I chose to install the Digitrax DN142 decoder in the Kato U30C because I had the decoder available and also because it would fit "easily" into this large loco. If I didn't have the DN142, I would probably rather have used a DZ121, or nowadays a DZ143, since it's smaller which means less frame milling, and it's also a little less costly.


The complete shell lifts off the frame easily. Remove it, and then undo the two screws that hold the frame halves together. With the screws removed, the trucks can be removed easily by just pulling them out. Also remove the frame wipers and the light boards.

With the frame lying on it's right side, prize the two halves apart carefully and lift off the left half. Mark the top of the motor with a permanent marker, to make sure you'll know which side is which when you get to soldering wires to it, and to reassembly. Then remove the motor and the drive trains, as well as the bearing retainers on the right frame half. Finally, remove the motor from the motor saddle.


Like with the Atlas GP30 and GP40, I put the decoder below the rear light board, and I milled out the rear end of the frame to 6 mm deep and 29 mm long to make room. This is done with a vertical cut straight down from the top of the frame, just to the rear of the slots into which the rear light board fits, and a horizontal cut just above the top of the rear screw recess. Apart from that, I also cut in under the rear light board's slots so the wireless end of the decoder can fit in there. This may make the rear light board slots untrustworthy for power pickup, but they will still serve to hold the light board in place above the decoder. On the picture I show you an unmodified (top) and modified frame (bottom).

With the shorter DZ121 and DZ143, this undercut would probably be unnecessary and the rear light board would still be usable as power pickup, as I did with the Atlas GP30 and GP40.

I cut a groove, 5 mm wide and between 1 and 2 mm deep, lengthwise along the centre of the top of the assembled frame halves. It runs between the lips of the light board slots, to accommodate the wiring that goes to the front light board and the motor. Again, the modified frame is the bottom one.

And I milled out recesses in the insides of the two frame halves where the brush caps in the unmodified model touch the frame halves for power pickup, to isolate the motor. On one frame half I also milled out a slot to accommodate the grey wire to the bottom motor brush. I also made a hole in the top of the assembled frames just above the brush cap position for both motor wires to pass through.

After milling, brush off and, if necessary, wash the frame halves thoroughly to remove all milling shavings.


On both light boards, cut through the trace that's next to the number on the board, halfway between the LED's solder spot (on the LED leg opposite the one that's connected to the resistor) and the contact pad that fits into the loco frame. Just a brief touch with a Dremel or Proxxon or similar tool's cutting disk will do the trick. On the opposite side desolder the end of the resistor that's closest to the contact pad. It's easy if you insert the tip of a small screwdriver between the resistor's wire and the light board, and touch the solder spot on the board with a heated solder gun until the resistor's leg pops out of the board. On both light boards, bend the half-desoldered resistor around 180 degrees so that the resistor is positioned next to the LED.


Unless you're very sure of your soldering abilities, do not attempt to solder to the brush caps directly without having removed them from the motor. If you apply heat for too long, (a second comes close to be too long!) the heated caps will melt the surrounding plastic parts of the motor and the motor may not work so good after that. Remove the brush caps carefully - remember that there's a brush and a spring inside that will certainly try to go into orbit if you're not careful! Remove the wipers and discard them (read: put them in your spare parts box), solder the grey and orange wires to the brush caps, and then reinstall the brushes with the grey wire to the bottom of the motor and the orange to the top.

Make doubly sure that the motor is properly isolated by installing it, in its saddle, first in one and then in the other of the two frame halves and checking for contact between the brush caps and the frames. Just to make doubly sure, I put strips of Kapton tape on the inside of the frame where the brush caps may touch, even though I had already milled out cavities. Much like wearing a belt AND suspenders! Your meter MUST show NO contact! If it does, go look for the contact and fix it before proceeding.

The grey motor wire gets routed around the side of the motor in the slot that was cut out for it, to be out of the way so it can't get pinched when the shell is replaced.

Assemble the two frame halves, with the bearing retainers, frame spacers, motor, drive worms, trucks and front light board in place, and with the grey and orange wires passing through the hole in the top of the frame halves. Leave the side truck wipers aside for now. Tighten the frame screws, and check again for shorts. If there is a short, open up again and find and fix it before proceeding.

The decoder can then be placed in position under the rear light board, with the wire end to the rear of the loco and the grey and orange wires folded over the top of the light board past the LED. Although the light board will perhaps no longer fit properly if it's slots were undercut in the milling process, I've found that the decoder and light board hold each other nicely in position.

Carefully part some of the blue wire's insulation at a point close to the decoder (about 10 to 15 mm away) so that the cleared spot will be able to reach the free end of the resistor on the rear light board, while still leaving some slack - I used my fingernails, but be very careful since it will cost you $17 plus shipping (or $23 plus shipping if you're in South Africa, like me) should the wire get separated from the decoder. Then solder that cleared spot on the blue wire to the free end of the resistor on the rear light board. Insulate the solder joint.

