Installing DCC in an S4 I have not done a QSI install on this loco yet, waiting for the new QSI USAT board. In the mean time, R.J. Deberg and I fitted an NCE D408 to the S4. Conversion of lighting circuitry: There are 2 boards that have the lights in the S4, a front board that has the front headlight and the number board lights, and a rear board that has the backup light and the cab light. The front headlight and number board lights are LEDs (2 for the number boards). The rear headlight is a LED and the cab light is an incandescent bulb. There are 3 wires coming from each board, and one wire is a common. To make the cleanest install and re-use the existing boards and lights, you need to convert the boards from a "negative" common to a "positive" common, since DCC boards use a positive common for function outputs. Front circuit board: You need to reverse the polarity for all 3 LEDs. You can either remove the wires from the board and re-solder in reversed , or cut the wires and resolder blakc to red. We are suing the decoder white and the blue wires. The dropping resistor should be on the decoder white wire.Connect the black wire from the lighting board to the blue decoder wire. Connect the yellow wire from the lighting board to the white decoder wire which has the dropping resistor in line. Rear circuit board: You only need to reverse the polarity for the rear headlight. The cab light is an incandescent bulb. Again, either remove the LED wires from the circuit board and reinstall in reversed direction, or cut the LED wires and resolder reversed. We are using the decoder yellow and blue wires. The dropping resistor should be on the decoder yellow wire.Connect the black wire from lighting board to the blue decoder wire.Connect the yellow from the lighting board to the yellow decoder wire which has the dropping resistor in line.