Decoder Wiring

A guide to quality installation part 2

Basic Decoder installation information.

OK - At last… Its time for a bit of information about the Decoder:

When it comes to decoders, size really does matter - Seems silly to say it – but “almost fits” isn’t good enough here, so before you commit to a soldering Iron or cutting wires, check it REALLY WILL fit.

It only takes a few thousandths of an inch or a tenths of a millimeter and the body won’t fit, or the pressure of the body on the decoder twists the chassis, rubs the drive shafts or the wires get tangled in the universals or gears – disaster!!

Check that there are enough functions: Lighting needs at least one wire for front light (usually white), another for rear lights (usually yellow), a perhaps a third for interior lights (green) another for class lights (purple) etc, etc… Of course, steam loco’s have it a bit easier, but I think you get the point – PLAN before you go too far, or buy the wrong decoder!




Power handling, what the wires are, what the colours mean, what the important specs are, good wiring practice and more.

POWER HANDLING - OK, so you have a decoder that says “two amp power handling” – what does that REALLY mean?

Firstly, understand that this is the total power draw that the decoder can handle. It’s the SUM of all active circuits… Motor drive + lights + smoke unit etc.

Run anything to MAX and it’ll not last long – so do try to be a bit kind here, and balance things. IE: If you have an inefficient motor that draws an amp or more pulling a train, be conservative with added functions.

If your loco performs at less than an amp you can feel free to add lots of lighting.

Here is a good set of “rule of thumb” guidelines

Decoder Motor Drive – Usually 1 amp – (sometimes more, sometimes less for smaller decoders. TCS still rate their small decoders for a 1 amp motor drive however Hornby and LENZ in particular rate their smaller decoders lower as they use far less power tolerant components).

Decoder Function Loading – Functions are usually OK at 10o mA each, however while they don’t make much of it in their paperwork, to increase reliability where customers aren’t so careful some brands such as TCS now use much higher current transistors to drive their functions, with up to 200 mA capacity on some decoders. This is a real boon to those who realise it!

check your decoder paperwork first – EVERY time – or ask us if you are not sure. Don’t just assume - it varies even in the same brand sometimes… So do double check or ASK: Best to be safe and plan well.

Motors: Allow the following: (S / HO / OO or 4mm scale). Less for N & Z, More for O scale and larger.

• Open frame motor / older pancake style motor – 300mA to 750 mA is typical. There will be exceptions - Older US Pittman motors are an example.

• Can motor / modern (last 10 years manufacture) & loco in 4mm scale – from appx 100mA to max 400mA. "Slipping" current is rarely ever over 600mA. There will be exceptions - for example older Athearn Locomotives

Each lighting function – allow the following:

• Incandescent bulbs = Max two per function – they can be as high as 100mA each, LED’s are usually less than 20mA, so max five per function.

• Smoke Units – designed for DCC = 100mA, but standard common models (ie Seuthe 22) = 200mA, so it MAY need TWO function wires working together to do its job well and not destroy the decoder. (Use white AND yellow for example). NOTE: Current TCS decoders will tolerate 250mA so are totally safe with most smoke units. Again, Lenz tends to have lower power handling on its functions.

• Memory wire circuits – usually about 50mA safely. (Note: you MUST limit the current available to memory wire - "over-power" it and it will lose effectiveness. Use the memory wire manufacturers spec and follow it for best result)

• Solenoids – These are wildly variable, but from say 150~200mA to 750mA for bigger low impedance coils. Use Ohms law to check these. If you measure the impedance of the relay coils and divide it by the track voltage, you will get the current draw!

• Relays - These can be excellent for making one function do two things and can also be obtained in tiny sizes that will fit into a loco. They are usually quite low current - between 10 and 20mA for a micro relay up to perhaps 150mA for a larger one. All but the largest of relays are safe to use with decoders.

• Digital uncouplers – Motor driven will be less than 100mA, but if they are relay driven it will depend totally on the relay impedance - CHECK first.


What are all those coloured wires for?

