Lionel Alaska GP7 (6-28857) - is this a K-line drive?

Well, that's the K-Line truck design if that's what you mean.  Truthfully, if you have a brass gear you can press on and will work and mesh properly, that's the one I'd use.  The plastic gear has probably split and that's why it's loose.  If you press a brass gear on, make SURE you support the shaft on the opposite side of the motor or you'll wreck the motor.

This engine is known for this problem. 100% failure rate on this model and I think it was the only time Lionel tried to use a plastic pinion gear on an engine. Within 6 months, replacement motors were available with brass pinion gears. I’ve replaced motors on probaly 30-40 engines over the years.

@Kelunaboy posted:

Just for personal knowledge, is this a K-line drive?

Not sure what you mean by "K-Line drive?" While this truck design was used by K-Line in the S-2, Alco FA, RDC Budd car and the MP-15, it was used by Lionel first in all of their starter set types of diesel locomotives.

Matter of fact, I can tell by the photo this is unquestionably a Lionel version (aside from the extra and different colored wires) because Lionel to my knowledge has always placed the traction tire wheels both on the same geared side of the engine. K-Line put them all over the place, but most frequently in a diagonal arrangement, with one traction tire wheel on the geared side of the loco, and the other on the other axle on the other side.

I think Lionel's traction tire arrangement is much better and doesn't cause the same amount of "gear growl" as do the K-Line engines mentioned above.

But Ryan Selvius is correct: On this particular 2005 cataloged engine, Lionel used a plastic pinion gear right off the motor, instead of the usual brass gear. And as such, there have been many documented problems with that gear breaking on this particular engine.

It would be easier to get a Lionel replacement motor with the brass gear. FYI, a similar K-Line motor will not work because the number of gear teeth on the pinion gear on the motor shaft is different on K-Line's motor versus the Lionel one.

Thanks everyone!   I used term the "K-Line drive" because it resembles the drives in my K-Line locos and I recall seeing "K-Line by Lionel" or similar phrase.

Anyway my pinion swap hasn't work so far.  Supporting the shaft, I tapped a brass gear on - went on with single light tap.  Mesh feels fine when turned by hand,
but when assembled and on the track I hear the motor spinning at high speed and truck gears are not engaged.  Not sure what is happening until I can power
it up on the bench.  My dad taught me to be a DIY guy so fixing this going to happen!

Looked up the parts on the Lionel site and the motors are available as is the truck assembly below.

I believe W/SNSR refers a sensor in the truck connected to the sound board.
The purple-orange-black ribbon cable plugs into a board with chip labeled "diesel",  in the lower left side of photo.

I'd just buy the truck, it's not that expensive.

Yep - just ordered the truck and a spare motor with gear - just in case.

Price was reasonable - first time ordering parts and the process was straight forward.

Alaska #1804 is out of the shop and running like new!   Both motors had a problem with the original plastic pinion gears, I replaced one with a slot car pinion I had, and used the new motor with pinion on the second truck.  Could have used a exploded view parts diagram to guide me, but I worked it out and only required one trip to hardware store to replace the "speed nut" holding on the middle gear.

Gaining confidence with train repairs and find it as enjoyable as running trains and layout building.

Thanks to everyone for the assist!

Alan