Decoder Harnesses & Cables
One of the areas of confusion in decoder installations is the issue of cables, plugs and harnesses.
NMRA Color Code
(revised Jan 2008)
RED from right-hand rail power pick-up (or center rail, outside third rail, traction/overhead wire) to motor or
interface
ORANGE from interface to motor brush (+) connected to right-hand rail (or center rail, outside third rail, traction
wire) [ed. the motor wire previously connected to where the red wire is]
BLACK from left-hand rail power pick-up to motor or interface
GRAY from interface to motor brush (-) connected to left-hand rail [ed. the motor wire previously connected to where the black wire is]
WHITE front headlight(s) power sink
YELLOW rear headlight(s) power sink
BLUE common (+) headlight(s)/function(s) power source
BLACK with WHITE stripe
common (-) power sink
NEM652 - NMRA 8-pin
The photo (left) shows the “bypass” version of this plug used
in “DCC ready” locos. With this plug in place, the motor and
lights are connected to track power. When this plug is removed and a decoder
plugged in its place, the loco can run on DCC. Since the locos are frequently
tested with this plug in place, the manufacturer has no way of knowing if
there are shorts in the wiring which this plug is masking. The user frequently
finds these errors when he plugs in a decoder and toasts it.
Bruce recommends that you use an ohmmeter and the drawing at the right
(click for large version) to measure the socket once the plug is removed.
If you find resistance below about 10 ohms between any two pins on the socket,
you probably have a short in the loco. Check this out BEFORE
you install a decoder. You should measure about 12 to 50 ohms between
pins 1 (orange) and 5 (gray) - the motor windings. You may measure something
between pins 7 (blue) and 2 (yellow) and 7 (blue) and 6 (white) - the lights.
Some locos use pin 3 for the green wire, although it is not defined by the
NMRA. You should NEVER measure anything less than many thousands of ohms
between 1 and 8 or 4 and 5. If you do, there is a motor to rail short in
the loco.
A plug is available from Litchfield Station, (right). This plug can be
wired to a decoder, using the colors shown in the drawing above. Then the
decoder of your choice can become a “plug-in” style.
This plug can also be used as a socket. You can wire to the pins (Bruce recommends you use shrink tubing to insulate between them) and make your own harness to connect a decoder into a loco which was not originally equipped with an NEM652 socket.
Since this product has male and female sections each of which are compatible with NEM652, it can also be used as a stand off to raise the connection to a plug equipped decoder about 1/8 inch (3.5 mm).
NEM651 - NMRA 6-pin
Many small locos made in Europe use the NEM651 6-pin inline plug and socket.
Decoders which fit this socket are rare, but Bruce tries to stock what he
can. Search the store using "NEM651" to see what is available.
An example of a decoder using this plug is shown (left). Since there are
only 6 pins, the connections to the rails (2), motor (2) leave only a single
connection to control each of the front and 
rear lights. The lights are connected
with one lead to track power and the other lead to a pin on the decoder socket.
Here is a photo of a socket commercially availalbe for this standard, showing NMRA compliant colors. The power source for the headlights is one of the rails, either red or black wire.
JST - 9 pin
Many medium sized (HO) decoders use a white 9-pin plug, known as a JST plug.
Some “DCC ready” locos are coming with sockets for JST connectors
already installed with a “bypass” board (as shown in the photo
on the right) connected for DC (analog) running. Using the colors of wire
in the harness which connects to this board, test for shorts, as discussed
above. Bruce has seen MANY locos with these plugs and shorts! A
few minutes with an ohmmeter may save grief and money, as some decoder
manufacturers will NOT replace “toasted” decoders for free.
Decoders with JST plugs are frequently supplied with a harness with about
7 inch long wires, although more and more manufacturers are selling the decoders
without wire harness, for users who want to plug them into properly equipped
locos.
One of the quick ways to get a compatible loco running on DCC is to use a custom JST harness. Many of these harnesses rotate the decoder for ease of installation. As an example the photo (below) is of one for the Stewart FT units - one of the easiest “plug-in” installations out there. Notice how the NEM652 is wired so that the decoder fits into the space available on the loco, as shown in the photo (click to view installation - use your BACK button to return).
NOTE: Installation information on this site represents Bruce’s experiences and opinions. They are presented without warranty as an example of ways to utilize DCC products. The user is solely responsible for any use made of this data.