Tap Creek

The Chicago & North Western Tap Creek Branch


DZ143 Decoder in GP18

Decoder Installation (DCC)
Digitrax DZ143 in an N scale Life-Like EMD GP18

Step 1 - Frame milling

Gently remove the shell and the trucks. The picture below shows the frame after this step, with the front of the engine to the left in the picture. The blue lines show where the frame will be milled or cut to make place for the decoder.

The above GP18 was originally a black Nickel Plate Road engine. It was my first attempt at painting and detailing a locomotive, and I think it came out alright as CNW 1779. But that is another story – Here I describe how I installed a Digitrax DZ143 decoder in that engine. The same principle ought to work for any brand of decoder, small enough to fit.

Take the frame halves apart and put the motor aside. Do not let it come near your work place while you are cutting and filing the frame. You would not want to let the metal dust get into the motor.

I used a cutting disc in a Dremel motor tool to cut the frame halves, one a time, as indicated by the blue lines in the picture above. Use a file to smooth any sharp edges. The picture below shows the result. Do also remove the small rectangular “tap” at the top of the right frame half (encircled in green). This will make the wiring easier.

Step 2 - Contact and motor housing preparations

Drill two holes, one in each frame half, and insert a small screw in each. The screws will be used to solder the decoder electrical pick-up wires (black and red) to the frame. This because soldering directly to the zinc alloy frame is more or less impossible. Since the frame is quite soft it is not necessary to tap the holes.

File some groves in the plastic motor housing – one horizontal grove at each end of at the top, and one vertical as shown in the picture below. The groves will later house some of the decoder wires.

If you look carefully in the picture above you will notice that the blue wire splits in two. The soldering that was done to achieve the wire split is protected by the black heat shrink tub seen in the picture. One of the blue wires will be used for the forward light and the other will be used for the rear light.

At this point you might take the engine for a test drive, just to make sure everything is OK so far. Any problems are more easily corrected now, before any lighting wires are in place and before the shell has been re-installed.


Step 3 - Motor wiring

Cut off the contact strips that originate from underneath each motor brush cap - you do not want the motor to make direct contact with the frame anymore. Then cut the grey and orange decoder wires to length and solder to the motor brush caps. The grey wire goes to the bottom cap and the orange wire goes to the top cap. Thread the grey wire along the grove you filed earlier and up to the top brush cap where it joins the orange wire. Reinsert the motor between the frame halves, and thread the wires between them and towards the rear.

Cut the read and black wires to length and solder to the screws earlier inserted into the frame. The red wire is soldered to the right frame half and the black wire to the left.

Step 4 - LED wiring

Now turn your attention to the front LED circuit board. Turn it upside down and, using a file, remove the metal contact leads used to make contact between the frame and the LED board. Make sure you remove enough to ensure that no electrical contact is made between the board and the frame when you reinsert it into its slots in the frame. The picture below shows the board after this step.


Compare this picture to what your LED board originally looks like and you will see what metal needs to be removed.

Before you reinsert the LED board, thread the blue and white wires between the frame halves towards the front of the engine. Then reinsert the LED board, cut the wires to length and solder them to the board as shown below.

Step 5 - Winding up

Using a file, remove the metal from the front of the rear LED board. Do this to ensure that it will not short-cut against the frame or the newly installed screws when placed on top of the decoder. Cut the so far unused strand of blue wire and the yellow wire to length and solder to the rear LED board. The blue wire goes to the front resistor leg on the left side, and the yellow wire goes to the right-hand LED lead.

Do also cut the green and violet wires so that they can be smoothly tucked away along-side the decoder (they will not be used now, but might be in a future project). Using some tape, secure the whole package—decoder, wires and LED board—as shown below.

The blue wire is soldered to the rear resistor leg and the white wire is soldered to the left (right in the picture) LED lead.

If necessary, file the top of the rear LED to make sure it reaches no higher than the frame itself. If it does, you will not be able to seat the shell properly.

Re-insert the frame into the shell, and – that is it!

After-thoughts: When I hade completed my decoder installation, and tested the lighting with the shell in place, I found the front LED to be quite weak. It hardly shows when lit. I might replace the front LED with a LED with a higher intensity. You might want to consider this from the very beginning.