How to Install a DCC Decoder

NCE decoder installed on an Athearn HO locomotive frame and motor
©2010 Ryan C. Kunkle

Many locomotives are available today with Digital Command Control (DCC) decoders factory-installed. But what about converting older locomotives? Installing a decoder is easier than you think.

Although there are many differences in locomotives, DCC standards make the process similar for most models. Once you identify the parts and decide where to place the decoder, the rest is straightforward. Many decoders simply plug into the boards on the model. If hardwiring is required, a little soldering is all you'll need to add a decoder in minutes.

There are many decoders available with a variety of special functions. Choose a decoder that has the functions you desire, but also one that is rated for the current draw of the locomotive.

Steps for Wiring a DCC Decoder​

Here's what you'll need to know to install a DCC decoder.

Difficulty: Easy

Time Required: 30 to 90 minutes (Time varies widely by model complexity.)

What You Need:

  • Decoder
  • Wire strippers
  • Soldering iron/gun
  • Solder
  • Flux
  • Heat shrink tubing
  • Small screwdrivers (for removing shell)
  • Electrical tape
  • Volt meter
  • Assorted resistors (for light bulbs)

Here's How:

  1. Remove the locomotive shell. It can vary greatly from model to model. If available, use the parts diagram to disassemble the locomotive.
    Newer locomotives often have circuit boards to control headlights or other functions. Many of these boards also include a standard plug that will accept a plug-in decoder. Other boards may have to be removed to make the connections and for space for the decoder itself.
  2. Isolate the motor. The decoder must be installed between the pickups from the wheels and the motor itself. Some models use the locomotive frame to transmit power to the motor. If necessary, the motor can be isolated from the frame with electrical tape.
  3. Connect the track leads. Attach the BLACK wire from the decoder to the electrical pickups from the left side of the locomotive. Attach the RED wire to the right side pickups. Depending on the model, there may be multiple contacts or only one for each side.
  4. Connect the motor leads. Attach the GRAY wire from the decoder to the left or negative motor lead. Attach the ORANGE wire to the right, or positive motor lead.
  5. Connect the headlight(s). Attach the WHITE wire to one lead of the headlight. Attach the YELLOW wire to one lead of the backup light if equipped. The BLUE wire is a common wire for the all of the lights.
    Low-voltage bulbs or LED's used in some locomotives may not hold up to the higher constant voltage of most DCC systems. A resistor may be inserted between the decoder and the bulb or LED. Resistor values vary. You could also replace the bulbs with LEDs.
  6. Connect any additional features. Ditch lights, Mars lights, sound or other features are found on some locomotives and supported by many decoders. PURPLE and GREEN wires are used to connect these functions if necessary. Most can be accommodated similarly to this marker light project.
  7. Secure the decoder. It is important to prevent the decoder and its leads from becoming entangled in moving parts or creating a short circuit. Decoders can be attached to the roof of the body shell with tape. For steam locomotives, the tender often offers the easiest installation.
  8. Reattach the shell. Replace the shell on the frame. Be careful not to pinch any wires. Do not try to force the shell. If the decoder or new wires cause problems, make adjustments.
  9. Program the decoder. Place the locomotive on your programming track and follow the manufacturer's instructions to program the address and other features.
    For a quick test of the decoder before programming, you can place the locomotive directly on the layout. Decoders have a standard default address of "3." Dial in the engine and test.

Tips for Installing a DCC Decoder

Use these tips for an easier installation process.

Heat Shrink Tubing

Heat shrink tubing is available from electronic supply stores in a variety of sizes. Cut a short length of tubing for each lead of the decoder and insert it over the wires before soldering. The tubing can then be slid and shrunk in place with the heat from the side of the soldering iron after making the decoder connections. Much more effective and easier to apply than electrical tape, the tubing will prevent short circuits from exposed wires.

Trim Wires

Most decoders come with very long leads. Trim these to length during installation. A little slack helps with installation, but too much becomes a liability when trying to avoid moving parts upon reassembly. Keep all those little extra wires; they'll come in handy in future decoder projects.

Take Notes

Keep a log of all the decoders you've installed and any special features you've included. These can be helpful years later when maintenance is required, and memory has faded.