Like most things, this looks complicated but it is really simple, and an excellent set of standards means that the coloured wires do the same on every decoder.

RED WIRE – this is to pick up power from the track. Traditionally, this should be connected to the pickups that collect power from the RIGHT rail

BLACK WIRE – this is also to pick up power from the track. Traditionally this should be connected to the pickups that collect power from the LEFT rail

ORANGE WIRE – this is to connect the decoder to one of the motor brushes. There is no “normal” left or right for this one, as motors do not always face the same way in the chassis. Don’t worry – if the loco runs backwards, simply swap for the Gray wire OR use the instructions to change CV’s to make the loco run the other way (easy and works every time)

GRAY WIRE – this is to pick up power from the other motor brush.

IMPORTANT – Red and Black wires must NEVER share any form of direct electrical  connection with the Orange or Gray wires. Nor should the Gray and Orange wires ever share a connection with the pickups in any way. Fail to check or ignore this and you will destroy your decoder immediately – The result of this contact, no matter how slight, is usually a puff of smoke and a black hole in one of the decoder IC’s.

BLUE WIRE – This wire is the COMMON wire for all functions. It is used with  other “accessory: or lighting wires. It is always POSITIVE, and is the common partner for every function. For example, front light = white + blue, rear light= yellow + blue.

WHITE WIRE – “Front lights/functions”. To connect the headlights or “front” function, use the white NEGATIVE wire and the BLUE POSITIVE wire. How your decoder controls this wire will be clearly stated in the instructions, but it is usually via Function Zero on your controller (Often helpfully marked “lights”)

YELLOW WIRE – “Rear Lights/functions”. To connect the rear lights or “Rear” function, use the Yellow NEGATIVE wire and the BLUE POSITIVE wire. How your decoder controls this wire will be clearly stated in the instructions.

Please note as follows:

* Rear light functions can often be used for things like firebox flicker on steam loco’s.

*  If the function needs more than 100mA, you can parallel two functions to make a higher power function – ie Yellow+ white to one side of the smoke unit, blue to the other side.

* Lighting can be configured to be manually or automatically reversed, plus MANY other options.

Check your manual, and if lights connected to the blue + white and/or yellow don’t light as expected, suspect the need to switch them on!

lights are always set to operate on function zero.

GREEN WIRE – “any other function” including lights or accessory control. Use the Green NEGATIVE wire and the BLUE POSITIVE wire. How your decoder controls this wire will be clearly stated in the instructions. Usually 100mA Max. On non sound decoders, this will be set ex factory for operation by function 1

PURPLE WIRE – “any other function” including lights or accessory control. Use the Purple NEGATIVE wire and the BLUE POSITIVE wire. How your decoder controls this wire will be clearly stated in the instructions. Usually 100mA Max.   On non sound decoders, this will be set ex factory for operation by function 2.

BROWN or STRIPED WIRE – “another optional function” for use with any lighting or accessory of 100mA or less. Rare, but useful for complex MU or diesel lighting SHORT Circuits and HEAT kill decoders! On non sound decoders, this will be set ex factory for operation by function 3.

Take Care when wiring the decoder into the locomotive:

Be sure that NO active part of a decoder (and that’s ALL if the circuit board tracks and components) and NO loose end or joins in a wire can short out on any other “live” part of the wiring OR the loco chassis.

Always solder every joint and use “heat-shrink” on wire joints Always insulate any adjacent chassis parts with tape, epoxy, a coat of nail varnish or a coat of superglue (let dry fully before proceeding further of course) or plastic sheet.

ALSO be aware that if you are “Maxing out” the decoder with a hungry motor and a load of lamps etc, it WILL be close to max output and WILL get hot eventually. Excess loading results in heat and Heat KILLS, so with regard to preventing decoder “heating”, we also strongly suggest the following.

(1) Make sure track voltage is max 15 volts or preferably lower at 13.5 volts on 4mm scale or smaller – no more is needed, despite the crazy voltages on some units (early ROCO for example is set at 22 volts!). Higher track voltages not only kill decoders, they also  encourage “runaways and strange control problems”.

