Trouble w/ Legacy GP-9  6-28573 

Actually, I believe that model does have the Liondrive trucks.  You need to zero in, which axle has the least play.  You'll find that one of them has just a little play I suspect.

Mike, since you’ve removed the truck and wheels will not move it’s definitely a gear mesh problem. The idler gears have a tendency to engage on the extreme outside of the mating gear. Look closely for a gear not engaged properly. Also with truck removed you can try to spin the motor shaft to ensure it’s free and not damaged. 

Mike I'm not a diesel guy, but as far as I know the LionDrive system is still being used by Lionel to this day. 

The problem I've read about on the forum is that often one or more axles have too much side-to-side play, which causes the gears to wear uneven, disengage or even come off of their studs.  This problem isn't caused by LionDrive per se.  Hopefully others will chime in with a solution, besides just buying a new truck block from Lionel.

The side play seems to be fine, there is no abnormal gear wear.  A little prying did not move a gear- so they are not loose, and I can't see any ballast or other foreign matter in the teeth, 

The motor turns OK,  I'll hook up the motor to DC and see if it is burned out. 

What else, guys??

You’ll have to disassemble the truck then to see what’s wrong. 

OK- I think I figured everything out.  There is a small gear third aft in the chain between the main drive gear and the wheel.  That third gear in the chain had somehow frozen on its shaft. 

I pulled it from the frame (it's held in place by a knurl) took the gear off the shaft and polished the shaft using a Dremel wire wheel.   Oiled it, and pressed it back into the frame.  It LOOKS like it will work.

The red circle in a photo below of an earlier GP-9 shows the location of the frozen gear.  We'll see if the re-press holds, or if I have to use JB Weld or something to hold it in place.

@RickO Mike re-used that photo from an earlier post to illustrate his point.  That's not his loco; his doesn't seem to have a wide axle.

I'm more curious about what caused the gear to seize up.  Lack of lubrication?  Or did it wear oval and get cocked because the rear axle IS geared too wide, allowing it to wobble and partially disengage.  Is there a lot of slop Mike?  Or is it still a tight fit on the stud??  It might be instructive to compare the gauge of the adjacent wheelset to the other three on your loco.

There was no visible damage or wear to the shaft or the inside bore of the gear.  But it DID appear "dry"- as if the mfg. did not lube it at the factory.  It seemed to polish up completely.  All the teeth on that gear and the mating one appear fine after cleaning (plastic-safe contact cleaner).  And of course I did lubricate it and all others (light oil) as I re-assembled.

I ordered a new truck ($42) from Lionel- two main reasons- should this one fail again, and the fact that I have TWO of these GP9s, both NOS but some 12 years old.

I really like these locomotives.  And I can't rely on a long-term availability of these trucks.

Lesson: be sure to follow my normal practice.  Run a new loco out of the box to make sure all is OK electrically etc., then do a full lubrication.  

Mike, 

Do either of these units have the enormous side to side slop of the axles? Maybe an image of your trucks?

Like Rick says, shims on the axles at the indicated points.  I'd also check the gauging of those wheels, they may not be correct with that much play in the axle.  Those gears are going to chew themselves up if you leave it like that!