In this second photo we can see that the red and black wires from the decoder have been attached to the tender pick-up tabs (foreground) and the original black wires to the motor have been removed and replaced with the orange and grey wires from the decoder. The blue and white wires from the decoder have been connected to the light, after removing the original black wires of course.
The remaining wires on the decoder are not used. The larger white plug is the JST 9-pin decoder harness that comes with the DH123. The smaller white plug is the "factory" connection between the loco and the tender.
In this first photo we see an IHC 4-8-2 "Mountain" locomotive with the boiler shell (foreground) and tender shell (background) removed so that a decoder can be installed. The original black pick-up wires are still attached to the motor on the loco frame.
These black wires will be removed from the motor in preparation for installing a decoder. The black wires from the loco to the tender and tender pick-ups will be left in place. This will provide electrical pick up from both the loco drivers and the tender trucks.
A Digitrax DH121 decoder is typical of the DCC "computer" that is installed in locos to translate the commands from the DCC throttle to DC voltages that control the loco.
Installing a DCC decoder in an HO locomotive is not a difficult task. It can be tedious, especially removing the loco shell or boiler, but with a little care anyone should be able to install their own decoders.
Decoders similar to the Digitrax DH123 shown at the right (Actually that may be a DH121 but they look alike.) cost about $16 discounted and provide the features that I want for my steam locos. These features include "silent running" (The motor pulses are at a supersonic frequency so you cannot hear the motor "buzz".), two functions for lighting and all of the "standard" control variables (CVs) for momentum, braking and other speed control functions. This may sound complicated but it's not really.
