See photo on Flickr.

After a year under a tarp in a driveway, my 1977 BMW motorcycle is (very Germanically) requiring me to prove my dedication before permitting me to ride to the train station in the morning. Now that the brakes are sorted, the headlight is acting odd. There are two switches that drive the headlights; one that allows you to turn the headlight on or off during the day; another to select the high/low/passing beam. Different combinations of switches provide different, puzzling (and very much not standard) results; it’s like a game of Mastermind with accidental blasts on the horn to liven things up. And, sometimes, blue sparks when I short things.

Fortunately, the Airheads list continues to be a fount of wisdom from people who see Airhead wiring diagrams on the backs of their eyelids when they go to sleep at night, and they’ve been giving me plenty of advice. For example, an airhead named Joe ‘Cuda says:

>> It’s dirt simple. Juice (+) comes from the battery via a Red wire to the Relay 30, out Relay 87 via a Yellow/White wire to the switch, to the headlight via a Yellow (Low) or White (High) wire, and then to ground via a Brown wire.

While that doesn’t really fit my description of “dirt simple”, the instructions I’ve been getting have been helping me to sort out the electrical connections. It’s a definite contrast to pushing pixels for a living: while it may look like a rat’s nest inside the headlight shell at first, it resolves to a rational (if complex) system of color-coded wires, each bit of which can be individually checked with a multimeter. So I think I’ve got it narrowed down to a bad headlight relay, which means another call to the Amazon of BMW parts.



Update: Just to fully illustrate the detailed, descriptive, and entertaining nature of the help the Airheads provide, here’s the response I got this morning from “Airhead John”:

Nice photos! I can see what you were talking about. In the first photo the headlight shell looks quite clean and the wiring and fuses look to be in good condition.

In the second photo I agree that the mystery wires are going to the turn signals. Rather than fuses, it looks like 2 into 1 connections. I imagine a previous owner (we’ll call him Sparky) put on an ugly fairing of some sort and spliced into the stock turn signal wires to hook up the fairing turn signals. When the next owner (we’ll call him Rico Suavay) bought the bike he wanted the wind blowing through his hair, so he took off Sparky’s ugly fairing and stuffed the turn signal extensions into the headlight. You don’t need that stuff. I would return it to stock. It appears Sparky snipped off the spade connectors on the blue/red and blue/black turn signal wires and crimped on his yellow bodges. If you leave it like you found it, it
won’t hurt anything (except my feelings).

In the third photo, the yellow/white (gelb/weiss) wire should be getting power with the ignition on. It supplies the headlight switch. Although the headlight relay and all the connections look quite clean, they do fail after a few decades. I concur with your diagnosis of a faulty headlight relay.

Good work.

I swear, can you imagine getting this much and this quality of help in other aspects of life?