‘G’ Scale Alco S4 to DCC

Fitting a sound decoder in G scale locos is not difficult due to the immense amount of space inside. Most problems occur in trying to break into them in the first place and sorting out the wiring. The model in question is a USA Trains Alco S4 Switcher. For this model I used the QSI Q2 Magnum Decoder. One of the new breed of programmable decoders. You can buy this decoder with the correct sound files already installed, or you can install the sound files yourself using a computer and QSI Programmer software.

Problems and Solutions
The main problem with this model is not the fitting of the decoder or speaker, but sorting out the wiring and lighting. This model is not DCC friendly, in fact it’s positively hostile. Wiring for Motors, pickups, lighting and smoke all need modification in some way for operation on DCC.

Strip Down

This is quite a small model by G scale standards, but it’s still quite a handful to work on. (Don’t go underneath without using axle stands). It’s best to use a loco cradle with thick foam padding to protect roof detail and walkways when working on the model. Six screws hold the long hood in place, some of them quite difficult to access as the trucks have to be swiveled out of the way to get at them. The hood also engages in the front of the cab so has to be eased out of there as well. A plug and cable connects the smoke unit and the front Headlight and number board lights. A plug has to be separated from the Switch PCB for the Cab light and rear headlight. Two more screws hold the cab in place. Then everything can be separated from the underframe. The trucks are self contained and identical, plugs connecting them to the Switch PCB. The pickups and track skates are connected to one plug and the motor to another. Only red and black wires are used, but it’s easy to tell which is which as 4 wires connect the pickups and only 2 for the motor. I un-soldered the wires from the Switch PCB leaving the plugs and sockets intact so I could disconnect the trucks easily for servicing if needed. I ditched the Switch PCB, it’s not needed for DCC and takes up too much room. If you intend to use the smoke unit disconnect it’s wiring from the Switch PCB as well.

Traction Wiring

I connected the wires for both trucks together. As one truck is aligned 180 degrees to the other it’s necessary to reverse the wiring, red goes to black and vice versa for both motors and pickups. Otherwise the right rail pickups will be connected to the left rail pickups and the motors will try and go in opposite directions, remember the trucks are identical. I added a short length of cable to each joint to go to the decoder. Red and Black for the pickups, orange and grey for the motor. Covering each joint with some heat shrink tubing. Less chance of connecting something wrong if you use the right wiring colour code.

Installing the Decoder

The QSI decoder fits neatly between the hood rails and is held in place by a couple of sticky fixers, wire connections are mainly by screw terminals. Personally I don’t like these, they are fiddly to use and it’s difficult to fit more than one wire, I would prefer solder pads. Connections for the speaker are by a board mounted 2 pin plug and socket.

Installing the speaker

The decoder is not supplied with a speaker so you have to choose an appropriate one and order it separately. I chose a 2″ diameter speaker, the largest that would fit in the hood. I found it was a perfect fit in a plastic Aerosol cap. The wires were lead though a hole in the cap top then the hole and speaker edge were sealed with silicon sealer to make it air tight. This makes a good speaker enclosure that improves the sound dramatically. With the speaker plugged in and motor and pickup wires connected to the decoder, the chassis can be tested on the programming track to check for shorts. If all is well it can be run on address 3.

In order to get the speaker in it was necessary to relocate the forward weight. A little metal had to be removed to clear the inside of the hood, then it was fixed to the chassis with silicon mastic. The speaker was fixed in place the same way. See Figs 6 & 7.


Before installing the speaker it is necessary to rewire the front headlight and number board lights. As it comes the cathodes of the front end LED’s are wired together, these had to be rewired to give common anode connections to allow the numberboards and headlight to be controlled separately. DCC control has a common positive connection (blue wire) with functions controlled by the negative side. This was achieved by reversing the LED connections on the PCB. While I was on I replaced the headlight LED’s with high intensity LED’s to give a brighter light.

The aluminium foil covering the LED’s visible in Fig.2 is to prevent light leakage, this is the way USA Trains do it. It is simple and it works, just make sure you have no bare wires under it. QSI in their wisdom only provide forward and reverse headlight outputs (F0) on their Magnum decoders. There is supposed to be an add on daughter board that plugs into the main board to provide additional outputs, however up to now it has not materialised so an additional decoder has to be provided for any additional lighting. I fitted a TCS 4 function only decoder to provide control of numberboards, beacon, cab light and smoke unit. The TCS decoders are capable of full function re mapping, well, up to function 12 anyway, so you can control these outputs with a function that does not interfere with sound control. You don’t want the cab lights coming on every time you blow the horn.

The cab PCB had to be modified like the front end one and an additional wire added for control of the beacon. See Figs. 3,4,& 5. Unfortunately it is neccessary to completely remove the cab interior for access to the lighting board in the roof. This isn’t too bad an operation, but putting it back in isn’t easy.

There are a lot of options for control of the headlights on the QSI board. On in direction of travel only, dim in reverse, dim when stopped etc,etc. The easiest way to program these options and the myriad sound control options is using Decoder Pro. This is a software program, available off the internet that interfaces with your DCC system. I cannot praise this program too highly as it makes all decoder programming a doddle. You can program without it, but wading through QSI’s multi CV options is not to be undertaken lightly. Threading your way through a maze blindfold is an easier pastime. If your DCC system does not have a computer interface, now is the time to change it. It’s worth it for this program alone.


The USA Trains smoke unit is a sophisticated little device. It’s fan driven heater is controlled by a voltage regulator and once the fluid is warm gives smoke as soon as the loco starts to move. In order to control it I used the TCS decoder to drive a relay. The smoke unit takes too much current to be controlled directly and anyway we need a way to control the smoke output. To do this the smoke unit is connected to the QSI motor output terminals (orange and grey) via the relay contacts. The relay is a 12volt relay with a contact rating of at least 2 Amps.

The original USA Trains control PCB is still used but now it is under DCC control. Once the function is turned on smoke increases along with motor speed. Not perfect, it should increase with engine sound output, but it looks pretty good. It can be seen operating in parts of the video.

Finally the model was given detail modifications to match the Prototype. The Santa Fe S4’s had a different exhaust stack, horn and canvas window shades, amongst other differences. See photos opposite. See below for a video of the model in action.

Note. Modifying the USA Trains locomotive in this way will nullify your warranty.