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I went for ride yesterday, tried the horn and it didn't work. So today I pulled the tank to see what was up. Here what i did and the results so far. Flushed switch with contact cleaner. Disconnected wires to horn, the black wire goes to switch, with meter it indicates good going to ground. Green wire comes from junction on starter relay, got continuity between it and the green wire of the horn. All those green wires go through fuse from ignition switch, but every other circuit on that fuse work so that can't be it (new style automotive plug in type) . When I put meter on horn wires and turn on key and hit switch, have 12v at those wires.
check horns to see if good with another voltage source, they work.
One thing I did notice when ignition on and horn button left depressed voltage did drop a little to below 12v.

More info: Pulled headlight, checked wiring, did find my solder connection for wire to new fuse has broken from lug on board, which could explain lack of current, repaired, but still no honk.

any ideas???

One thing to remember, the horns pull a lot of current in order to operate, a meter does not. So you could have voltage indicated by a meter and still not have a good enough connection to supply all the required current.

The way to get around this is to unplug the GREEN wire from the horn. To that same terminal connect a 3ft long wire. The loose end of that wire will become a probe, that when touched to any Positive voltage source will sound the horn. Sound is always better than looking at a meter.

Start with the battery Positive terminal. Then check both ends of both fuses. Then you can poke around the horn relay.

Wobbly,

I have run across that situation before. I did find 2 loose connections that the solder had broken free on on each of the wires going to the new style fuses from the terminal board. Resoldered them and still no honk. I was thinking of doing the green to positive on the battery next, that way I could see if the horn button was part of the problem as it should provide the ground path if I do it that way. Puzzling as everything else on bike working fine, but horns do draw a lot of current.

If you hook battery directly to a battery...does it work?

ZM, yes horns work, hooked them up to voltage source, hit switch, loud noise startled me. I will do a few more tests today to see if I have a wiring problem, switch or if the starter relay has gone south on me.
Since we are scheduled for heavy rain, good excuse to play with bike. I was looking at replacement relays, dealer has one for $65.00 which is OK, decent price. Found one at EME which says theirs replaces the old electro mechanical version for $69.95. Do any of you have any experience with the newer version of the relay, sounds like theirs is totally electronic. Electronic versions have good features but also drawbacks as when they fail it is absolute. The old electro mechanical has a deminishment of operation, but will still partially work like in my case.

More info: Found a schematic showing the internal wiring of the starter relay, looks like the wires in question are part of the pull circuit for the relay, so it looks like the horn wire is just there being tapped at that point for a 12v source.

Did some more troubleshooting. Removed bundled green wire from relay, applied 12v to it and hit button, no horn. Removed green wire from horn, 12v to it, hit button, no horn. Attached jumper wire to black wire on horn, turned on key and hit ground with wire, HORN.

So even though I have continuity through the switch and voltage gets through, it has enough resistance to limit current.

Maybe you need to let those horns run for a bit. Ear plugs?

OK, I spent some time trying to figure out where in the system the horn was failing. It all came to the button not touching ground to complete the path. So this morning I went out looking at what point the switch accomplished this, it is done through the top mounting screw that holds the switch in the housing. There is a contact the screw comes in to touch to complete the circuit when the button is pushed. In my case the plastic supporting the contact had broken away and the contact had moved away from being touched, my quick fix was to tear off a bit of paper shop towel roll it into a ball and shove into the switch lifting the contact into place. Assembled and now it HONKS. As far as I can figure the readings I got earlier with continuity and voltage where the screw was just barely touching the contact and didn't have enough of a connection to allow enough current to activate the horn.
I did order new switches for both sides, since these were only used on a 74 R90 and earlier /5, I figure I might as well get spares from BMW while they are available.

I've done that too Jagarra (ordered new switch clusters) even though mine are still working fine.

jagarra wrote: ↑Tue Apr 10, 2018 11:57 am OK, I spent some time trying to figure out where in the system the horn was failing. It all came to the button not touching ground to complete the path. So this morning I went out looking at what point the switch accomplished this, it is done through the top mounting screw that holds the switch in the housing. There is a contact the screw comes in to touch to complete the circuit when the button is pushed. In my case the plastic supporting the contact had broken away and the contact had moved away from being touched, my quick fix was to tear off a bit of paper shop towel roll it into a ball and shove into the switch lifting the contact into place. Assembled and now it HONKS. As far as I can figure the readings I got earlier with continuity and voltage where the screw was just barely touching the contact and didn't have enough of a connection to allow enough current to activate the horn.
I did order new switches for both sides, since these were only used on a 74 R90 and earlier /5, I figure I might as well get spares from BMW while they are available.

I'm dealing with this now. That being the broken plastic that holds the metal grounding tab. Been racking my brain on how to fix. Forgot how to keep it simple. This will at least give me more time to think about it while I ride. -Thanks