Shimano Non-Series EW-JC304 Di2 SD300 4-port Junction Box in Black

The EW-JC304 is the junction box that makes 12-speed Di2 configurations work. It provides four SD300 ports arranged in a compact inline housing designed to sit inside the frame's downtube or bottom bracket area.

Most 12-speed Di2 installations use a simple three-wire setup: one SD300 wire runs from each shifter lever down to a junction inside the frame, then a single wire continues from that junction to the seatpost battery. The four ports give flexibility in how the system is routed. In the common three-wire setup, both shifter wires plug into one end while the battery wire plugs into the other end, leaving one port empty. That empty port needs a dummy plug, which Shimano includes with the junction box. All unused SD300 ports in the E-Tube system require dummy plugs to maintain proper sealing and electrical continuity.

The EW-JC304 can also be used to extend wire lengths by connecting two pairs of SD300 wires through the junction, essentially splicing four wires together while maintaining full power and data transfer. This is useful on larger frames or unusual routing situations where standard wire lengths fall short. The junction box is small enough to fit through most internal routing ports and tuck away cleanly inside the frame.

Installation requires the TL-EW300 plug tool. The SD300 connectors are smaller and more delicate than the older SD50 style, and trying to push them in by hand risks damaging the connection. The plug tool ensures clean insertion and removal without stressing the wires or ports.

Features:

• Four SD300 ports in a compact inline housing
• Includes dummy plug for unused ports
• Enables wire length extension by splicing wires
• Fits through most internal routing ports
• Requires TL-EW300 plug tool for installation