You cannot measure this directly from the rails with ANY normal multi-meter. To check rail voltage, set your meter to DC volts and turn on the light function of a loco decoder. Measure the voltage across the blue positive wire and the white headlight wire. The voltage read here plus between 3/4 and 1 volt will be your track voltage (the 3/4 to 1 volt is adding back voltage dropped in the bridge rectifiers which are part of the light function circuit).

(2) If it can be done safely, try to position decoders hard against a large metal lump like the chassis – Whilst it MUST of course be insulated from the decoder electrically, the mass of metal will help to draw away some heat.

(3) If you MUST load the decoder up, do NOT mount it on the motor – motors heat up too, and the combined result may be enough to cause premature failure.

OUR opinion – Important things about decoder choices!

• Make sure the decoder brand is NMRA compatible or compliant, and generally AVOID stripped down cheapies and most decoders from trainset brands—they always have compromises in quality, features, tune-ability or power handling… so their low cost comes with real compromises in quality or adjustability.

• Stay with specialised DCC brands, not train-set brands, no matter WHO claims to have made them. Think about it – if a brand that costs X makes a decoder for another brand, and they then sell it at LESS than X – what has been left out?

• Avoid “Badge engineered” decoders – (Means made by one company, sold under another companies name) Just like the “train-set brands” the same decoder direct from the primary manufacturer MUST be better value cost wise – there is one less profit margin getting in the way after all.

• Avoid ANY decoder without silent/supersonic/high frequency drive. Motors run sweeter, quieter, better and much cooler with a silent drive decoder.

How cheap is a non-silent drive decoder if your locomotive motor lasts less time because of the coarse drive pulses – and it WILL also be annoying as low frequency drive decoders create an audible grumble from the motor…  Silent drive is a MUST for good performance and long term reliability.

• Make sure the decoder has a 4 digit address ability. Low cost decoders that only allow a 2 digit address are always going to be a cause of frustration and regret.

• Make sure it has a high quality, fully adjustable Back EMF ability. There is absolutely no comparison in the quality of slow running achievable—a Back EMF decoder will give such superior performance that NOT having it is unthinkable.

I often have US modellers complain that the loco’s fight each other in consists with back EMF enabled, however they simply haven’t adjusted it correctly—if the rate of back EMF influence is adjusted and it is set to turn off after the loco has exceeded starting speeds, then they would have no problems at all—and making those adjustments is dead easy!

• SIZE Matters – For smaller installs I’ve personally standardized on the very small TCS M-1 and MC-2 or for “DCC ready” loco’s, the totally wireless DP-2x and DP-2x-UK because they has all the features I need for my hand-built brass loco’s AND combines excellent Back EMF performance with a superb motor drive and it has a ton of power handling. SMALL RULES when it comes to decoders – A great small decoder is a valuable find as an easy fit means easy installation and wiring after all! If you need more functions, then the TCS MC series have up to 6 function variants available (MC-6 for example)

• Look at the Warranty and backup very critically.

Decoders are usually reliable products but exceptions do happen, so warranty and backup are important. Warranty on decoders varies from “totally Goof proof” to “until you install it”. The difference is extreme, & as we ALL make mistakes, good manufacturer backup or a bit of free “Insurance” is well worth having!

• Avoid decoders with only ONE function. Apart from the fact they will NOT be silent drive, they will be very limited in application. Fully functioning decoders such as the TCS T-1 have everything at the same price or lower than those “stripped down jobs” that are so often taken up by new DCC’ers, and offer better value, better performance and better flexibility in application.

• Look for DECODER LOCK as a function – it makes multiple decoder installs a breeze, and is a great aid to control for things such as “loco plus coach lighting”.

There are LOTS more issues, depending on your needs. Do YOU have an opinion on this – please feel free to email me and “add your thoughts” to the list.

OK – time to look at “The Second 5” of this list… Now we are looking for more specific issues, and planning to get the tools out and do some install work. Hope this helps!

Back again to the locomotive….

