For old OO models where the motor is located in a steam train's tender, or for modern models where there is a decoder
socket in the tender, but the motor is in the loco, it is often simpler to add a 2nd decoder into the loco part of the model
to provide DCC control of functions up-front in the loco, such as loco lamps, firebox glow and smoke.
This is often more desirable than trailing lots of wires between loco and tender as such wires tend to spoil the
look of the model and can even affect running of the loco causing derailments in the most extreme situations.
As a general rule, if you have more then 3 or 4 wires joining loco and tender, this is the threshold where deploying
a 2nd decoder is preferable.
Examples of such models include many of Hornby's current DCC Sound models where the sound decoder and speaker is in the tender
and a small 4-way connector is already used to join the loco to tender.
Hornby's modern 4-pin micro-connector joining loco to tender...
There are a variety of function-only decoders suitable for this job, including YouChoos' favourite, CT DCX33 7-function decoder,
and Zimo MX681, or TCS FL4. However, almost any decoder will do - even Hornby's low-cost decoder can be used for basic lighting - just
don't bother connecting the motor output wires to anything!
Essentially the only connections you have to worry about are the track pickups, which are the red and black wires of the
decoder. Simply join them into the loco's pickup wires somewhere, which may mean cutting the existing pickup wires and joining
the 2 cut ends plus the decoder wire together.
After that, use the function output wires of the decoder for whatever purpose you require, remembering that the BLUE wire
is the common positive and other wires are the negative side of each output (WHITE, YELLOW, GREEN etc.). All manufacturers
of decoders use the same colour scheme for the 1st 3 function outputs (WHITE, YELLOW, GREEN), but beyond that there are
various schemes used. Many use PURPLE for the 4th output, such as Hornby and CT, but others used BROWN, such as Zimo. Read
the decoder's documentation to find out which is which!
Decoder's RED and BLACK (pickup) wires connected to loco's pickups...
Fitting DCC sound to the tender is straight-forward, but don't be afraid to
modify the hidden parts of the tender to get the speaker where you want it...
Remember when connecting LEDs for lighting that each LED should be protected by a resistor (something between 680ohm and 10k
depending upon how bright you want the light) on the positive side i.e. connect a
resistor between the LED's positive side and the decoder's BLUE wire. You can share resistors between multiple LEDs, but each
LED will be affected by the others if you do this e.g. if you have 2 cab lights connected on different DCC functions, switching one
cab light on will cause the other light to dim slightly, so best to use a separate resistor for each LED.