The NMRA DCC specification was released in about 1993 but DCC didn't become really common until after 2000. When DCC was introduced, NO locomotives were specifically designed to accept a DCC decoder however, wiring one in was, and is, not very difficult. After around 2000, most locos were designed to accept a DCC decoder through some sort of plug and play method. Many come with a DCC decoder preinstalled.
This page is here to provide some guidance on how to install a decoder in a loco that didn't come with one.
There are basically four types of installation, which one you will end up doing is partly based on what you find when you crack open a loco an partly by your own personal preference. However, there is a basic set of steps that you can follow if you don't have a preferred method. Note that there is no hard and fast way of doing the installation, it is possible to hack pretty much any decoder into any loco as long as the decoder has the electrical capability to handle the motor in that loco.
This checklist is partly an introduction to the sections below that have more details.
RTFM. If you loco came with some kind of manual or instruction sheet, read the thing. Until you poke around inside, this is your best source of information.
DC Test. If a loco doesn't run well on DC, then it will likely not run well after DCC is installed. Take some time and evaluate your loco and fix whatever ails it. Pay special attention to reliability.
Visual Inspection. You will eventually need to open the loco to do the installation so that is next step. You will want to determine what kind of wiring that the loco has already and how much room there is so that you can make a proper selection of a decoder.
Decoder Selection. There are literally hundreds of decoders that you could use in any given loco, but only one or two types will be the easiest to install and provide the best results. Which one is best depends your loco and your preference, if you have one.
Wired. Using a wired decoder is usually the least expensive and, in the smaller scales, often the only option. You'll need a soldering iron and basic tools. Don't be afraid of doing it, it can boil down to just 4 wires.
Plug In. Some locos have a socket that can accept a decoder with a plug or socket on it. This kind of installation is mechanical only, no soldering required but the decoders sometimes cost a little more.
Drop In. Some locos have a printed wiring board (PWB) that can be outright replaced with a decoder designed for that kind of loco. These tend to be less expensive as they have no hand assembly steps, but they are a bit of a pain to install. Soldering is generally not required.
Sound. Sound decoders are more expensive, but most of them sound pretty good with a good speaker. The decoders are larger and may have a storage capacitor to mount as well. You also have to mount a speaker. These installations are usually the most work (due to the speaker) but, IMHO, sound is good. Sound decoders may be in any of the interfaces described above.
Either from information in the manual or by visual inspection, it is pretty easy to tell what kind of interface the loco has. The most common types are an 8 pin NRMA socket, a 9 pin JST plug or a "drop in" PWB.
Most locos will have just one type, but some late 90's locos had two kinds. This photo shows both. At the left is an 8 pin plug wired to a decoder. The board has a corresponding 8 pin socket, 2 rows of 4 sockets each. At the right is a white 9 pin JST socket. There might be a small jumper board plugged into either one. The jumper board makes the right connections if a decoder is not installed. The jumper board will be removed but should not be discarded. They come in handy at times for troubleshooting.
Some types of decoders can be ordered with an 8 pin plug as an option. However, HO type decoders probably already have a 9 pin JST plug integrated at the base of the decoder. In that case, any harness that came with the decoder is not used and the decoder plugs directly into the 9 pin JST socket and you are done.
Some more modern locos come with a PWB inside that looks something like this. This board also has an 8 pin socket. The only other parts on the boards are some diodes that deal with the headlights. An 8 pin decoder can plug in or the entire board can be swapped out for a "drop in" decoder. There will be drop in types for specific locos, check the part number carefully when you order. With a drop in decoder, you discard this whole board and replace it with a similar one but with all the DCC decoder parts on it.
If you want to use a drop in decoder, you will be swapping wires from this board to the same connection locations on the drop in decoder. Note that this is a tedious process but it is not difficult.
