Daytona 675 Aftermarket Brakelight Switch/Ignition Switch Interposer

What it does

The interposer preserves all six original switch circuits while adding two branch wires to the brake/rain-light circuit. The aftermarket brake switch can therefore be connected without cutting the BSD switch harness or the original Triumph wiring loom.

The modification is completely reversible: unplug the interposer and the aftermarket ignition switch can be connected directly to the motorcycle again.

BSD ignition switch
        │
   6-pin connector
        │
        ▼
┌──────────────────────┐
│ Custom interposer    │
│                      │
│ 4 circuits ───────── │
│                      │
│ Brake A ───┬──────── │
│            └─ switch │
│                      │
│ Brake B ───┬──────── │
│            └─ switch │
└──────────────────────┘
        │
   6-pin connector
        │
Triumph main harness

Why use an interposer?

• No cutting or stripping of the aftermarket ignition-switch harness.
• No permanent modification to the Triumph main loom.
• The brake switch is integrated cleanly into the existing brake/rain-light circuit.
• The whole modification can be removed or rebuilt independently of the motorcycle.
• It creates an OEM-style serviceable sub-harness instead of an irreversible splice inside an expensive loom.

Materials

Tools

• Open-barrel / F-crimp terminal crimper
• 20 AWG wire stripper and cutters
• Soldering iron and electrical solder
• Heat gun
• Multimeter with continuity mode
• Small depinning pick or 1–1.5 mm precision flat screwdriver
• Small needle-nose pliers

Construction

1. Choose the interposer length. Cut six equal pieces of 20 AWG TXL approximately 4–6 inches long. These form the six circuits that pass between the two HM090 connectors.
2. Record the original connector layout. Photograph both sides of the original connector and keep the latch orientation consistent. Label the six cavities before inserting any terminals. Every circuit must remain cavity-for-cavity through the interposer.
3. Identify the two brake/rain-light conductors. On the BSD plug-and-play switch these are color coded. Only these two circuits are branched. The other four remain simple straight-through wires.
4. Crimp the HM090 contacts. Strip the 20 AWG TXL and use a proper open-barrel crimper. The forward wings capture the bare conductor; the rear wings support the insulation. Do not solder the HM090 terminal crimps.
5. Build the four normal circuits. Connect each of the four unaffected circuits directly from the BSD-side housing to the corresponding cavity in the Triumph-side housing.
6. Build the two brake-light Y-junctions. Each Y-junction contains three 20 AWG wires: BSD side, Triumph side, and brake-switch branch. Crimp all three into one Splice 4x18 terminal. Repeat for the second brake-light conductor.
7. Seal the Y-junctions. Cover each open-barrel splice with adhesive-lined heat shrink and shrink it fully so the metal joint is sealed and supported.
8. Extend the brake-switch leads. If the microswitch wires are short, solder in 20 AWG TXL extensions. Make a mechanically sound inline joint before soldering, insulate each conductor individually with adhesive-lined heat shrink, and keep the solder joint away from a location that flexes with every lever movement.
9. Add harness sleeve. Use roughly 6.5 mm sleeve around the main six-wire bundle and 4.5 mm sleeve around the two brake-switch extension wires. Leave enough free wire at the connectors and junction to avoid strain.
10. Insert and lock the terminals. Push every HM090 terminal into the correct cavity until its locking tang engages. Gently tug each wire afterward. Depin and correct any misplaced terminal rather than forcing it.

                    ┌──── Triumph harness
BSD brake wire ─────┤
                    └──── Brake-switch extension

                    ┌──── Triumph harness
BSD brake wire ─────┤
                    └──── Brake-switch extension

Test before installation

Use a multimeter in continuity mode before connecting the interposer to the motorcycle. Verify that all six cavities pass straight through to the matching cavity and that no unrelated circuits are shorted together.

A → A   PASS
B → B   PASS
C → C   PASS
D → D   PASS
E → E   PASS
F → F   PASS

Then verify that each brake-switch extension wire is connected only to its corresponding brake/rain-light conductor. Operate the brake switch and confirm that the intended circuit opens and closes correctly.

Installation

Plug the BSD switch into one side of the interposer and the Triumph main harness into the other. Route the two brake-switch wires toward the master cylinder, leaving enough steering movement and keeping the harness clear of the throttle, brake line, fork, and steering stops.

Test the brake light with the bars at full lock in both directions before riding.

BSD SWITCH
     │
 [HMU-6]
     │
     │    CUSTOM INTERPOSER
     ├──────── four circuits straight through
     │
     ├── Brake A ──┬──────────────
     │             └── Brake switch
     │
     └── Brake B ──┬──────────────
                   └── Brake switch
     │
 [HMU-6]
     │
TRIUMPH MAIN HARNESS