Stuck - Repair/DCC Install in LGB 20851 Mallet



Active Member
Hi Folks - I'm doing a DCC conversion plus repairs on someone's older LGB 20851 two motor block Mallet locomotive which has an existing Phoenix Model 97 DC/DCC Sound Decoder installed. The loco's owner stated he probably striped the idler gears in the rear motor block during a recent railway crash, and when I removed the motor, the two damaged idler gears were obvious. I've done the following: Installed a Massoth XL Power Decoder; installed Massoth DCC Pulsed Smoke Generator; Installed 2 new Idler Gears in each motor block (front motor block also showed some slight damage); installed new LGB/Marklin E126050 motors in each motor block which required bending the motor's electrical pins for proper motor block fitment (see photos of older LGB angled motor pins v. new Marklin motors); replaced two defective carbon brushes in front motor block.

Problem: The loco's motor blocks run smooth when operating on my Piko DCC Central Station test stand or when tested with DC power. But when I test operate the loco on my indoor DCC layout powered by my Massoth Command Station/Massoth Power Supply, the loco doesn't operate smoothly. There's very slight continual surging (or jerking if you prefer) noticeable at Navigator speed setting 1, 2 , 3 etc. I took the bottom covers off both motor blocks and realigned the wheels/gears to ensure there's properly meshing, no binding, and the loco still doesn't run smoothly. I can't figure out why this loco isn't running smooth as silk like all my other LGB locos. What diagnosis am I missing.......advice please!



DOGS, Garden Railways, Steam Trains, Jive Dancing,
25 Oct 2009
St.Neots Cambridgeshire UK
I am assuming that when you say the Motor Block runs smoothly will this be without the Coupling rods and valve gear fitted? If so this sounds like a perfect description of not properly aligned setup. You need to carefully view the Axles of the Wheels at a certain point say the first one on the balance weight. Check the next geared set, if they are minutely different the cause is that the Cogs on one gear could be one or two cogs out of alignment. To fix undo the wheel base support move just one the geared Axles till it is in perfect alignment. The Coupling Rods will take care of the Centre Axle. Check this out for both Bogies..


retired at last
27 Apr 2015
I have had this on my Sumpter mallets and I installed new gearboxes and gearbox base plates. The lugs on the base plate that hold the axles wear and don't push the axle gears up to mesh with the idler gears. Old and new pics.....


Active Member
This morning I replaced the Massoth XL Decoder with a brand new one. The one I had put into the Mallet was originally used on my test bed loco and may have made some CV adjustments to it. The Mallet operates almost perfect now for well used older model.

Since the decoder was the problem, I also pulled the new Marklin motors out and put the old LGB motors back in........they still have some good life in them. Originally the rear motor block wheels were going much slower than the front motor block wheels, so I replaced the motor in the rear motor block. And I had originally only replaced the rear motor block idler gears but then later some slight damage to the front motor block idler gears too and thens replaced them. So, the loco is operating pretty smoothly now........I've been pulling a set of three RhB passenger cars for 15 minutes now and she's doing fine.

This has been a very frustrating incremental process dealing with these two old motor blocks. This Mallet has been used quite a bit based upon the condition of the wheels, and this is the second owner. Eventually, when the motors need replacement, it will probably be best to replace the motor blocks due to the normal wear and tear of the axles on the supports. l also appreciate dunnyrail's advice on the axle/gear/wheel alignment which I've encountered with Mogul repairs.
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