This is yet another kind of decoder interface. In this case, the loco is wired to a small PWB with an 8 pin socket that faces downward. It connects to 8 pins facing upwards from the PWB. In this case, the original PWB is discarded and a decoder PWB with a similar set of pins fits where the old PWB was and then the little connect board is pressed over it. Alternately, a decoder with a short 8 pin harness can be plugged into the little board. Be sure to insulate the top of the frame weight in either case so that none of the connections on the decoder or plug can touch the frame weight.
If you don't see any of these kinds of interfaces, then your loco was not designed with DCC in mind and you'll be installing a wired decoder.
The 8 pin NRMA plug typically supports 2 functions. The 9 pin interface can support 4 if the decoder can handle it.
There are lots of decoder types available, but under the hood, they are all pretty much the same. The biggest electrical difference is if it has sound or not.
Decoders are usually grouped by scale, G, HO, N, or Z. O scale DC locos will use G scale decoders. G scale decoders are usually rated for higher motor current, up to 8 amps continuos. HO and smaller decoders are usually good for 1 to 2 amps. Most small scale locos draw less than 1 amp so that you can pick pretty much any one that will fit.
HO decoders are usually just wide enough to fit in the shell of an HO loco and typically won't fit in N or Z locos. They also usually have a 9 pin JST interface at the decoder proper with an option for plain wires on an 8 pin plug harness. Pick any recent model that you like. One that have BEMF (back EMF) and "silent" running are better, they are newer designs that tend to work better. Many older decoders are very sensitive to power dropouts and misbehave very badly on dirty track. The newer ones do much better. TCS and Digitrax make the most cost effective basic decoders and both have a 1 year "goof proof" warranty. HO decoders will typically operate at the NMRA maximum track voltage of 27 volts so that the can be used even in smaller G scale locos where the current requirement is verified to be within the range of the decoder.
N scale decoders are typically physically smaller, but electrically equivalent to HO decoders. They tend to cost just a little more because of the smaller parts that they use. Also, no N scale decoder has a 9 pin JST plug, it's too big for N. Also many won't have an 8 pin plug either as many N scale locos don't have room for the socket. However, they are very useful in cases which are constrained for space, a common problem with steam locos.
Z scale decoders are smaller yet. They often have a somewhat lower current rating, but they are good enough for almost any HO loco. They also usually have lower track voltage ratings. However, for the typical HO track voltage range of 12 to 16 volts, they will do fine. In many cases, N scale decoders are falling out of favor because the Z scale decoders can do the job, are easier to fit in constrained spaces and the manufacturers would like to make fewer types.
You can find out more about some decoders at my Tsunami Tips, LokSound Tips, and Digitrax Tips. A lot more generic DCC info can be found at DCC Tips. Information on programming interfaces can be found at Programming Interface Tips.
If your loco is old or brass it will probably require a wired installation. In this case, the decoder typically comes with 7 to 9 wires that you connect to specific locations within the loco. An example of a wired installation can be found at my Installation of a Wired DCC decoder in an Athearn "Blue Box" loco page. Most other locos will be similar. Follow the instructions that come with your decoder to determine which wire goes where. The most important single part is that the motor and any lights MUST BE ISOLATED FROM ANYTHING ELSE. Failure to do this properly can smoke a decoder.
Basically, the steps go as follows:
Red Wire. Locate the wire or connection between the motor and the engineer's (right) side of the loco. Mark that motor connection as ORANGE and disconnect it from the motor. Connect the red wire from the decoder to the wire that you just pulled from the motor.
Black Wire. Locate the wire or connection between the motor and the fireman's (left) side of the loco. Mark that motor connections as GRAY and disconnect it from the motor. The motor should not have any connection to anything at this point. Either the red or black wire MAY connect to the frame if your loco uses the frame for power pickup on that side. Connect the black wire from the decoder to the wire or contact that you just pulled from the motor.
Orange Wire. Connect the orange wire to the motor terminal marked "orange."
Gray Wire. Connect the gray wire to the motor terminal marked as "gray."
If you are not wiring up any headlights then trim and insulate all other wires. You are done with the wiring.
