1993 Liftmaster professional mod. 1345. Have adjusted door balance, spring tension, new safety sensors (correctly aligned). With trolley disengaged from door the trolley will run approx. 8 - 10 inch. & stops. Have adjusted chain tension, no luck. I suspect a problem with either gears or logic board. Any ideas?

issue with counterbalance springs. when you release trolley, door should balance at half way open. if i rises all the way on its own, or falls closed, then the springs are an issue and motor and trolley are working very hard in one direction or the other.

Because we live in a free country the government says garage door openers manufactured after 1993 have to have an inferred safety reversal system (UL325).
You need to adjust the receiving sensor until you have a steady green light, then the garage door opener will function properly again.

The torsion springs might need adjustment.

Pull emergency release, and manually lift door to half-open position.

When you release the door, it should stay in place.
If it almost "slams" down, the springs are not doing their job to "balance" the weight of the door itself.
The opener isn't designed to overcome the weight of the unsupported (no spring assist) door.

Over time, the springs take a "set", and might need a readjustment
THIS IS A JOB FOR PROFESSIONALS!!!
The springs are under tremendous pressure loading, and can even kill someone if they do not know what they are doing!

You said you adjusted the up limit already. Is it adjusted as far as possible? Usually the screw will become harder to turn. You mention a new door and spring but is the opener the same? My guess is you have a new opener as well and it is out of sync with the trolley. Try turning the up limit back and see if the trolley will stop shorter. If it does, loosen the chain or belt so that you can remove if from the opener's drive gear. Bring the trolley to your stop limit and put the chain/belt back on the gear. Adjust the chain/gear tension and try door operation again. If it was a sync problem, the door should close closer to your limit. Adjust up and down travel as necessary to fine tune it. This worked for me so good luck.

You did not sync the chain correctly. If you have not touched the travel limits, take chain off sprocket and run door to either up or down position. Put chain back on with trolley in up or down, accordingly. Tension and make final travel adjustments.

You can adjust the length of the arm between the trolley and door so that it is in a vertical position when the door is closed. This will prevent your problem.

It sounds like your chain is out of sync with the limit switches. Loosen the chain enough so that you can remove it from the sprocket on top of the motor unit. Now run the opener until the sprocket runs counterclockwise and stops. Now with everything attached and the door in the open position replace and tighten the chain. You are now in sync. You may need to tweak the up and down limit controls which are on the side of the motor unit.

If the door operates freely without any binding, while operating it manually then It would appear that the obstruction sensor mounted across the doors path is ether out of alignment, lens dirty,or malfunctioning

two problems. why aren't inner and outter trolley connecting. did you leave rope is disconnect position. Second problem is door springs do not seem balanced. you should be able to lift door halve way up and let go. it should pretty much stay there; go up or down slowly ok. fly up or crash down; need to adjust springs/cables or maybe wrong springs.

There are adjustments on the unit itself that control the distance door travels to close as well as the force it uses to close. The info and location of controls is indicated in the manual that came with your unit.