electrical question, 75 R90S

[lightfighter]

can anyone ID a green/red and a black wire enter the headlight bucket and go nowhere and appear nowhere on the diagram. they are in the main loom, so to speak, and although they have different connectors than the rest, appear to belong there. No guarantee this is the factory loom BTW.



this is a side note to poking around in the bucket yesterday to see if my horn was hooked up correctly, and probably cuasing a short in the brake light circuit since it only just then started blowing fuses.

[Bill Harris]

The GN-RT (green-red) wire is a brake light wire, probably to/from the front brake switch, or maybe the main harness back to the brake light light. The SW (black) wire probably is te one that goes between the "unhot" terminal of the horn (or horn relay, if installed) and the (grounding) horn button on the bars.

Time to break out a schematic and start tracing wires. A lot can happen in 35 years, and more can happen if to PO changed anything.

[lightfighter]

the horn is an interesting comment, since the two wires dangling where the hold goes are green/red ( which is igniton hot), and brown/white, (which is hot when the horn button is pressed.)

i see in a wiring diagram for 74 some clues to this being a brake switch for a front brake switch. (tho that shows green/red and green/black) beside which my brake circuit appears to be all correct.

http://r90sclub.dudley.nu/Articles/74R90_Wiring.pdf

im not sure if this is a 100% accurate for the 75, but it does not include that pair.... my bike was produced in mid 1975, FWIW, and I am fairly sure that I've gotten out all the PO non stock wiring. it wasn't even close to looking this native.

http://r90sclub.dudley.nu/Articles/R...ng+Diagram.pdf

[chasbmw]

If your bike is mid 75 and you have earth wires on your front indicators , then you have the later wiring harness, the 75s can be a mix of parts!

[lightfighter]

so, i just hooked up a horn to the brown/white wire, and grounded it, ala "not the way its sposed to be", and it is functional. so thats not BMW(not that i care, im just trying to not have random hot wires flopping around cuasing problems later.)

in the mean time, i accidenty shorted that ignition hot horn wire to the frame, and killed everything... for a minute, then it came back. i see no breakers in any diagram im looking at, and i didnt hear any clicking. i did it again, on purpose, and repeated the results. WTF?

[lightfighter]

Originally Posted by chasbmw

If your bike is mid 75 and you have earth wires on your front indicators , then you have the later wiring harness, the 75s can be a mix of parts!

my front turns are missing...

[disston]

There should be two fuses in the headlight bucket. They are those pointy European type fuses, older drivers and mechanics will say, "Oh, those types in VolksWagens?". Same type fuses were used in aircooled Bugs. Small Amperage, maybe 7 or 8, I forget. One of these will kill your ignition and the other will kill some of the lights, like brakes, I think. They, these fuses, appeared in the later years of /5 production according to one manual I have but I suspect this may not be exactly correct. Some /5s do not have any fuses, I've heard.

As long as you are poking around in there it's a good idea to clean the tits of those tiny fuses and the spades they contact with. Evan a small dab of Dielectric grease would be nice, if you have it handy. Every once in awhile we have a rider show up with a mysterious electrical problem and he traces wires for several days until these contacts are checked and it is discovered that the whole problem had been a tiny bit of corrosion on the tip of a fuse.

If you do not have any fuses at all? Then I would think you have an early /5 harness.

[Bill Harris]

The BR-SW (brown-black) is the wire that goes from a starter relay to the starter button on the bars-- the button grounds this wire, energizing the relay.

Time to break out a schematic and start tracing wires. A lot has happened in 35 years, and more oddities can and will happen if the PO changed anything.

[wirewrkr]

Don't Use The Diagram from 1974, it's wrong for a 1975/76 HERE IN THE USA.

Trust Me

[lightfighter]

How about the other one wire? Thoughts on its validity?

My fuses are the clear glass automotive type.

[bmwrench]

The black wire may be connected to the headlight relay to turn the lights off when the starter is being used. You'll find the other end of said wire at the starter relay at terminal 87. To use the black wire for this, remove the brown wire from the relay and replace it with the black.

The fuses control switched hot items. The Green to Green/Blue fuse powers the turn signals, horn and brake light. The Grey to Grey/Black fuse powers the parking light amd instrument lighting circuit. The ignition circuit and headlights are not fused. Nor is the clock, should you have one. I suspect more than a few BMW harnesses have gone up in smoke because of this.