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Installing a DCC decoder


Installing your first or your 51st decoder


Installing a digital decoder in a locomotive is not difficult and is very rewarding. The key to any successful installation is to take some basic steps to check what you have done BEFORE you run the locomotive.


Most current generation locos are DCC ready. This means the manufacturer has made provision for fitting a decoder, usually a socket built into the loco circuitry. The socket is fitted with a jumper plug or clips so it can run on DC. Remove the jumper and plug in a decoder (plug and play) and your up and running. Seems simple enough, however some manufacturers do it much better than others!


Older locos and those not DCC ready require you to hardwire the decoder, by soldering wires directly to the loco wiring for the motor, power pickups and lights. Often you can use a wire harness with a socket to take a plug in type decoder. The photo shows you some of the wiring harness fitted with sockets that accept standard decoder plugs. Also shown are the thin decoder wires that match the DCC colour code protocol. We can supply most of these to help you with your decoder installation.


Overview of a Decoder

A DCC decoder is simply a device installed in the loco that can receive messages and send back messages. These messages can change the speed and the direction of the locomotive, as well as turn on and turn off lights, smoke units, sound and other things. Lights, smoke, sound etc are known as functions. Decoders are often referred to by the number of functions that they have. A four function decoder can operate four things. Front headlight and rear head light are two functions, so you can add two other things like a smoke unit or a cab light.


Decoders are made by a number of manufacturers, in a variety of types and fitting and sold at various prices. We suggest that you fit the best quality decoder that you can afford. The money you saved in buying a cheap decoder is soon forgotten if the loco runs poorly, the decoder won’t programme properly or it fails because it does not handle the motor current! I certainly won’t waste time installing a cheap decoder knowing that I may need to replace it because it doesn’t run properly or it fails.


Types of Decoders

So which decoder fits what? Decoders differ in the way they connect to the loco as well as the number of functions they have. They differ in the current they will handle (continuous and maximum current), if they are a motion decoder or just a function only decoder. Some decoders also make sound.  You can see in the photos some of the different decoder types. The most common fittings are listed below.


  • Decoders with just a wiring harness. These are designed to fit locos that do not have a socket or wiring for DCC, they are not DCC ready. To install DCC you need to wire directly into the locomotive wiring. These are often older locos but there are times you find it easier to hard wire a decoder than try to use the factory fitted wiring. If you have a decoder with a plug, you can easily cut it off!  


  • Decoder fitted with 8pin NEM652 male plug. This decoder has a wiring harness ending with an 8 pin male plug that fits a matching socket in the loco. This socket is often part of a circuit board. Pin 1 on the socket is usually marked telling you which way the plug goes because it can be installed 180 degrees around in which case forward becomes reverse and lights wont work. Athearn, Atlas, Auscision, Austrains, Bachmann & Branchline, Hornby, Proto2000, Trainorama and many others use this type of socket. Just because the loco has a socket does not necessarily mean there is room for the decoder or that it is wired correctly in the factory. More about this later!


  • Direct fit decoders, as the name suggests, plug directly into the loco socket without any wires. If your loco has room to take this decoder type, they are a good choice. Easy to fit and no wiring to get caught up in the mechanism. If the decoder requires more height to clear the circuit board components, you can often install a riser. Some Broadway Blue-Line steam locos take this type and include a riser in the package. Direct fit decoders can also be used in the Austrains DL and T class and the Auscision A and B class, EL and N.


  • Circuit board replacement decoders made by NCE and ESU and others are designed to replace the existing circuit board found in Athearn, Atlas and other popular brand locos. The board has small male pins that accept the existing wires held on by plastic caps and clips into the same place. We use these in the Trainorama SAR930 class as there is limited headroom for the standard decoder.


  • JST socket decoders have a 7 pin female socket that plugs directly into a matching male plug. Most of the later Athearn locos include this plug option, often along with the 8pin socket. NCE and ESU make a decoder with JST socket in the package and you can also buy a wiring harness adapter from 8pinNEM to 7pin JST.


  • NEM651 decoders are made for N and smaller scales and are fitted with a 6pin male plug, as a direct fit decoder or with a wiring harness with 6pins. You can buy a wiring harness to hard wire a decoder with 6pins.


  • 21 pin decoders as the name suggests plug into a 21 pin female socket. These sockets provide wiring for more functions and for sound to be easily added to a DCC ready loco. Some Bachmann Branchline and the Bendigo Rail Models T class use this connection.



Never assume the manufacturer has got it right. I learnt the hard way and it cost our shop a quality Lenz Silver Direct decoder. Just because the loco has a factory fitted socket doesn’t mean that it is wired correctly in the factory. Find out more about this under Programming.


Pre-testing your loco and running in

There is no point installing a decoder into a loco that doesn’t run or runs poorly. It is sound practice with any loco to first check that it runs well on DC. If it is new, then ‘running in’ can be done at the same time.


