Fuses

A fuse is a small part with a thin metal strip that melts if too much current flows. Most modern bikes use plastic blade fuses in a fuse box under the seat or side cover; older ones use glass tube fuses. There is usually a main fuse near the battery or starter relay, plus smaller fuses for each circuit.

What it does

If a wire rubs through and touches the frame (a short circuit), current rises very fast and the wire could melt and start a fire. The fuse melts first and cuts the circuit. A fuse does not wear out in normal use. When one blows, something caused it: find that cause before fitting a new one.

Signs it is worn or failing

• Fuse keeps blowing.
• Nothing happens when the key is turned on.
• All lights stop working or one group of items dead.
• Corroded or loose fuse holder, or a fuse that looks fine but has no continuity. MAVCC warns never to trust a fuse by its look.

When to replace it

When it is blown, or the blades or holder are corroded. A holder that is discoloured or melted means poor contact or overload; replace it.

What to match when buying

• Fit the same rating (amps) and same size and type (mini blade, standard blade, glass). The rating is printed on top. Never fit a larger fuse, a wire or foil: the wiring can burn.
• Blade fuses are colour coded by rating, but always read the number.
• Cheap fuses may blow late or not at all. Buy fuses from a known maker in sealed packs.
• Carry spares of each rating on the bike.

Fitting notes

• Rider job: Check fuses and relays. Key off first.
• If it blows again, look for a short: Find a short circuit.
• When adding accessories, give them their own fused supply: Wire an accessory safely.

Reference (K-Light 125)