1975 R90 wiring question. Little black wire at coils.

[mattcfish]

A friend of mine is wiring up a 1975 R90/6. I was wondering if anyone could answer his questions.
I know about the little black wire, I think mine is disconnected. It doesn't show up on some diagrams.

"Yo! Quick wiring question. I have three wires that need homes. You have same year as mine so if I could ask you a favor?can you look at your bike for these?
One. Black wire from main harness comes out right next to coils under tank ( not coil crossover wire or wire from condenser). Is that a coil wire? Can't remember.
2. In headlight bucket , a black wire from main harness, mine has a plastic cap, not an angled connector for the board, as well as a brown wire with a dull orange tracer (same color as the brake switch wire) both are in the headlight bucket, what's throwing me off is both have the plastic spade cover,
Thanks!"

[Garnet]

The black wire at the RH coil should go to the condensor and then on to the points.

Inside the headilght

[mattcfish]

The black wire at the RH coil should go to the condensor and then on to the points.

Inside the headilght

Thanks, but it's not the condensor wire, he's got that figured out. I seem to remember this wire on mine not appearing in the Haynes manual. My wiring has changed due to aftermarket upgrades, so I can't be positive. I think it's something that some early /6's have that other bikes don't.
My theory about the wires in the headlight is, that they went to extra guages or a hazard switch.

[Kurt in S.A.]

1975 might be a bit early, but at some point, BMW was looking towards the electric tach and began to include that in the wiring harness, whether it needed it or not. Certainly, I would think that the '77 /7s (still mechanical tach) got the electric tach wire. Maybe that's what the coil wire is for?

Kurt in S.A.

[melville]

Mine has two black wires in that area, one for the starter relay and one for the coil IIRC. I'll give it a look when I get home. 12/74 production R75/6.

[Garnet]

That is where a tach wire would go, from the pionts side of the coils to the insterment pannel.

[djkilmer]

This is my bike we are discussing. Tonight I will check continuity between the two black wires. I mainly have owned /7 and up so this is my first /6. On Friday I am able to look inside the same year /6 headlight, but the bike is disassembled so the black wire at the coil will still be in question.

As far as the other extra wire within the headlight I am curious about, I suppose it is for the parking light on the headlight? I do not have the original headlight on so that is not applied to my bike.

[melville]

This is my bike we are discussing. Tonight I will check continuity between the two black wires. I mainly have owned /7 and up so this is my first /6. On Friday I am able to look inside the same year /6 headlight, but the bike is disassembled so the black wire at the coil will still be in question.

As far as the other extra wire within the headlight I am curious about, I suppose it is for the parking light on the headlight? I do not have the original headlight on so that is not applied to my bike.

I've got it hooked up to the hot side of the coil set, but I don't think the other end has a place to go. On my Haynes wiring diagram, there's a black wire marked "US Models" where the other end goes to the lights relay, post 85 or 86 (signal side of relay) but that made no sense, as that should be the ground side of that relay circuit, and black is "switched-on hot" in the DIN system. I have a proper brown wire hooked up at the lights relay, and the black wire taped up and set aside int he headlight bucket. Thinking about it now, hooked up to the hot side of the coils the black wire is possibly acting as a redundant power source to the coils, and one that does not shut off with the kill switch.

Perhaps while I'm avoiding family this weekend I'll go out to the shop with a multimeter and see what's attached to what, and what's hot in what circumstance. I'm liking Kurt's theory about the black wire. I do know that I have an extra wire for a brake light switch in the harness, one that would be used with a front drum brake, so a wire for an anticipated electric tach wouldn't be too far fetched.

[Chuey]

No. Clearly, if they were planning ahead for devices to be included at some future time, it would have been the wire to power the onboard computer display unit.

Chuey

[Major Softie]

No. Clearly, if they were planning ahead for devices to be included at some future time, it would have been the wire to power the onboard computer display unit.

Chuey

You are so helpful.