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Old 02-07-2013, 05:46 PM   #1
Taverner OP
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No horn on 1975 R90/6 maybe bypassed fuses

I purchased a 1975 R90/6 several weeks ago and just today realized that the horn does not work.

I tested the leads going to the horn and did not get 12V when pressing the horn button. I opened up the headlamp enclosure to make sure it was not a blown fuse and I wasn't too happy with what I found. I see wires soldered to the exposed fuse posts. Shouldn't they be soldered to posts on the back of the circuit board? I have to go pick up the 15A fuse which corresponds to the front brake light and the horn, but I'm skeptical at how well it will sit in the posts with these wires soldered to it. I attached a photo of the fuse post. I'm comparing mine to this photo I found on another post here (http://img.tapatalk.com/d/12/10/23/dasu3y4a.jpg)



I discovered or caused another issue when pulling wires back to expose the fuses, I suppose I found a short, as the head lamp and the brake/taillight turn on (key is not in). If I push on the black plastic part of the ignition, they turned back off, but when I start moving wires and cables around it goes back on. I attached a photo of the back of the ignition, including the black plastic piece.



This is my first motorcycle, and I don't even know where to begin troubleshooting. I have the Haynes and Clymer manuals, but only Haynes here, and can't find any info on how to troubleshoot the latter issue.
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Old 02-07-2013, 05:48 PM   #2
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Sorry, I meant to remove the note about bypassed fuses in the title once I realized that it doesn't appear to be what's going on, just missing fuse (I hope).

Thanks in advance for any help anyone can provide.
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Old 02-07-2013, 08:51 PM   #3
Bill Harris
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You've got a 38 year old vehicle which has the wiring tinkered with by a PO (Previous Owner). Not unheard of, but it's be best to get a schematic and go through the wiring and straighten everything out. And possibly replace the circuit board since it appears "terminally tinkered". That is a tall order, but more shadetree work on top of that shadetree work can't end well.

The ignition switch may or may not be a problem, but I'd be suspicious of the Red wire with the new hand-crimped terminal. That Red wire is the primary bus coming in from the battery.

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Old 02-08-2013, 07:07 AM   #4
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Quote:
Originally Posted by Taverner View Post
This is my first motorcycle, and I don't even know where to begin troubleshooting.
Here is one way to begin - http://www.p-thomas.com/shared/BMW/7...0%20Wiring.pdf . Start with understanding the circuits and then begin to trace them on your bike. The horn circuit isn't too terribly difficult, but some of the others are torturous. The schematic above is for a 1974; it should be fairly close to your bike. I have '74, '75, and '76 bikes and there are distinct differences. I have several wiring schematics - http://www.colorwiringdiagrams.com/bmw.htm - and they do not always agree with each other. The best one is in the owner's manual.
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Old 02-08-2013, 08:30 AM   #5
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Originally Posted by Bill Harris View Post
You've got a 38 year old vehicle which has the wiring tinkered with by a PO (Previous Owner). Not unheard of, but it's be best to get a schematic and go through the wiring and straighten everything out. And possibly replace the circuit board since it appears "terminally tinkered". That is a tall order, but more shadetree work on top of that shadetree work can't end well.

The ignition switch may or may not be a problem, but I'd be suspicious of the Red wire with the new hand-crimped terminal. That Red wire is the primary bus coming in from the battery.

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Terminally tinkered, I hope that is an intended pun

Quote:
Originally Posted by craydds View Post
Here is one way to begin - http://www.p-thomas.com/shared/BMW/7...0%20Wiring.pdf . Start with understanding the circuits and then begin to trace them on your bike. The horn circuit isn't too terribly difficult, but some of the others are torturous. The schematic above is for a 1974; it should be fairly close to your bike. I have '74, '75, and '76 bikes and there are distinct differences. I have several wiring schematics - http://www.colorwiringdiagrams.com/bmw.htm - and they do not always agree with each other. The best one is in the owner's manual.
Thanks for the advice both of you. I really was hoping to avoid having to redo the wiring as electronic circuits really are not a strong skill of mine. I nearly failed every circuits/electronics course I took in college... it just confuses me all to hell.

I have the circuit diagram for the 75 /6's in the Haynes and found a scan online to print and laminate, so hopefully I can look it over and redo this. In the mean time maybe I can at least get a 15A fuse to stay in the terminals so I can pass inspection and get title transferred now rather than a month or two from now, or however long it takes me to work this all out.

By the way, what do people who are customizing their airheads with third party headlamps do about this birds nest of wires? Is it typical fashion to put a small box somewhere else for all this circuitry? If I am going to have to redo all the connections, I'm tempted to go ahead and move it out of the head lamp since I plan to replace it down the road anyway. It seems there may be more modern and compact methods (including newer style fuses). Then again, without a kit of some sort, that would probably be well above my current skill level.
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Old 02-08-2013, 08:31 AM   #6
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Originally Posted by craydds View Post
Here is one way to begin - http://www.p-thomas.com/shared/BMW/7...0%20Wiring.pdf . Start with understanding the circuits and then begin to trace them on your bike. The horn circuit isn't too terribly difficult, but some of the others are torturous. The schematic above is for a 1974; it should be fairly close to your bike. I have '74, '75, and '76 bikes and there are distinct differences. I have several wiring schematics - http://www.colorwiringdiagrams.com/bmw.htm - and they do not always agree with each other. The best one is in the owner's manual.
Thanks for the tip about the inconsistencies. Like I said I have the Clymer and Haynes, but if there is the chance of any errors in both, I'll revert to the diagram in the manual available online.
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Old 02-12-2013, 08:18 AM   #7
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Well, I have looked and looked through Max BMW and cannot find this replacement part, which Clymer calls the fuse plate. Can someone point me in the right direction? I'd like to at least get an idea of new MSRP before I look at buying one off ebay.
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Old 02-12-2013, 08:58 AM   #8
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Old 02-12-2013, 09:17 AM   #9
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The horn gets its power through a relay. The first thing I would check is the horn relay. Good luck!
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Old 02-12-2013, 09:22 AM   #10
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I dont think that there is a horn relay on a /6 unless the bike was fitted with twin horns which was an option on some models.

The early /6 bikes did not have earth wires in the front indicators, easy enough to check.
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Old 02-12-2013, 09:34 AM   #11
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Originally Posted by chasbmw View Post
I dont think that there is a horn relay on a /6 unless the bike was fitted with twin horns which was an option on some models.

The early /6 bikes did not have earth wires in the front indicators, easy enough to check.
I think you are correct. Per the /6 wiring diagram at Snowbums site, no horn relay is shown: http://bmwmotorcycletech.info/schematic1975,1976.htm

Now the fun begins! Good luck!

Edit: My '78/7 doesn't have a horn relay either, whoops. I would look at the horn switch next.

Stan_R80/7 screwed with this post 02-12-2013 at 09:52 AM
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