It doesn’t matter whether its Z, N, HO, 00, O/7mm, G or any other scale or gauge… it MUST run OK on DC if its to run OK on DCC. That means reasonably good pickups, Wheel back-to-back set OK for smooth running on the rails and through turnouts/points, clean wheels.

It also means a clean motor (no oily build-up, clean commutator, clean slots in the commutator, good brushes, oiled gearbox, oiled axle bearings, no binding in steam loco valve gear…etc.

If you HAVEN’T given your loco a “Pre-DCC service” and test run, please do it NOW. Before you add an expensive decoder, and before you create a problem because your loco wasn’t ready for the transition to DCC to happen.

The following is a big list that looks a bit intimidating. I’m NOT trying to frighten you off – just give you the chance to learn what MIGHT await the unwary installer. DO bear in mind that MOST of the loco’s you will install have NO problems at all – this is just a set of precautionary checks that MAY be needed especially if your loco is “older” or “hand-made brass”.

Most “off the shelf” loco’s are very DCC friendly and install will be EASY.

NEXT: Take the top off and LOOK carefully. Where does the wiring go to/from. Which pickup goes to where. Are the brushes insulated OK, where can I put the decoder, what are the lights, LED or bulb – do I want to ADD more lights – where will I run the wires?

Take a photo of your loco with your digital camera before you pull it apart so you have a reference for reassembly… or if you’ve not got a camera, then just make a simple drawing and notes – and a list of things to do.

This check list is a really helpful thing.

A Mechanic makes one or one fills out when he services your car – He does it so he can bill you as much as possible…. but YOU should do it because if you miss a step you could end up with a frustrating failure – or a dead decoder.

Things to watch with older locomotives

Do check the current draw!

If the motor draws over an amp, choose a 2 amp max decoder such as the TCS T Series. Service the motor and see if it runs smoother – if you take the time to do so it will probably also run smoother and at a slightly lower current draw.

Be careful how many other accessories you add to a loco with a high current motor – remember what we said earlier… A decoder power rating is always for the TOTAL of motor drive + accessories.

If the loco motor draws much over an amp – consider replacing it if possible with a modern can motor. Running will get better, and your decoder will appreciate it!

Does the loco use incandescent bulbs? If so, plan to replace them with LED’s – MUCH less current draw, incredible reliability, better quality lighting, ZERO heating problems to damage plastic loco bodies or brass loco paintwork.

Are the lights in the loco installed so one side goes to the chassis and the other to a single wire? You CAN still use this method with DCC (for example for front light  leave one side to chassis, connect the other to the White wire) but this results in half wave power to the light and this is not ideal.

A much better way is to replace the bulb with a Prototype white LED from DCCconcepts (CLICK HERE) or at very least to insulate the bulb from the chassis totally and connect via both blue and white wire.

Does the lighting rely on “reverse of DCC” or track voltage? If so, plan to rewire all lights so ALL lighting is via the decoder as DC type light operation won’t work with DCC – It is worth making this effort as full control, adjustable light operation including dimmable levels and a much better “realistic operational possibility” will result.

Is there a capacitor or suppression circuit on the motor? If so, cut it out or remove it as it will most definitely stop you achieving the best possible  slow running, and it will interfere with any motor drive options such as Back EMF.

Is it an open frame motor? If so, check that both brushes can be insulated from the frame of the motor. 99% of the time this will be easy – that 100th time, consider replacing the motor with a modern “Can” motor for easier DCC conversion and of course better and smoother running.

Is the chassis used to collect power? Not a problem as most do in some way, but if so, think how to insulate the motor brushes from the frame 100% reliably. Also explore pickup options – can you make insulated pickups to render the chassis dead – NOT always necessary, but a VERY GOOD idea if you hope to double-head or “Consist” the loco with others!

Live “chassis to chassis” is all very well if loco’s are done consistently, but if one is live chassis to left rail, the other live chassis to right rail, then if you connect them via metal couplers, you will create a dead short on the layout—this sort of “loco to loco created short circuit contact can MELT couplers and damage pickups or even the locomotive driving wheels!

