Fix Tip: LiftMaster 3800 loose position sensor gear

pattenp · Jul 8, 2013

Fix Tip: LiftMaster 3800 loose position sensor gear loosing travel limits

Just an FYI. I spent about half a day trying to figure out what the heck went wrong with my Liftmaster 3800 opener. When I tried to open my garage door it went up a couple of feet then stopped. I tried to close it and it shuttered going down and stopped a few inches off the floor. I went back and forth trying to get it to open and close with no success. So I thought I needed to reprogram the travel limits. I tried to reprogram the limits and the same thing; the door will only go up and down a foot or so. After a lot of trying different things and fiddling I opened up the opener. After observing how things operate inside I noticed that the gear that turns the position sensor was slipping on the shaft. I tightened the set screws on the gear and problem was solved. So if anyone has a problem where the opener is not opening and closing all the way, check to see if the position sensor gear is loose on the shaft. It was just dumb luck that I noticed the problem. I was about ready to buy a new logic board.

See Set screw marked by red.

View media item 54906

richj11 · Dec 29, 2014

Lift Master 3800 loosing travel set points

Wow. I wish I had seen this post sooner. I started having issues with one of my three 3800 openers. It would try to shut past the set point, then re-open due to excessive force.

I would reset the travel setpoints. It would work for a day or two then start with the same problem.

After lots of reading on the internet ( this post never surfaced in my searches ) I figured I would swap out the logic board. Yep, that didn't help

While monkeying around inside the opener, I also noticed the position sensor gears would move indecently of the torsion bar. The tiny little setscrew was loose. Tightened that up and door works great now.

I went and checked the other two and they were also able to be tightened a bit.

So, hope this post will save someone else some time and/or frustration.

Heres a picture of the gear and setscrew. Sorry, it is a little dark...

pattenp · Dec 29, 2014

Yep.. that little set screw on the flat part of the upper gear in your photo is the little ****** that was loose. Sorry you didn't find my post prior to buying a new logic board.

gnpenning · Jan 21, 2018

Always check door balance first. If the set screws and coupler is tight. Look at item #2 in the parts break down. This is the absolute encoder. Common failure when travel limits change and won't hold. Replace it. There have been multiple revisions and part numbers through the years. This is true for the 3800/3900,8500 all have had this problem. For some reason tech support doesn't know this and will try and sell you something else.

patrickoneal · May 8, 2024

pattenp said:
Fix Tip: LiftMaster 3800 loose position sensor gear loosing travel limits

Just an FYI. I spent about half a day trying to figure out what the heck went wrong with my Liftmaster 3800 opener. When I tried to open my garage door it went up a couple of feet then stopped. I tried to close it and it shuttered going down and stopped a few inches off the floor. I went back and forth trying to get it to open and close with no success. So I thought I needed to reprogram the travel limits. I tried to reprogram the limits and the same thing; the door will only go up and down a foot or so. After a lot of trying different things and fiddling I opened up the opener. After observing how things operate inside I noticed that the gear that turns the position sensor was slipping on the shaft. I tightened the set screws on the gear and problem was solved. So if anyone has a problem where the opener is not opening and closing all the way, check to see if the position sensor gear is loose on the shaft. It was just dumb luck that I noticed the problem. I was about ready to buy a new logic board.

See Set screw marked by red.

View media item 54906

Thank you for posting this, I think I may have fixed my opener.

This is still a problem with LiftMaster jackshaft openers. I've got an RJO20 that's been in service for a little over 5 years and it seems to lose the travel limits more and more frequently. It would close and then immediately reopen, I guess after trying to close too far. I reset the travel and force limits multiple times, loosened and retightened the coupling and reset the travel and force limits yet again and still it would only work correctly for a week or two.

After unplugging the opener from power, the cover on the RJO20 comes off easily, there are just two phillips head screws on the bottom and the cover lifts right off(you can also use a 1/4" nutdriver). This can be done with the opener still mounted on the wall. I located the two screws on the positioner gear you mentioned and sure enough one of them was definitely loose. I snugged both set screws with a 1.5mm hex driver, reassmbled, plugged back in, and went about reseting the limits yet again. The door seems to be working correctly at this time.

This video shows the setscrews at this timestamp:

dtalbot · May 9, 2024

Kudo's for the video! It was actually this topic, found by a Google search, that got me here more than a year ago.
Since then, I've been lurking around in the background and realized that there's a wealth of info here in the forums.
So much so, I joined in, and happy that I have.

Mike07042 · Aug 31, 2025

patrickoneal said:
Thank you for posting this, I think I may have fixed my opener.

This is still a problem with LiftMaster jackshaft openers. I've got an RJO20 that's been in service for a little over 5 years and it seems to lose the travel limits more and more frequently. It would close and then immediately reopen, I guess after trying to close too far. I reset the travel and force limits multiple times, loosened and retightened the coupling and reset the travel and force limits yet again and still it would only work correctly for a week or two.

After unplugging the opener from power, the cover on the RJO20 comes off easily, there are just two phillips head screws on the bottom and the cover lifts right off(you can also use a 1/4" nutdriver). This can be done with the opener still mounted on the wall. I located the two screws on the positioner gear you mentioned and sure enough one of them was definitely loose. I snugged both set screws with a 1.5mm hex driver, reassmbled, plugged back in, and went about reseting the limits yet again. The door seems to be working correctly at this time.

This video shows the setscrews at this timestamp:

pattenp said:
Fix Tip: LiftMaster 3800 loose position sensor gear loosing travel limits

Just an FYI. I spent about half a day trying to figure out what the heck went wrong with my Liftmaster 3800 opener. When I tried to open my garage door it went up a couple of feet then stopped. I tried to close it and it shuttered going down and stopped a few inches off the floor. I went back and forth trying to get it to open and close with no success. So I thought I needed to reprogram the travel limits. I tried to reprogram the limits and the same thing; the door will only go up and down a foot or so. After a lot of trying different things and fiddling I opened up the opener. After observing how things operate inside I noticed that the gear that turns the position sensor was slipping on the shaft. I tightened the set screws on the gear and problem was solved. So if anyone has a problem where the opener is not opening and closing all the way, check to see if the position sensor gear is loose on the shaft. It was just dumb luck that I noticed the problem. I was about ready to buy a new logic board.

See Set screw marked by red.

View media item 54906

I could not tight it, my Allen wrench does not fit!!!

:-(

pattenp · Sep 27, 2025

I can't remember if the set screw is SAE or Metric.

Fix Tip: LiftMaster 3800 loose position sensor gear

pattenp

Well-known member

richj11

Well-known member

pattenp

Well-known member

gnpenning

Well-known member

patrickoneal

Well-known member

dtalbot

Member

Mike07042

New member

pattenp

Well-known member