DCC decoder & locomotive installation overview

This page is not intended to be a tutorial for how to install decoders. Installing decoders is a bit of an art, and most people learn by doing. In my opinion, you need to have basic skills in electronics to do this well. It CAN be done by a list of instructions, but this takes different instructions for each loco and decoder. I won't live long enough to do that on this site.

Instead, I'd give the concepts. You need to be able to measure voltage and current. Get a cheap digital meter and learn how. There's TONS of resources out there on the Internet for this.

DCC installation steps

I like to think of hooking up a decoder in 3 "stages":

  1. track to motor
  2. lights
  3. smoke and others

In fact this is how I install and test, step by step. Many people fail because they try hooking EVERYTHING up and then power on... the "magic smoke" is usually released, and it becomes a mess. Also it does NOT save time trying to do everything in one fell swoop, since we are all human, and make mistakes. Doing things step by step lets you catch errors quickly and easily and again helps avoid the release of the "magic smoke". Take my advice, you are not saving time doing this all at once, and I cannot tell you how many people have burned stuff up by trying to short cut the step by step I give here.

Connect track inputs to decoder and motor

It's pretty hard to debug functions if you don't have power to the decoder. 

First, find, verify and connect the track pickups to the decoder. This is often the failure where the wiring is wrong and destroys the decoder in later steps. You MUST get this step right.

I also am of the opinion to NOT "rip out" existing electronics unless necessary. There is that impulse to do it, and many people on forums espouse this view. Most of this comes from the battery folks that need every cubic inch of space inside to stuff in batteries and large electronics. Take my advice don't do it. 99% of the time it is not needed. Also, ripping everything out usually makes it MORE difficult to do the install, because the wiring connections to the existing circuit boards identify the wires.

1. Find the track pickup wires

USA Trains:

Visually inspect the loco and see if you can find where the wiring is. on USAT diesels, you can see the wires from the trucks, and there are 2 sets of wires, one pair to the motor, and 2 pairs that go to track pickups (they have redundant pickups).

LGB:

Many locos have 4 wires coming from the "motor block", 2 for the motor and 2 for the track pickups. Older LGB locos have 3 wires and one of the motor leads is connected INSIDE the motor block to one of the track pickups. This requires opening the block and breaking the connection and adding a 4th wire.

Aristo-Craft:

Most Aristo locos have the "socket". There should be a 12 pin socket and a 10 pin socket. The 12 pin socket will have the motor and track connections you need. Unfortunately, there is a history of many errors in wiring, as Aristo NEVER tested the socket for correct wiring, but tested the loco on DC with the motor leads to shorted. What happens is the socket pin for the motor could be miswired as the track pickup and vice versa. Since in DC mode with the shorting plug installed, they are connected together, this miswiring is not detected at the factory. This miswiring typically blows up DCC decoders instantly. This is VERY common with Aristo.

Bachmann:

Newer Bachmann locos have the Aristo socket, and are rarely miswired. Older units you have to follow the schematics, and there's often a ton of screwy wiring.

2. Isolate the track pickups from the motor wires

USA Trains:

Just disconnect the plugs from the motor blocks, each motor block has 2 connectors, one for motors and one for track pickups.

LGB:

This has more variation depending on the age of the loco. You have to basically figure this out unless you have a DCC ready loco where the wires are alreay isolated.

Aristo:

Remove the shorting plug from the socket

Bachmann:

If you have a socket, remove the board from the socket, if not, disconnect the track pickup wires.

 

3. Test for track pickup isolation

You MUST do this! This is the #1 issue with installs.

Get an inexpensive voltmeter

 

4. Test motor

 

5. Connect to decoder and test

Test for direction, both DCC and DC, correct DC direction

Lighting

Remember that "common" for DCC lights on decoders is almost always positive, not negative. (This is because it's easier to make an electronic component that switches a "pin" to ground, as opposed to supplying voltage.

To repeat, the common on the DCC decoder is positive, normally 5 volts or track voltage.

Driving LEDs

Limit current:

Use a resistor. Calculate the resistor based on the voltage from your common to ground, and subtract the nominal voltage of the LED and use the "remaining" voltage and Ohms law to calculate the current.

You should have the nominal working voltage and the maximum current in hand. If you don't know, then use 3 volts and 20 milliamperes.

