The easiest solution to decoder installation in the N scale Atlas GP40 would be to get hold of an Atlas GP40-2 or GP38 frame. A perfect fit in the GP40, it's decoder ready and just needs a Digitrax DN146A or DN163A0 dropped in. Then maybe you can negotiate to sell the old GP40 frame to someone like John Claudino at Aztec. Or you could get a decoder-ready modified frame from Aztec - John does excellent work. Described below is the quicker and cheaper, but slightly more difficult route, which entails some frame milling in order to make room for a Digitrax DZ121 or DZ143 decoder.


Like with the GP30, I put the decoder below the rear light board, and I milled out the rear end of the frame to 8 mm deep and 20 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 bearing retainer.

Since I had three GP40s to do and I did them one at a time, I was able to take pictures of a milled frame next to an unmodified one, for comparison.

I also cut a groove, 5 mm wide and about 2 mm deep, lengthwise along the centre of the top of the assembled frame halves between the lips of the light board slots, to accommodate the wiring that goes to the front light board.

And I milled out the two humps on the insides of the two frame halves where the brush contacts touch the frame halves for power pickup, to isolate the motor. This also resulted in a hole in the top of the assembled frames just perfect for the motor wires to pass through.

After milling, brush off and wash the frame halves thoroughly to remove all milling shavings. Use petrol, paraffin, or whatever will do the job. (Since model railroaders are sharp people, I believe it to be unnecessary to include a warning here about being careful when working with flammable liquids.)


Note that on the GP40 the front and rear light boards are NOT identical. The front board is marked "607-X003". I've seen two types of rear boards, one marked "607-X001R1" and another marked "607-X001". 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 to let the resistor's leg pop out of the hole.


Cut the gray decoder wire to a length of 100 mm measured from the end of the decoder's shrink wrap, and the orange wire to a length of 70 mm. This is much longer than necessary, but it's always good to have some slack when assembling or disassembling in the future. The rest of the wires will all be soldered to spots on top of the assembled loco frame and it will be easy to determine where to cut them then. Just remember to provide for a little slack in all the wires for the occasion when the loco may have to be disassembled again.


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! Solder the grey and orange wires to the wiper ends, and then reinstall the brushes with the grey wire to the bottom of the motor and the orange to the top. (The bottom of the motor is marked by a little hole in the nickel-plated motor jacket. One side of the bottom magnet is also painted.)

Make doubly sure that the motor is properly isolated by installing it, in its saddle, first in one and then the other of the two frame halves and checking for contact between the brush caps and the frames. It's a good idea to fold the wiper ends, to which the wire ends are soldered, around the cross members of the motor saddle the way it was before it was disassembled, and to cover them with 8 mm x 10 mm rectangles of insulation tape to make sure there's no way they can contact the loco frame. Your meter MUST show NO contact! If it does, go look for the contact and fix it before proceeding.

The next picture shows how the grey motor wire can be routed around the end of the motor to be out of the way so it can't get pinched when the shell is replaced.

On the rear light board, bend the half-desoldered resistor around 180 degrees so that the resistor is positioned next to the LED. Then assemble the two frame halves, with the frame spacers, motor, drive worms, trucks and both light boards in place, and with the grey and orange wires passing through the hole in the top of the frame halves. Note that the front light board is fitted with the LED on top, and the rear with the LED at the bottom. 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 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. You'll find that the rear light board holds the decoder firmly 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 could cost you $17 plus shipping 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 front light board's LED leg that's NOT connected to the resistor.

Next, cut the black wire at a suitable length (remember slack!) and solder it to the solder spot on the rear light board where you had desoldered the one end of the rear light's resistor (on the left side of the loco). (Refer to the previous picture.)

On the front light board, solder the red wire (remember slack) to the light board pad on the right side of the loco (the pad next to the spot where the trace had been cut). Solder carefully here so that the whole pad does not get covered in solder or it will not fit back into the frame slot.

Next, solder the yellow wire to the rear light board's LED leg that's NOT connected to the resistor. Simply 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. (Refer to the previous picture.) If you're installing a 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.)

Finally, solder the white wire to the free end of the front light board's resistor. It's not necessary to bend the front resistor around 180 degrees like with the rear resistor. Just bend it slightly towards the centre of the light board so it's clear of the spot where it was desoldered from. And insulate the solder joint. (The positions of the white/blue and of the yellow/blue wires on the two light boards may sound wrong when compared to each other, but trust me on this - remember, the boards are not identical and one is also installed upside down.

And if you're installing a 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/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!

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!

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!


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

Atlas Plug & Play Locomotives
Atlas EMD GP30
Bachmann GE C40-8 & C40-8W
Kato EMD SD40
Kato GE U30C
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 Story of a Kalahari Telephone Pole
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Created in January 2002. Last updated on 11 September 2003.