Then string the rest of the blue wire along the top of the frame, cut it at a suitable length (remember to leave a little slack - about 10 mm will be more than sufficient) and solder its end to the free end of the resistor on the front light board.

Next, cut the yellow wire to a suitable length, remembering to leave some spare for slack, and solder it to the rear light board's LED leg that's NOT connected to the resistor. In other words, solder the wire to the solder spot on the rear light board that's right next to the cut that was made in the circuit board's trace. If you're installing a DN142 or DZ143 and do not need the green and violet leads for other functions like ditch lights, you can also solder the green function lead to this same spot. (See below.)

Then cut the black wire at a suitable length (remember slack!) and solder it to a spot on the front light board just to the rear of the spot where you had cut the trace (on the left side of the loco). Next do the red wire (remember slack). It goes to the spot on the front light board where you had desoldered the one end of the resistor. Finally, solder the white wire to the front light board's LED leg that's NOT connected to the resistor. In other words, solder the wire to the solder spot on the front light board that's just ahead of the cut that was made in the circuit board's trace.

And if you're installing a DN142 or DZ143, you can also solder the violet function lead to this same spot. This way it is possible to program directional blinky light functions of your choice to the F1 (green) and F2 (violet) leads. Then, with F0 off and F1 and F2 on, the loco will have the blinky lights of your choice, and when F0 is turned on (without having to turn off F1 and F2), the lights will be on steady. (On DT100 throttles F2, being intended for the horn if sound is used, is on only while pressed. To keep it latched on, press F2 and while holding it down, also press Run/Stop. Pressing F2 again will unlatch it again.)

Tuck the wires into the groove cut in the top of the frame. And that should be it!


Install the side truck wipers. Then fit the fuel tank - it helps to hold these wipers in position. And install the headlight cover, making sure that all wires pass on the sides of the LEDs so that the light will not be obstructed. Leave the shell off till later.


While you program your loco with its shell off, watch the lights - the headlight will flash each time the decoder acknowledges to the programmer, and the motor will also jerk or turn slightly. But if both lights start flashing at you repeatedly, you have a short, which you will have to fix before proceeding. The decoder will refuse to be programmed anyway while the short still exists. After programming your loco on the programming track or PR1 setup or whatever you use, it's time for the final safety check.

Take the loco to your layout, and while leaving the loco OFF the tracks, dial up its address on your throttle, and make sure that the speed is at 00. (Zero, nil nil, double-oh, zilch..... nothing else!) Then put the loco on the track and watch its lights. Again, if they both start flashing at you, remove the loco from the tracks immediately and go look for the short that you missed! Whatever you do, leave the throttle at 00 while the loco is flashing its lights at you, since if you should turn the throttle clockwise in the slightest, the decoder will send power to the motor, and there will be some very pretty lights of brief duration on the decoder and you will actually see $17 worth of smoke waving goodbye at you! (Follow the same procedure after you've finally replaced the shell - see below.)

If the decoder's lights don't flash at you, however, there should be no short and you can test-run the loco.

And you're almost all done! Replace the shell, check for shorts AGAIN (flashing lights) - I've seen how the pressure that the shell has on the sides of the frame can activate a potential short, by pressing items together that were almost but not quite touching with the shell off. If all is still well, add the loco to your fleet of decodered machines and have fun!


In the milling process the rear set of raised "buttons", one on each side of the frame, that clip into indentations on the inner sides of the shell are lost. The result may be that the loco tends to lift its rear end because the shell is lifted off the frame by the decoder, or by the decoder wires, or because the rear LED touches the numberboard lens, or whatever. It may help some to prevent this if you tape the rear light board and the decoder in position on the frame. If that doesn't help, apply a very small spot of silicon rubber on each side of the frame below the decoder, install the shell and wrap some masking tape around the assembled loco to hold the shell in position. Leave it overnight with the masking tape bandage to let the silicon set. The shell will then stay in position so that the loco does not look like a hyena any longer. Just be careful not to get silicon into the motor flywheel cavity or drive shaft assembly or the truck gears in the process!

If anything about the procedure is perhaps still unclear, or if you used this guide and it worked for you, please drop me a note.

Good luck, and keep them on track!


Terug na die Afrikaanse blaaie

The following pages contain decoder installation guides for some other N scale diesel locomotives:

Atlas Plug & Play Locomotives
Atlas EMD GP30
Atlas EMD GP40
Bachmann GE C40-8 & C40-8W
Kato EMD SD40
Life-Like EMD GP38-2
Life-Like EMD SD7 & SD9
Life-Like EMD SW9 & SW1200

And the following pages contain guides to servicing or performance enhancement of some N Scale diesel locomotives:

Atlas & Kato Diesel Truck Servicing
Life-Like SD7 & SD9 Power Pickup

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The Kgalagadi Transfrontier Park
The Story of a Kalahari Telephone Pole
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Created on 9 June 2002. Last updated on 11 September 2003.