Things to watch with Newer locomotives

Does the wiring use those small black plastic “clips” to hold it in place? If so, plan to replace them with soldered connections – They are only there as it makes them cheaper to make the loco, but unsoldered wires are always a recipe for unreliable performance!

Do what the maker should really have done day 1 and solder every connection!

Is there a capacitor on the motor? If so, cut it out or remove it – it will stop good slow running, and will interfere with quality motor drive options like Back EMF.

NOTE: Newer loco's may have "chip-type" capacitors (surface mount) on the PCB, and there MAY be more than one. These can be hard to spot. If you are NOT sure, ask a question on a DCC E-group about that particular loco - someone will have done it, and they'll be able to tell you where to look.

Does the loco have a DCC socket? – if so, it will probably be fine but please DON'T assume it is OK - Sadly, Mfrs sometimes mess it up, so still check carefully to be sure there are no bad solder joints etc, before U plug in.

ALSO – be aware that not all DCC ready loco’s have DCC ready bulbs in the light circuit. If you loco HAS an incandescent bulb, and you really don’t want to replace it, that fine but be careful... 

Install the chip and run the loco without the body on and see how hot the bulb gets on your DCC track voltages – if it gets more than medium-warm, it will get hot with the body on. Replace it with an LED or add a (say 10 ohm) resistor – HOT bulbs don’t last long – and very hot bulbs DO melt plastic bodies!

Even better than adding a resistor to the original incandescent bulb,  plan to replace them with LED’s – They aren’t expensive, run cold at any track voltage, have much less current draw, are incredible reliability, give better quality lighting, and have ZERO heating problems to damage plastic loco’s or brass loco paintwork.

Click HERE to see DCCconcepts very wide range of LED’s with correct colouring for modelling use in a range of sizes (In Prototype white / Golden white/sunny white)

Does the lighting rely on “reverse of DCC” or track voltage? If so, plan to rewire so ALL lighting is via the decoder – full control, dimmable levels and a much better “realistic operational possibility” will result.

Is there a complex PCB with lighting and other functions in place? If so, and you are a bit confused as to the next step, check the manufacturers web site or use the WEB to get data. Email me if you like, or do a search using Google.. for example “DCC install in Proto 2000 E7”. OR – why not join a DCC “user group”.

There WILL be diagrams or advice available showing how to do the modification, photo’s too perhaps – as well as many users out there with the answer… and it’s a great way to “meet other DCC’ers” and learn more about DCC!

To start you off, we have compiled a lot of information on LEDs and lighting here on this website. Click on this link to read our “lighting advice pages”


The ability of the locomotive to cleanly and reliably contact the track to receive information and to provide power for movement is THE critical area for DCC and good loco performance. Most poor running loco’s are in fact fine – they just have lousy wheel or pickup contact because of accumulated gunk, corrosion, oxidization or even protective coatings applied in manufacturing (Yes, MANY loco’s are “ready to fail” out of the box because of careless cleaning or added protective coatings ex factory!)

Here are a few tips:

Inspect your loco carefully, and clean the wheels: I prefer a combination of cotton buds with isopropyl alcohol, MEK or similar solvents, followed by a fiberglass brush to burnish them or remove stubborn spots.

• Clean the wheels treads carefully – pay attention to cleaning the fillet where the flange meets the wheel too

• Clean the back of the wheels. This is usually where pickups run so its important.

• Clean any pickups, and make very sure they are contacting the wheel reliably. Tweak them outward slightly to improve contact if it looks like they even MIGHT lose contact occasionally

• If you have some to hand, apply a TINY bit of “contact enhancer” like rail Zip, or similar product sold in electronic stores for cleaning and protecting switch contacts.

Polish all pickup surfaces:

To work at their best, pickup surfaces must be smooth as well as clean. If there is any form of pitting or plating  on a pickup surface such as the back of a wheel (those pesky short-circuits as the wheels cross turnouts/points are a prime cause of pitting) then you should polish it smooth.