Blue Wire. This is the common + wire for headlights or other functions. Connect it to one lead of any lighting that you are using. Note that if you use an LED, you will need to connect the + (long) lead of the LED to the blue wire through a current limiting resistor (470 ohms up depending on how bright you want the headlight, lower values result in brighter LEDs). If you are not using it, insulate is separately from the remaining wires.
White Wire. This goes to the other lead of the forward headlight.
Yellow Wire. This goes to the other lead of the reverse headlight.
Green Wire. If you use it, this is for whatever you want on Function 1.
Purple Wire. If you use it, this is for whatever you want on Function 2.
These installations are the easiest to do. If your loco has a 9 pin socket already wired in it, then use an HO decoder. Just unplug the harness from the decoder body by grabbing all nine wires and giving it a tug. Then plug remove the jumper plug and plug in the decoder. Locos with both 8 and 9 pin connections may have a jumper plug in either connection. You don't want a jumper plug in EITHER connection.
8 pin NMRA connections are almost as easy. However, the plug is not keyed and it will go in two ways. If you get it in backwards, the loco will run backwards and the headlights won't work. Simply unplug it, reverse it, and plug it back in.
Drop in decoders are mis named. They don't just drop in, they require some tedious work with individual wires. If you have big fingers, then working with the connections can be very frustrating. Further, most drop in boards can "drop in" in four different ways, only one is correct. Consult your decoder instructions carefully to determine which end is "forward" and which side is "up." Then, one connection at a time, remove the wire or wires from each connection and transfer them to the same connector position on the decoder board. The little black plastic cap has a groove in it, this is to allow the wire to enter the terminal. The wire will be folded across and pressed against the back side of the terminal when you push the cap back on the decoder. Note that on some locos, the cap won't fit back on the decoder because the tab on the decoder is slightly wider than the cap. If that is a problem, then either carefully file down the width of the terminal or just solder the wires. The motor tabs may not be on the same side as ones that were on the original board. If the loco ends up running backwards, reverse the wires that enter the side terminals.
The connections to the drop in boards are pretty standardized. The ones in the corners are for the power pickups. The wires from the front and rear trucks go to the front and rear contacts. The motor is connected to the side terminals. The inner four are for the forward and reverse headlights. If you are using LEDs, consult the instructions to determine which is the positive one. That goes to the longer lead (or smaller terminal INSIDE the LED). Some Digitrax drop in decoders have a constant current source for the lighting and will provide 15 mA to the light. These don't need a resistor, check your instructions. They will also work with 1.5 volt 15 mA bulbs. If your bulbs are 30 mA, then you can cut a jumper on the drop in decoder to reprogram the current limiter to 30 mA.
Sound decoders will have the same set of interfaces and configurations as basic motor decoders. However, sound decoders contain a lot more parts and are more expensive ($80 up as compared to $15 up). The decoders are typically larger to accommodate the extra parts and there may be large off-board parts. An external storage capacitor is common to allow the decoders to ride through short power dropouts without sound glitches and there will be a speaker to deal with. Most locos have room for a Z scale motor only decoder somewhere without serious mechanical modifications. Sound decoders will often require some mechanical changes to the loco to make room, especially for the speaker.
Some sound decoders are more difficult to program. The Tsunami is one of those which is why I have used only one. Some ESU decoders (v3.5) require a 100 ohm speaker. Some Digitrax decoders (SDH164) don't like some 8 ohm speakers and require a 32 ohm speaker or at least a 5 ohm resistor or so in series with the speaker to prevent overload and shutdown when the whistle is blown. Programming the operating functions of a sound decoder can be done on a regular programming track (Tsunami excepted) however, changing out the sound program itself usually requires some custom interface hardware and software (different for EVERY manufacturer).
One particular caution is to insulate or protect the terminals of an off board storage capacitor. These are connected to points in the decoder that are especially sensitive to being even momentarily connected to any other circuits in the decoder or the loco frame. Smoke is a usual result of a short.
This page has been accessed times since Apr 2, 2012.
© 2010 George Schreyer
Created 2 Apr 12
Last Updated April 2, 2012