Repowering an old AHM GE Center Cab Small HO Diesel


I asked a few questions on "another" publication's forum about this topic, but I am still seeking some solid advice on this first loco rebuilding project.
First- I picked up an old AHM Center Cab for literally a few bucks at a recent local swap meet. The shell was in very good condition, and I used my "purple stuff" to get every bit of the original paint off. I was lucky to find a recent "makeover" physical rebuild and repaint article on the Tyco forum about this same loco, and have made significant appearance improvements so far, as I have just finished making wire rails and giving it a primer coat.
Second- the loco ran haltingly on my test track, so I used 400 wet/dry paper to clean the wheels, the metal contact wipers and the original Mehano can motor contacts , as well as the small spinning thing ( forgot it's name) just ahead of the armature. The windings look very bright and no carbonization or soot appears on them. I also thoroughly cleaned the gears and other internal areas and have auto trans fluid ready as a relubricant. I have already made plans to install two LEDs with appropriate resistors to power the front and rear lights in place of the single small build that was there in the front. I have already also set up fresh multi strand color-coded wiring for the motor.
Third-this model runs at light speed and has poor slow speed performance, as per my experience with it before breaking it down-very similar to an MDT Plymouth Hustler or Model Power Porter, whether new or old. I have a loco servicing/ freight yard, so slower speeds are important to me.

Now- having said all of this, I was curious if it would be a good idea to get one of those electronics supply website small 12v DC motors, instead of a more expensive NWSL one to repower the loco. I found two with top (12vDC) rpm of 11000 and 8600, as well as a single spindle, as my present original has, but have no clue about the characteristics of my original- other than the warp speed nature of it. The plastic worm and gearing are in good condition, but may be very helpful in reducing the gearing ratio. Would a lower top rpm speed motor be more helpful? I saw where an Atlas metal "single lead" worm gear was suggested as a speed reducer.
We are only talking about spending less than $10 at this point, as I made the upgrade physical parts of scrap styrene and .020 steel wire already in my parts bin and I have plenty of paint choices on hand in my workshop.

I am learning a great deal with this project and fellow modeler's advice would be helpful.


Any advice or suggestions would be appreciated

DKRickman's picture

DC or DCC?

First of all, are you a user of DC or DCC?  DCC has some advantages here, as it will let you tune the voltage curve that the motor sees relative to the throttle position.  I do not know if the same curve is applied when using a DCC decoder on DC track.

In my humble opinion, the only viable way to reduce the top speed is to remotor the model.  A single start worm will work exactly the same as the (almost certainly) single start worm in the model right now.  A double start work would cut the speed in half, but I do not know of ANY double start worms available for the HO market.  A lower speed motor would do a lot.

If you cannot find a lower speed motor, and/or you do not want to use a DCC decoder, the only other option is to re-gear the drive.  You need to increase the total gear ratio, by adding another reduction or something equally drastic and creative.  Re-gearing is not easy to get right, and will almost certainly require that you have a ready supply of gears (from scrapped models?) on hand.  The only other option is a NWSL or similar re-gear kit or completely new gears - either of which could easily cost more than the original model.

My suggestion?  Use the model as-is, using a DCC decoder or resistor to slow it down.

Ken Rickman

Danville & Western HO modeler and web historian

AHM Center Cab Project- Pt. 2

To Ken Rickman:

Thanks for the input. I am using DC at this point, until I join my loco servicing/freight yard to a larger layout segment.

My research into a similar, but lower max. rpm motor seems to be along the lines you suggest, but how would using a resistor in the basic motor electrical connections work? Please describe that aspect (and how to do it), as it seems to go along with the low cost route I have been taking.

Many thanks for your help.


A resistor in series with the motor reduces voltage.

A resistor wired in series to one lead going to the motor will cause a voltage drop at the motor, thus reducing the speed.  The voltage drop will be a percentage across the speed range of the motor, but whether it will change the tendency of the motor to act like an off-on device, I'm not sure.  Certainly a can motor will help.  You could try the cheaper option first to see if it works, and definitely get the motor with the slower speed.  I would try the can motor first, and only install a resistor if the motor starts running too fast as soon as the throttle is cracked.

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