiring and connections look good. The bulbs are properly rated. The back one is super clean. I haven't replaced it because I don't have a spare of the heavy duty incandescents that I use at this time. Without a front light bulb the opener has worked flawlessly. Not a single hiccup. I can only speculate as to why. Some sort of voltage issue related to that bulb's connection giving the logic board a fit? Harness/wire/connector problem?

I could do a tear apart and more but that's more than I want to put into it. This is a perfect excuse to get that nice quiet belt drive opener I'd like. So until I see the one I want on sale, the old chain drive will do just fine without one light bulb. Perhaps I can find a tinkerer locally who would like to buy it cheap for a "project".

Interesting! It sounds like the bulb socket (low bid supplier?) was defective and not making good contact. The arcing that resulted probably created enough electrical noise to confuse the digital control circuit.

The amount of arcing may have depended on such variables as moisture in the air and the ambient temperature. In some circumstances, the contact was good enough to keep things quiet, and the opener ran fine.

Most likely the opener has some noise filtering to keep out power line hash, but the manufacturer probably didn't count on noise coming from inside the opener!

Some folks may not agree, but in my book, chain drive wins hands-down over belt drive, mostly for durability. I have a Sears (Liftmaster) chain drive opener in one of my rental houses. I'd estimate that it's at least 40 years old, and the only repair that's ever been done on it was a motor capacitor replacement. (However, one of these days I'll have the opener replaced with a modern one, for the sake of the improved safety and security.)

Assuming the no-bulb work-around keeps the gremlins away for good, you could replace the bad socket, just leave the bulb out, or leave a burnt-out bulb in that socket. It sure is cheaper than buying a new opener!

If the contact wasn't hitting the bulb.....turn the power off and then use a screwdriver to pry the contact up into the socket for more positive contact with the bulb. Might be able to have the bulb work and the opener function properly.

If it's been arcing for a long time, there's a better than even chance that it's already too late to just pry up the contact. Most likely it's been eroded by the arcing, and still won't make good contact. This is especially true since heat destroys the temper in the metal, so it loses its springiness.

I guess it wouldn't hurt to try it, but if it were mine, I'd just replace the socket.