Installing your first mobile decoder

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Don't let the smoke out! - Installing your first mobile decoder

You can buy locomotives with DCC decoders already installed, or you can install your own decoders into your models.

Electricians have an old saying: 'Don't let the smoke out.' If you notice a component smoking, it is most probably ruined! So always take care not to let the smoke out.

One way to protect your investment and do a good installation is to read the manual that came with the decoder. Installing a decoder without reading and having the manual handy is asking for trouble.

For our first installation we will use an old, reliable Athearn locomotive. It is a good choice since it is not "DCC ready" and does not come with an NMRA 8 pin plug. It does not have an electrically isolated motor.

Also read the Decoder Installaton page for additional important details.

Installing a Decoder In An Athearn Locomotive

Steps to installation:

  1. Run the loco on a Direct Current layout if possible. Does it run well? DCC decoders will not make make bad running locomotives into good running locomotives. If it runs fine, go to Step 2. If it does not, check out the model to see why it does not run well. It may need lubrication, or cleaning, etc.
  2. Remove the shell.
  3. Remove the metal strip that runs from the front truck, running over the motor to the rear truck. You will not need this part.
  4. Remove the motor from the chassis and try to not damage the rubber motor mounts, as we can re-use them. Gently rock the motor back and forth. Replacement parts are available from Athearn. Sometimes these mounts become brittle and must be replaced.
  5. Isolate the motor from the chassis. The metal of the motor must not contact the metal frame in any way. We can use electrical tape or Kapton tape for this. Place a piece of tape on the metal frame where it contacts the motor housing. To be safe, do the same thing on the bottom of the motor housing.
  6. Look at the manual that came with your decoder. There should be a diagram depicting the actual decoder, and the different colored wires and what they are for. Decide in advance which wires you will use for which lights.


Here are the 'standard' wiring configurations:

    • Note: Decoder Wire Color Code
      • Red Wire - Right rail (Engineer's side)
        Black Wire - Left rail (Fireman's side)
      • Orange Wire - Motor +
      • Gray Wire - Motor -
      • White Wire - Forward Light (F0 FWD)
      • Yellow Wire - Reverse Light (F0 REV)
      • Blue Wire - Function Common (+)
      • Green Wire - Function 1 (F1)
      • Violet Wire - Function 2 (F2)
      • Brown Wire - Function 3 (F3)

Basic Installation

  1. Solder the gray wire to the bottom motor strap.
  2. Solder the orange wire to the top motor strap.
  3. You may wish to drill a small hole in the frame while the motor is removed for a later step. Now carefully press the motor back into place.
  4. Solder a piece of wire (preferably red) to the metal piece standing up on the front truck to the metal piece standing up on the rear truck. This jumper will pick up one side of track power.
  5. Solder a piece of wire (preferably black) to the frame. Some modelers drill a tiny hole in the frame and use a press pin or screw to attach the black wire in order to have a better electrical connection. This will pick up the other side of track power.
  6. Solder the red wire from the decoder to the red jumper wire you previously installed.
  7. Solder the black wire from the decoder to the black wire attached to the frame.
  8. Solder the forward light to the blue and the white wire. The light bulb or LED must match the current output of the decoder or 'smoke will be let out.' Use resistors if necessary.
  9. Solder the back light to the blue and the yellow wire.
  10. Solder any other functions to the blue wire and the function.
  11. Test the model.
  12. Reassemble the model and enjoy it.
If the locomotive runs backwards, that can be corrected by swapping the Orange and Gray wires, or changing the NDOT setting.