I had a similar issue with a GN F3a. It turned out to be a stripped gear with the nylon gear that engages the motor worm gear. Fortunately the part was pretty cheap ($8) and pretty easy to replace as the nylon gear is just a press on to the gear axel.

hope this helps,

Kevin

Most likely the worm gear is spinning on it's axle. Replacing it should solve the problem.

Part number 15 in this link may be what you need: https://www.lionelsupport.com/...2d-9c73-24438a61787a

I got it partially apart.  The worm gear seems tight to the motor shaft.  The gear the worm gear meshes with shows signs that it is getting chewed up.  When on the track, this motor is sloppy and you can wiggle it with little effort and feel the worm gear engage and disengage from the gear it meshes with.  So, I am perplexed.  All screws are tight and all gears spin smoothly when the motor is detached from them.  I don't know how to keep everything together so that the motor worm gear engages (and stays engaged) and causes the truck gears to spin and the wheels to turn.  The loco will run forward for a few inches before the worm gear disengages with it's mate.  (I wish I knew what to call that gear).  The entire set of truck gears seem to be rather sloppy starting with the first gear that meshes with he worm gear.  The whole assembly seems to be sloppier than the rear motor and gear assembly, which seems tight.

Bob,you say the motor is sloppy did you check the screws that hold the motor to the mount? I've had a few engines with this problem.

Doug

Yes, I have checked the motor mount screws.  They are tight.  After further investigation, I have found that there is a LOT WRONG with this set.  The worm gear will not stay engaged with the gear it is supposed to drive, no matter how many times I reset them.  The Loco will only run for a few inches before the gears become disengaged. They are very sloppy.  Secondly, the sound board is erratic in it’s operation, most often only making a click-click-click sound.  Thirdly, the rear trucks hang up on an O-31 curve and cause the two A units to derail, every time, without fail.  If I were able to correct the first two problems, these units will not navigate a curve without the metal stairs being removed.  

It appears as though this set was a mess from the factory.  The first owner got a lemon, Trainz got a lemon, and I got a lemon.  If it can be fixed, I am not sure if it’s worth fixing.  It is a beautiful set, but I am not willing to put a bunch of money into it at this time.  And, I don't display trains, so it can't be a shelf queen here.

UPDATE:  It turns out that the motor mount screws were loose.  After finding them, I tightened them and the loco now runs fine.  Removing the ladders at the end of each cab eliminated the derailing problem.  The only remaining issue is the sound board.  It only goes clickclickclick without a battery.  Instilling a battery causes the board to work, but only sometimes.  Also, at shutdown, the soundboard continues to remain active until I remove the battery.  Sometimes, it works perfectly and shuts down on it's own.  I may put a kill switch in for the battery.   I kind of like the sound, but don't want to install battery for each operating session.  

Thanks for the posting the solution. Too many times a solution is not posted leaving everyone thinking it’s another factory design error.

Good to hear you got the running part fixed. For the sound issue I would try re seating the boards a couple times,there maybe oxidization of the pins. Also check to make sure the two chip on the railsounds board are fully seated.

Doug

I suspect that reseating the two socketed chips on the board may fix the sound issue.

Well, I removed both boards sound and power, wiped down contact pins and replaced boards.  I don't know how to get the two chips out of the sound board, but they seemed to be seated tightly.  Put everything back together, and now there is no sound at all, nothing, nada.  I found replacement boards at Lionel, but they'll cost $85 plus shipping.  I don't think I want to invest that much into $150 set.  The board part numbers are 691PCB1110 fro rail sound 4 conventional diesel, and 691PCB104D for power board.  I think I may have shorted them when I took them out.  I forgot to remove the battery first.  Anyway I was wondering it anyone has some used boards they might be willing to part with.  

You need a PLCC Puller to remove and reseat the chips on the RS boards.  I find this to be a rather common issue with the older locomotives.

Okay, if anyone is following this, I still need help.  I replaced both boards, the sound board and the power board.  Fired up the loco and all I get for sound is a sort of rata-tat-tat sound that can be increased or decreased with POT.  I checked the speaker and it is moving with the rata-tat sound.  this is the same sound I got before I changed the boards, and before I shorted out the old boards out.  Gunrunner john confirmed that my old boards were bad.   The new boards came from Lionel.

I was thinking perhaps the motherboard might need to be changed. I can only see one electrical component on the board and it does not appear to be cracked or broken anywhere.  

Does anyone have any ideas?   

Did you try doing a engine reset?

Doug

This is a conventional engine.  Don't know if any reset is possible.

Sorry about that,I was thinking it was TMCC.

Doug