Headlight Relay Installation

By Robert Laposta
June 2001

I just wired up head light relays on my K75. The posted instructions were way too much like rocket science and I didn't understand it until Scott Conary put it in slightly simpler terms [see below]. When I finished I got on photoshop and made this diagram, printed it out to a .pdf file for mass viewing. Hope it helps. Feel free to post.

Scott's words...

You run a big fat fused power lead from the battery. Split into two, one line for each relay - one relay for hi, one for low. Use automotive type relays rated for 30 amps. I bought mine at Radio Shack.

Let's play with the high beam, we'll pretend that its power comes through a white wire (it might, I don't remember). The brown wire going to the headlight is the ground for both beams.

Each relay has 4 'plugs'.

(The relays have a nice diagram on their underside and the back of the package)

Cut the white wire going to the headlight. Take the piece running from the bike side and attach to 'C'. Run a ground line to 'D'. The relay is now a working switch. When the headlight is turned to hi-beam, power runs to 'C' then through the relay and out 'D'. As it goes through it closes a switch allowing power to run from 'A' through to 'B'. Turn off the hi-beam switch on the handlebar, and power stops flowing through 'C' to 'D' thus breaking the circuit for the BIG power ('A' to 'B'). Nice.

But you still need to hook up the big power. :) Attach one of the Big lines from the battery to 'A'. Attach the white wire that is still attached to the headlight to 'B'.

You might beef up the wire if you're feeling industrious. While you're there, also beef up as much of the brown lead to the headlight as you can.

And now your hi-beam is run through a relay. Because you used nice big wires, and it doesn't have to run through the dirty nasty little switch on the handlebar, you can run much more juice to the headlight.

Now do the same for the low beam.

I used spade type connectors so that I could remove the relays if I wanted to. I mounted them just above the headlight, attaching them to the underside of the top center piece of the fairing (the bit the windscreen attaches to). Fuse the main power lead, and use the thickest wire all around that you think is reasonable. Even if you stick to a stock size bulb, you should see a nice improvement in brightness from the headlight.

Robert Laposta
iBMWr ; IBA #7268 ; LPR #71
'87 K75S - "Maybelline"
'75 Honda CL360, which threw me, wench...
'72 Indian ME-70cc dirt bike