Check fuses and relays

A fuse is a thin metal link that melts when too much current flows, so the wire does not overheat (Unit 2, PDF p. 33). A relay is an electric switch: a small current through its coil pulls in a contact that switches a larger current, for example to the starter, headlight or fuel pump. When a whole circuit is dead, check these first.

Safety

• Turn the key off before pulling fuses or relays.
• Never replace a fuse with a higher rating, a wire, foil or a nail. The fuse protects the wire. Without it, a short can melt the harness and set the bike on fire.
• A fuse that blows again straight away means there is a fault. Fuses blow for a reason (MAVCC PDF p. 313). Go to Find a short circuit.
• Any jumper lead you use to power a relay coil must have its own inline fuse.

You need

• The fuse box and its label. Fuse boxes are often under the seat, near the battery or behind a side panel; some bikes also have an inline fuse holder near the battery.
• Spare fuses of exactly the same ratings and type (blade, mini blade or glass tube). The K-Light 125 wiring diagram shows a 20 A main fuse and two 10 A fuses (PDF pp. 74 and 278). The manual we used does not give a full fuse-to-circuit list, so check the label on your fuse holder.

Steps

1. Look first, then measure. Pull the fuse and look at it. A broken link or a dark, burnt window means it blew. But a fuse can look good and still be open inside, so always test it with the meter (MAVCC PDF p. 312).
2. Test with the meter, fuse in place. Key on. Meter on DC volts (V with a straight line, 20 V range). Black probe on battery negative. Blade fuses have two tiny metal test points on top: touch the red probe to each one. - Battery voltage on both sides: the fuse is good. - Battery voltage on one side only: the fuse has blown. - 0 on both sides: no power is reaching the fuse. Check the main fuse, the battery and the ignition switch.
3. Or test the fuse out of the bike. Key off. Meter on continuity (beep or lowest Ω). Probe both legs of the fuse. A beep means good. No beep means blown.
4. Replace a blown fuse with one of the same rating. Turn the key on and use the circuit. If it blows again, stop and go to Find a short circuit.
5. Check the fuse holder. Green corrosion or loose clips cause faults that come and go: Clean and seal electrical connectors. Melted plastic means a bad connection was heating: replace the holder.
6. Find the relay. Use the diagram. Relays are small plastic cubes, often next to the fuse box. The K-Light 125 has a starter relay, a main relay, a headlight relay and the flasher under the seat and side panels (PDF pp. 74 and 278).
7. Swap test. If two relays in the box are the same part, swap the suspect relay with one from a circuit that works. If the fault moves to the other circuit, the relay is bad.
8. Click test. Key on, operate the circuit (press the start button, switch on the headlight). Touch the relay. A click means the coil is getting power and earth. No click: check the coil feed and earth with the meter, or the control switch (for the starter: Test the starter relay).
9. Bench test. Relay out. Find the coil pins and the switch pins on its label or in the manual. Meter on continuity across the switch pins: normally no beep. Connect 12 V across the coil pins with fused jumper leads: it should click and the switch pins should now beep (K-Light starter relay method, PDF p. 94). If not, replace it.

Check your work

• The circuit works and keeps working after a 10 minute ride.
• No fuse or holder is warm after the ride.
• You have a spare fuse of each rating stored on the bike.

If it did not fix the problem

• The fuse blows again: Fuse keeps blowing.
• Fuse and relay good but the circuit is still dead: Test a lighting circuit or Find a wiring fault.

Reference values (K-Light 125)