Headlamp Relays

Dec 5, 2005
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I am updating the old Lucas wax candle on my 1973 750 Commando to a Halogen unit. I have acquired a H4 headlamp unit that also has provision for a pilot light. I will be using a 60/55 watt halogen globe. I am hoping that the stock RM 21 alternator will be up to the task. I have improved the earthing by running a new earth from the headlamp back to where the main earth connects at the head steady. To further guard against voltage drop I intend to fit a relay. I assume that I will need to fit relays in the high beam power lead as well as in the low beam.

It would be appreciated if a forum member was able to provide me with a wiring diagram showing the correct placement of the relays in the circuit.

I have been considering the idea of fitting an LED as a replacement for the pilot globe. My thinking is that if I can get an LED of sufficient wattage it might make a suitable "daylight running light" and save the need to ride with the headlamp on in daylight.I ride with the headlight on in daylight because of previous encounters with drivers who say "sorry mate I didn't see you." One problem is that with the existing setup the pilot light is also illuminated when the headlamp[High and Low] is in use. Is there an easy way to isolate the pilot light so that it does not illuminate when the headlamp is on?


When you run the ground back to the battery or the common ground point, run a fused power lead off the battery up into the headlight as well. Then buy some nice new three way common female bullet connectors and plug in the two new wires. Now the new little cube relays get velcroed into the back of the headlight short wires plug into the new female connectors to the power side of the three new relays one for dim,one for Bright. one for the horn. All you have to do than is plug in the switch leads for each idem onto the other side of the relays. If you also want to relay the kill and key switch to the coils via the E.I. a place by the battery box works best.
You will be pressing that alt. to keep up I would be stepping down that bulb a bit. I use the 20 watt halogen pilot bulb 11.00 at Walridge. When you have it all done if you wire in a AMP gage to the positive lead on the battery and rev the bike to 3000 or so with the headlight on you stll need one AMP positive to keep the battery happy.
Speaking of relays, I've heard that Radio Shack has suitable ones. Does anyone know if that's true, and if so, when the 16 year old sales clerk says "can I help you?" what would I ask for?

Or, if Radio Shack isn't the place, where can I source some?

Stan Smith at Rocky Point cycle carries the mini relays. They are used in cars so a junk yard with late models is a cheap source. You need the extra thick furry velcro for the curved surface on the inside of the head light. Both the four pole and the five pole will work. They do make a mini that can control two switches it's just a bit larger. The switched side uses 3/16 spades and the power side is 1/4 spade. Relays like this are not polarity sensitive.
Bazz - Don't use an LED bulb as the pilot unless you can isolate it. They don't like the heat put out by the H4 bulb and won't last long........
For the Victor I am building I have purchased a Halogen 23W pilot bulb for daylight running, an LED would not have enough power for daylight and also it is very focused so can be seen well only from straight on. As I am looking to be seen by cars waiting to pull out from a side road I am figuring an unfocused light like the 23W pilot is the way to go. Main bulb will be a 35/35W Halogen.