To do this, run it forward on the track at medium speed for 5-10 minutes, then in reverse at the same speed for the same time. A timer is handy here. Then change to a lower speed and do the same, in both directions.


Run again at a slightly higher speed, then at a lower speed, until it frees up in both directions after about an hours running. During this running in, check for unusual noises and any problems before you start pulling it apart. Why both directions? You want it to run nicely in forward and reverse so run it in both directions. You may already have a loco that runs great in one direction only!


Often the instruction sheet or exploded diagram will show where the decoder socket is in a DCC ready loco. This will also give you clues on how to remove the body. When the body is removed on an older loco, it is a good time to service. Pecos service cradle holds both N and HO locos and protects the body from accidental damage.


Check that motor bearings, axle bearings and gearboxes are lubricated, wheels and pickups are clean and the motor brushes and springs are sound. Replace the motor brushes and springs if worn. Peco and Faller have plastic compatible oils and Atlas has grease suitable for gearboxes. XXX have Teflon grease too for quiet running. These lubrication checks should also be done on a new loco too, ‘cos sometimes the factory forgets.


Decoder Wiring

Decoder wiring looks complicated because there are more than two wires but we can demystify this quite easily, cos it ain’t rocket science. Most decoders are fitted with 8 wires, some with more functions have more wires, often 10 and sound decoders can have 12.

The wires on quality decoders conform to a colour standard to help you get it right.


In a DC loco, track power from the rails goes directly to the motor. In a DCC loco, track power has to go into the decoder first and then to the motor.


Two wires on the decoder are for rail pickups, left rail and right rail. Another two wires are for the motor. And that’s it. Four wires are all that are needed for your decoder to run the loco.


Wiring colours are


  • red              - right rail (R for Red, R for Right)
  • black          - left rail (L for Left, L for bLack)
  • orange       - where right rail pickup went
  • grey            - where left rail pickup went


But what about the other colour wires? If the loco has lights, then


  • white         - front light (positive +ve)
  • yellow       - rear light (positive +ve)
  • blue           - common connection (negative –ve) to all lights and functions
  • green        - aux 1 (another function, positive +ve)
  • purple       - aux 2 (and another function, positive +ve)
  • pink           - aux 3 (and yet another function, positive +ve)
  • brown       - speaker wires for sound decoders (positive +ve & negative –ve)


So to wire a loco with a headlamp, connect the white and blue to the lamp. If it has a rear light then connect yellow and blue to the rear. If it had a cab light or ditch lights, you would connect the green, purple or pink and the blue.


Any wires not connected can be shortened and the ends insulated over with shrink wrap tube or tape to stop the smoke coming out. (this prevents accidental shorting if that function is turned on)


Lighting tips

If the loco has globes for lighting then consider replacing them with Led’s. Globes draw more current, especially when cold, they get hot when left on for a long time and often fail because they are getting full voltage on DCC, unlike when connected through the motor on DC.

Besides we make money selling you Led’s. We have both prototype white for steam and early diesels and sunny white for modern diesels, in a range of sizes. (see Modelling Tips – Fitting Led’s) You will see in the photo Led’s installed in place of the factory fitted globes.


Where do you stick the decoder?

Now’s the hard bit. If you can not fit a Direct Plug decoder, then you need to find space in the loco. Steam locos are usually ok as the DCC plug is often in the tender. Some like the Hornby Flying Scotsman have a plug on the loco chassis, so you need to find space there.


Early Austrains C class and early NRs have a cut-away section at the rear of the chassis weight that will fit most decoders. Some N gauge locos can be interesting and may need the chassis modified or have a wagon permanently attached for a speaker, if fitting sound.


Use a good quality double sided tape to secure the decoder making sure the wires are well clear of flywheels and rotating drive shafts. Make sure there is enough free air space around the decoder for dissipating heat. I’m not a fan of sticking decoders to the loco or tender body, as they do get hot, hot enough to melt plastic. Use shrink-wrap tube to cover any soldered joins to stop smoke coming out. Good for keeping wires bundled together too!  


Decoders and noise suppression

Some locos such as Hornby are fitted with noise suppressors in the wiring circuit. This is usually fitted across the two motor connections and works great on DC. DCC often ‘sees’ this as a ‘dead short’ and wont program. Other times the locos just run poorly while fitted, so I often remove these suppressors and bin them. Noise suppressors can be seen in the photo of the Hornby steam loco. These will soon get the flick.


Fitting Sound Decoders

Sound has added another dimension to model railways with more and more modellers wanting to enjoy sound. Nothing better than to listen to a steam loco sitting in the yard, boiler hissing, safety valve blowing off and Fireman Fred adding a shovel full of coal to keep up steam. Hear the generator cut in as you turn on the headlight and the compressor pumping up air pressure for the brakes. A diesel engine at idle with cooling fans turning on and off at random, air compressor cutting in and pressure safety valve blow off, all just adds to the realism. Just sitting there, they are alive!


