For model year 2004 Harley-Davidson began using a serial data bus to communicate information to devices like the Speedometer, Tachometer and Malfunction Indicator Lamp (Check Engine Light). Basically what this means is that a single wire on the bike has information related to the vehicle speed, engine speed and check engine light status (on or off). The speedometer, tachometer and check engine light monitor this wire to determine what their readings should be. In previous model years, these devices were controlled by separate wires. In other words, the speedometer used the signal from the vehicle speed sensor to determine how fast the bike was going, the tachometer used the tach signal wire or the negative side of the coil to determine how fast the engine was running and the ECU grounded a wire to turn the check engine light on. S&S has updated the IST so it is completely compatible with 2004 and up Harley Davidson motorcycles. S&S is currently working towards having the VFI system compatible with 2004 and up Harley Davidson motorcycles by the fall of 2006. If you are building a carbureted custom bike, feel free to use any year wiring and components in conjunction with the IST. If you are building a custom bike using the S&S VFI system, please be sure to use pre-2004 wiring and components to avoid any issues.