Posted by: bgtwindad | May 27, 2011

Model Power 4-6-2 Decoder Install

Untitled by BGTwinDad
Untitled, a photo by BGTwinDad on Flickr.

Over the last two weeks or so, I have been working to install a Digitrax DZ125 decoder into the tender of a Model Power N-scale 4-6-2 “Pacific” locomotive.  This was a “commissioned” job for a friend, who had the locomotive and decoder shipped directly to me from the store, and I will be forwarding it on to him shortly.

Even though I was using a Digitrax decoder, the installation instructions from the TCS website submitted by Jeffry Maurer of Sacramento CA — thanks, Jeff! — are perfectly suitable.  Jeff used a TCS MC2, I used the Digitrax DZ125.  I’m quite sure a TCS M1 or any other small hard-wire decoder would work just fine and install in the same way.  The tender is a bit cramped if you leave the weights in place, so you’ll want to use a small one.

(Note that Jeff claims that this install will also work for the Model Power 2-8-2 “Mikado”.  I can’t prove this but I have no reason to believe it won’t work.)

The basic decoder install is very straightforward, but the lighting update was a bit tricky.  Model Power has set this locomotive up to be “DCC Friendly”, and it is.  Power from the drivers is passed back to the tender via a red/black wire pair across the drawbar, combined with power from the tender wheels, and then returned to the motor in the boiler via  a separate grey/orange wire pair.  This little bit of extra wiring work on their part makes the decoder install very simple.

Inside the tender "Before" ...

To add the decoder, all you have to do is remove the tender shell, clip the grey and orange wires free from the black/red wires, and solder the color-matched wires (grey to grey, orange to orange, red to red, black to black) of the decoder to the tender wiring, tape everything down, and replace the tender shell.  Anyone with even a little bit of soldering skill should be able to do this much.

Inside the tender "After" ...

At this point, though, the (incandescent) headlight is still directly powered from the rails via contact strips on the driver axles.  It cannot be turned off, and will be fairly bright.  If you want DCC control of the headlight, we’ll have to do some minor surgery.  On this particular model, I decided to replace the incandescent bulb with a 1206 size SMD (surface mount) LED.  Unfortunately, I failed to take very many photos, so you’ll mostly ave to take my “word” for it.  The good news is that the instructions provided by Jeff above are exactly what I did with only a few very minor modifications.

The basic process is as follows:

  1. Separate the boiler and cab from the locomotive drive train
  2. Clip off the contact strips from the headlight assembly
  3. Solder two wires to the headlight and run them back to the tender
  4. Attach the wires to the blue and white decoder wires
  5. Reassemble the loco.

I found only one minor variation to Jeff’s procedure, and that is that apparently on the version of the model I have, the cab is not a separate piece, but comes off with the rest of the boiler.  Other than that, the process is the same.  Instead of painting the decoder wires black with a Sharpie, I cut the blue and white leads short and replaced them with lengths of black wire.

I also only had to make a few minor changes to swap in an LED as well.  First, back in the tender, I added a 1KΩ resistor on the blue wire.  This limits current to the LED and protects it.  Second, you replace the lamp with the LED, making sure to keep the polarity correct.  The cathode (“negative”) terminal of the LED is usually marked with a dot or other mark, and this should go to the white wire, while the anode (“positive”) terminal goes to the blue wire.  Keeping this straight is especially important if you have replaced the decoder wires with black wire.  To be sure of this – and to make handling the LED a bit easier, I soldered short lengths of blue and white wire to the actual LED pads, and then connected these blue/white lengths to the long black wires back to the tender.

SMD Led with blue and white leads attached...

One other change.  The headlamp (mounted on the drive frame) sticks up through a hole in the bottom of the boiler.  To get the 1206 size LED and its wires to fit, I had to square up the hole just a bit with a needle file.  I did so just enough so that the LED now press-fits into the hole in the boiler instead of mounting to the frame.

LED installed in the Pacific boiler

The basic decoder installation is very easy, assuming you know how to solder wires together.  A reasonable level of care and attention to detail will do fine here.  Connecting the headlamp up requires considerably more work, but even so, most of the trouble is mechanical – routing the wires unobtrusively beneath the running boards, removing the incandescent lamp, and reassembling the engine.  Having completed this one I concur with Jeff’s assessment, if an easy drop-in is a “1”, then the basic install is a “3” and the headlight work is a “6”.



  1. Many thanks for a great ‘how-to’. I have several of these MP Pacifics, they are fine runners as is, but I’m making the head-first leap into DCC.
    Mike DeSensi
    Pittsburgh, PA
    An ATSF modeler in the land of PRR

  2. Thanks, you saved me a lot of headaches. George in Highland village TX

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