|How to Wire Relays
Basically I took some lengths of wire that I connected to the battery. I installed a fuse holder (a 15 A fuse will be strong enough… might even be a bit too much) in each wire, within a few centimetres of the battery terminal - remember, you want to protect the wire in case of, say, chafing of the insulation, not the actual bulb. (Note: Guzziology suggests getting the hot wire from the existing fuse box - I wanted to eliminate as much wire as possible and so ran the hot wire straight from the battery).
This hot wire from the battery is connected to terminal 30 of the relay. The existing headlight wire from the handlebar switch is cut and connected to terminal 86. The new hot wire from the relay to the headlight connector is connected to relay 87. The terminal 85 of the relay is connected to ground.
A normal four-terminal relay will do; I think a five-terminal relay, such as a starter relay will work as well, as long as it has these terminals. There might even enough room for the relay in the headlight bucket.
Now the power from the handlebar switches via ignition lock and whatever else only powers the relay control circuit while the actual voltage for the headlight is run through the new, unobstructed wire and the relay.
One circuit is needed for each filament of the headlamp, so that if you want to run both high and low beam through the relays, you'll need two relays, fuseholders etc.
I'm sure somebody will next come up with a circuit to eliminate one of the relays and still run both filaments, but this is an easy way to do it.
Hope this helps.
-Topi Kuusinen, Finland
'86 LM IV