Driving bicolor LEDs:

There are 2 different kind of LEDs that have 2 colors, one where there is a common and an individual lead for each color. In this case, you need a common anode LED and most you will find will be common cathode. (Again the same reasoning as why DCC function "outputs" are really inputs to ground.

If you have a common cathod LED, I suggest you get a common anode replacement.

The other type is where there are only 2 leads to the LED and the 2 LEDs are connected in parallel, and in reverse polarity:

Using existing LEDs in the loco

 

Using existing incandescent bulbs in the loco

Connecting smoke and other items to the decoder

This is a separate section for two reasons, the first is that higher current devices might exceed the capability of the decoder and you might need to add a relay or other device to turn things on and off.

A second reason is that smoke units have changed significantly in recent years, from simple heaters that have no smarts, to electronically regulated heaters, to systems that allow independent control of the heating element and fan, such that the smoke output can be varied according to load, and the most requested feature, that the smoke "puffs" in synchronization with the chuffing of a steam locomotive.

 

In many other scales, "how" to install DCC is not a question. The decoder is connected directly to the track pickups, motor and the lights (almost always LEDs). There is often an NMRA standard connector.

But, in large scale, it seems that nothing is designed to work with DCC properly.

Aristocraft has a socket in it's locos that could work fine, but in many cases, even Aristo does not follow their own standard in regards to wiring, and lately even the alignment of the pins.

Also, a fundamental problem is that virtuall all decoders (DCC, AirWire, Aristo Revolution) have the lighting "outputs" actually connect to ground when activated, so the "common" to all the lights is positive. Many locos are internally wired that the common "ground" is negative.

So, how to deal with this?

1. "Quick and Dirty" installation

2. "Traditional" or "full" or "complete" installation

Quick and Dirty:

basically you drive the existing "main board" on the loco from the motor outputs of the decoder. The locomotive operates as though it was on DC track power.

Another way to say this is that you put the decoder in the loco between the track pickups and the rest of the electronics.

If track powered, the track pickups go to the decoder, but the motor outputs are then fed back into the main board of the loco.

If battery powered, you would feed the battery to the decoder and the motor outputs to the loco main board.

The advantages are:

  • very quick and easy to do
  • easy to return wiring back to stock
  • you retain the stock lighting with no modifications.

The disadvantages are:

  • you won’t have constant lighting, it will go vary in brightness as the speedcanges and go out when the loco is stopped
  • you cannot take advantage of any specialized lighting effects from the decoder
  • often locos are not wired correctly so you may get the reverse lights on and the motor going forward

More on lighting wiring:

In USAT locos, unfortunately the wiring “ground” is negative, but in all decoders, like the Revolution, DCC, etc. the “ground” is positive. The USAT lighting circuits are complex, often using 3 different voltages.

Aristo locomotives are worse because there is often a complex voltage regulator that is actually a microprocessor running a transistor for output capability... this system goes nuts on low duration pulse width power.

Traditional method:

This is where you wire a decoder the way it was intended, the independent parts to the decoder outputs:

The advantages are:

  • Constant lighting and constant power to other items
  • Full independent control of motor and lights
  • lights controlled by decoder logic, not polarity
  • lighting in consists can be automatic and operate differently than single locos
  • get rid of all old regulators and internal boards

The Disadvantages are:

  • more work
  • normally takes soldering
  • normally it's best to replace lamps with LEDs

 

More installation notes overall:

On the Aristo socket, there are track pickup wires, but they do indeed go to the socket, and furthermore, you need to establish that when you slide the track/battery switch that the track pickups are not connected to the battery input plug you need a meter to do this if you are trying to use the socket.

This is a mess because Aristo basically has voltage regulators and usually runs them from the motor circuit. If you are not doing a “quick and dirty” you can either figure out the wiring and connect in, or just replace them with LEDs and wire the new LEDs to the G3.

Figuring out the wiring and finding how to substitute resistors (for LEDs) or voltage regulators (for bulbs) and then “breaking” the negative common ground and converting to positive common ground takes some experience. I do it myself, but it’s not for most people.

The Aristo wiring can vary, how it is connected and what colors, so you won’t find any how to’s on the web, and for good reason.

To top it off, the 2 pins on the Aristo socket that SHOULD control the lights often do not, recent versions of the SD45 have nothing connected to them, much to my chagrin.

 

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