Bear with me this might get kind of long. This is a 1993 R100GS it's had it's main wiring harness replaced, and the main relay, horn and load shed relays replaced. Currently I have a horn only about 40% of the time I've looked into it and here's what I've found The horn tested off the bike works every time. The horn on the bike works about 30% of the time Testing voltage across the horn wiring I get 12+ VDC every time I hit the button, IF the horn is not hooked up. Hook up a horn and it's back to only about 30% of the time. The switch is good and clean. A different known functional horn exhibits the same traits. Wiring all looks good, all connections are clean and do not show any signs of overheating. Here's where it gets weird about 1200 miles ago I replace all three "main" fuses, that was due to a random idiot light, headlight has to be off for bike to start deal. Today while trying to figure things out I find that if I swap the two relays the bike functions but the idiot lights don't come on and there's no headlight but the bike will fire. Swap them back and the idiot lights are normal. and I get the horn only about 30% of the time. So I'm leaning toward replacing the two smaller relays, but what I'm wondering is what's going on that's so bad that I'm running through these relays so fast.
You've got a big voltage drop across the horn wiring. Pull a schematic and check everything from the horn connector back through the points of the relay to wherever the horn wiring connects to the main 12V bus. Even if the connections are nice and bright and tight, check where the wire is crimped to the terminal-- sometimes there can be a loose crimp or oxidation. You may have to peel back the safety insulating cover on the terminal. Check for the under-load voltage drop at every connector point along the chain-- frequently BMW will daisy-chain connections and a connector that supplies voltage to another circuit may have a bad connection/splice at that tag. Why were the main harness and the relays replaced?
Did this start after the relays were replaced? After the new wiring harness? Is it a brand new wiring harness? New relays or used? I like Bill's idea, but it also could very easily be the relay. Circuits that carry a bit of current like the horn - I like to solder the connections to absolutely ensure no corrosion build-up that might hinder the flow.
And ones that are critical-- like ignition or the the main harness. With factory ("proper") crimps, it's not a big deal to flow just a spot of solder at the bottom of the wire/crimp. Think of it as the belt-and-suspenders approach. Be careful to not flow a lot of solder into the wire-- it can wick up past the crimp and into the wire, making it less flexible (brittle) and prone to break at that point.
Hey thanks all for the help The wiring was replaced because at some point in the bikes life before me someone miss routed the main harness so it was doubled back and folded against the fuel tank, it then wore through the insulation and shorted the wiring and melted a good section of the main harness. The relays were replaced as an attempt to solve a weird headlight, starting idiot light thing. after replacing them it was fixed. This horn issue is just the latest thing to come up. I should mention that when I was doing my testing the battery was only showing about 12volts, the bike has been sitting and it needs a charge, but even with a full charge the horn was showing the same symptoms. With the voltage tester hooked up when the horn honks it's pulling 12+ volts, when the switch is pushed and it doesn't honk it's pulling much much less. I don't think corrosion is the issue, or the connectors, but I'm going to double check the plugs at the relay box. Everything was double checked when I rewired the bike, all the parts that were left behind were cleaned and everything got a smidgen of dielectric grease. It weirds me out that this bike is having these relay issues after having total electrical system meltdown
After pulling the relay I noticed that there is a build up where one of the spade connedctors connect. It looks like I must be getting some kind of arcing. So I'll be looking into the harness side plug more fully just to see if there's an issue where the female plug (really the relay base) are not snug enough
I'm thinking that's next... I did find out that the relay is also common to a Benz relay But those cost $34... the BMW part is only $14. Of course if I could find an automobile cross for the relay I'd be looking at around $7. a friend and owner of MG classics down in Long Beach hooked me up with a known functional used but 5 pin relay So I can at least see if that's my issue. Next up I'm going to dive into the switch and make sure it's ok and test the system with a load rather then just with the multi meter.
OK So I cleaned up and tested the old BMW relays (which were in fact not old to the bike, I got them sometime between 07-09 One was bad, out of the bike they tested fine, when I first put them into the bike they worked statically, then when I fired the bike up and went to use the turn signal the charging light came on and neither the horn nor the blinkers would function. I pulled the old relays out, put new known working used ones in and everything works as it should. I'm thinking that the harness for the horn is not strong enough for the horn current. I'm thinking I might rewire the horn using the stock switch and heavier gauge wiring and relays and bypass the whole harness all together. I did clean and make sure all connections were nice and tight from the handlebar switch to the horn. I'll see how long this lasts, but i suspect that there is some design flaw within the horn, load shed circuit.