This “cleaning also means getting rid of the wheel blackening often applied to the back of wheels… It looks fine, but DOES compromise optimum pickup quality!

I do this by running the loco upside down and gently applying a polishing stick made from a bit of thin strip-wood or styrene which has had 1200 grit wet and dry paper stuck to it. (Also good for polishing the commutator of open frame motors – you’ll be amazed how well they run when polished perfectly)

Clean axles and chassis bearing surfaces if these are used for power pickup: If the loco can be disassembled to do this, great... if not, be sure that as far as possible any accumulated gunk is removed. Perfect power requires a clean axle in a clean bearing for reliability.

Make temporary connections permanent:

If the loco and tender share pickup in any way, and there is a “live drawbar” connection relying on a contact between pin and hole or anything similar, you will greatly help reliability by permanently wiring the loco and tender together. If there are a couple of diesels that will be permanently coupled, I strongly suggest that perfect performance will come from a quality permanent wiring between them or perhaps a set of quality connectors linking all pickups.

DCCconcepts make both 4 and 6 wire Mini-Connectors for this purpose. Click Here

This will have the added bonus of allowing accessory functions from a single decoder to provide all required lighting etc.

ADD pickups wherever possible: If the steam loco picks up power from one side and the tender the other, or the loco picks up power and the motor is in the tender, ADD pickups. This is particularly helpful with locomotives that do not have any form of equalization, as in a rigid framed loco the slightest twist in track means that only two of the main wheels will be in contact…and if they aren’t the two with pickups… the loco will stall or hesitate.

DCCconcepts Pickup Springs will help!

DCCconcepts now make a superb easily installed friction-free pickup spring that will fit both HO and N scale rolling stock and most loco pony or trailing truck axles!

These are supplied at no extra cost with our “Flickerfree” packs, and are available in economical packs at very low cost for general use. Click HERE learn more.

If your coach, caboose, guards van or tender has axles with one live wheel and one insulated, then pickup has just become super low friction and very easy to install!

Simply remove one wheel and slip the phosphor bronze spring onto the axle, then replace the wheels. Solder a fine flexible wire to the “tail” of the spring and run through a small hole in the vehicle floor. Connect to the device that needs power and you are done.

If your vehicle has metal wheels but both are insulated, then it takes ONE more step but it’s an easy one! Add the spring as above then clean any blackening from the back of one wheel. Keeping the spring clear, just add a small amount of silver conductive paint or conductive Glue from the axle to the wheel.

(to see this conductive Glue & Paint Click here).

Complete the install by soldering the wire etc as above.

Fine Phosphor bronze strip is available as follows:

UK – Mainly trains in Watchet, Somerset (perhaps the best mail-order “parts place” on the planet). They can be found on the web at

USA – Tichy Train Group of Plainview NY. They have an excellent range and sell high quality phosphor bronze wire and strip in USA. They can be found on the web at

SUPER TIP: Better quality pickups can be made by buying some plain gold “keeper” type earrings from a jeweler, and soldering a small segment (only about 3mm / 1/8” ) to the tip of the phosphor bronze pickup contact. Using gold sounds extreme, but it never corrodes, and anyway, this option is cheaper than you think, as one pair of simple gold earrings costing only $20~$30 will make many, high quality pickups.


Finally on this page - Do you know what the gunk on your rails is?

You know the stuff, you see it every time you run your finger along the rail top and get a black stripe on your finger tip…. The stuff that stops the trains running if you don’t keep it under control!

I’ve heard lots of comments as to what it is, but most are wrong, and here’s definitive proof - a high level analysis of track gunk done by the experts at a University with a million dollar + machine to play with!

Surprising, isn’t it! 57% copper, lots of Zinc and a little nickel.

All of it simply part of the chemical makeup of the nickel silver rail that’s been leached out via electrolytic action and oxidization.

Who says Nickel silver is a good choice for rail—certainly not me, I only ever use steel rails on my own layouts!


Do you still have questions? No problem – just click here to email us and ask, we’ll be happy to help!


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