Fitting sound into locos can provide another challenge because you also need to install a speaker where it will be heard. Austrains T and X class, CLs and NRs, Trainorama 930s, Auscision A and B class and the Ixion On30 Coffee Pot can all be fitted with sound with good results. Most steam locos make great sound projects because of the tender.


We advise that you fit the biggest speaker that will fit and in a position that allows the sound to escape. They work best in an enclosure or chamber where the front cone and rear body are separated just like a hi-fi speaker. Nothing is more disappointing than a sound loco that can barely be heard over the mechanical noise because of poor speaker selection and location.


T, X, A & B class locos all have an empty fuel tank (you wonder how they ever run being empty) that makes a great speaker box. Coffee Pots have the water tank under the body that makes a good size box, even better if you remove the centre metal chassis rail. The photo shows where this has been done.


Most Hornby and Branchline steam loco tenders make great speaker chambers by removing the coal load and fitting the speaker facing up. A piece of grey or black sponge cut to size makes the new coal load. A speaker sounds best if the chamber is sealed, white glue is good.

The coal load in the tender below has been removed carefully with a Dremel and the rectangular ESU speaker sealed with white glue. Why white glue? If it ever fails, the old speaker can be easily removed by dissolving the glue in water. Try doing that with CA!!!


Often you need to assess the loco and think outside the square. The older Austrains C and Auscison EL can have the chassis or weight milled to take a speaker and the decoder. The NR with the centre two side windows removed (looks like they are in the open position) allows a speaker fitted in the cab to be heard. The speaker holes already machined in the CL weight cab be bored to accept a standard ESU 23mm speaker with great results.



So you have installed your decoder and you are keen to put your loco on the track. STOP right here. There is a test procedure that I strongly suggest that you follow if you want to save time, save money and save your decoder.


Before you replace the loco or tender body, check over your installation to make sure that


  • the decoder is wired correctly to the colour code
  • the decoder is secured with double sided tape
  • the decoder has adequate air space for ventilation
  • all wires are well away from flywheels, drive shafts and gears
  • resistors are fitted in the lighting circuit if you have fitted Led’s
  • any function wires not used are shortened and taped over
  • all solder joints are shrink wrapped or taped to prevent the smoke getting out. (is what happens if you have a short circuit)
  • wiring is secure and will not get squashed or caught as the body is replaced


This next part is critical. After you have installed it in the locomotive, try to program the address using the program track only (PLEASE DO NOT PROGRAM ON THE MAIN) Even if the loco is DCC ready, please do not use the main track yet.


This makes sure that the installation is correct and the DCC system and decoder are “talking” to each other. If you cannot program the address there is a good chance the installation is not right. You have a chance to re-check and correct the problem without destroying the decoder.


With DCC ready locos, we have found some manufacturers have incorrectly wired the DCC socket in their locomotives. So please don’t assume they have got it right!!!!


If the installation is incorrect this means the decoder will usually be damaged beyond repair when the loco is placed on full power on the running track. This also means that we can sell you another decoder. You will not be very happy!!!!


If your DCC system can read the decoder and you can program the address, the locomotive wiring should be correct. Further programming can now be done via the Main Track.


Decoder testers and sound programmers

If you think you will be installing lots of decoders or you want it to be your life, consider investing in a decoder tester. It is a simple test unit made by ESU that will accept most decoder types, including 21pin and sound decoders. It allows you to test out the decoder and the functions before you install it in a loco, so that you know that it works. You can also make your own tester using a surplus loco mechanism fitted with a DCC socket.


Again you would connect the decoder tester to the program track first and try to program the address. You can also program start voltage, top speed or sound downloads before it goes into the loco and before it connects to the main track. All this testing may seem tedious at first but it will reassure you that the decoder you are about to install is working as it should.



If you are serious about fitting aftermarket sound, then ESU’s Lokprogrammer allows you to download ESU sound files into their sound decoder. The Lokprogrammer is the interface between the loco fitted with a sound decoder and a computer. Downloads for steam, diesel or electric locomotives sounds are available free from their website as well as programmer updates and firmware updates. New sound files are always being added too!


Once the sound file is downloaded into the decoder, you can make changes to the address, starting voltage, momentum, lighting, sound volume and much more. The program includes a Drivers Cab that allows you to test run the loco after you make these changes. ESU sound decoders can be downloaded again and again if you want to change to a better sound file or install it in another locomotive.


To quickly summarize,


  1. test the loco on DC before you start the install.
  2. test the decoder using a decoder tester on the program track.
  3. if it responds to programming test on the main running track.
  4. if ok install into the loco and then check carefully the installation.
  5. replace any globes with Leds fitted with resistors.
  6. check your installation on the program track and change the address.
  7. if it responds to programming test on the main running track.


If you purchase your decoder from us we are more than happy to advise you and to help you with all of your decoder installations.