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Old 09-24-2008, 11:33 AM   #1
Dieselboy OP
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F800GS CANBUS Stebel Quandary

Need a perspective:

I put a Stebel Compact Airhorn on my F800GS last night.

Horn worked...once...then caused the CANBUS to shut down the aux plug circuit.

Why?

CANBUS reset and the horn worked again...once.

An overly simplified diagram:






A word picture of the circuit:
1- BMW accessory plug above the battery connects to 2-Relay connects to 3a- battery and 3b- AP1 Fuse Panel connects to 4-Horn Relay connects to 5a- OEM horn and 5b- Stebel Compact

Why would the horn shut down the AP1. Obviously the horn is overpowering the CANBUS, but why. It is isolated from it by two relays. I don't understand. I've used this set up on my R1200GSA with no issues.

AP1 is also running the Zumo450 and Starcom1 Advance. They worked fine for a week prior to my adding the horn. They still work fine since I've disabled the horn circuit pending further experimentation.

Help is requested.
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Dieselboy screwed with this post 11-21-2012 at 07:47 PM
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Old 09-24-2008, 11:45 AM   #2
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You need to obtain the wiring diagram for the relay for this horn used on the 12GS. I've had mine on there for several years and it works perfectly well without being shut down by the Canbus.

The wiring however, is not how you'd normally wire a horn using a relay. I don't have the schematic handy, so I can't tell you how to wire it so it works, but there is a way of doing it. I do recall there are 2 leads to the battery, 1 to the bike's +ve horn harness..
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Old 09-24-2008, 12:00 PM   #3
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Quote:
Originally Posted by Gadget Boy
You need to obtain the wiring diagram for the relay for this horn used on the 12GS. I've had mine on there for several years and it works perfectly well without being shut down by the Canbus.

The wiring however, is not how you'd normally wire a horn using a relay. I don't have the schematic handy, so I can't tell you how to wire it so it works, but there is a way of doing it. I do recall there are 2 leads to the battery, 1 to the bike's +ve horn harness..

Gadget Boy, there are actually multiple wires not depicted in the diagram. I pull the horn "signal" from the oem horn to the relay. When I push the horn button on the handlebar, that signal tells the horn relay to send power from the AP1 to the Stebel and then the stebel fires. That actually worked. But in the process of drawing power from the AP1, the BMW Aux Plug that feeds the 2nd relay (the one that tells the battery to send power to the AP1) shuts down.

Now why would drawing power from the battery--through the AP1--to the stebel, cause the CANBUS to shut down. The horn should not put any additional load on the CANBUS system at all.

The stebel intersects the CANBUS in two places: at the OEM horn and at the Aux Plug. In both cases, the connection is to a relay and that is wired to post 85 on the relay which should be milliamps of current. The only thing that current is for is to tell the switch to close and send power from the source to the device.

Source= battery Device= AP1
Source=AP1 Device= Stebel

I need ideas on how to isolate the problem. My next step is to wire the horn to the battery. That will remove one of the relays and one of the CANBUS connections. That won't however tell me why it is doing this in the first place.
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Old 09-24-2008, 05:05 PM   #4
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I don't know much about the BMW system, but I do know that when you fire a relay the induction pulse can be large and maybe the system doesn't like it. Might help to put a filter across the relay coil to dissipate the spike..
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Old 09-24-2008, 06:50 PM   #5
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Quote:
Originally Posted by rapidoxidationman
I don't know much about the BMW system, but I do know that when you fire a relay the induction pulse can be large and maybe the system doesn't like it. Might help to put a filter across the relay coil to dissipate the spike..
Filter, eh? From my electronic meanderings here, I seem to recall that configuration mentioned at some point. I'll do a search.

If anyone happens to have a make, model, source, I'll take that.

If the pulse is the culprit, I'm curious as to why this bike and not the R1200GSA? I suppose it's magic.

Thanks
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Old 09-24-2008, 09:30 PM   #6
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I've got a voltmeter on the Strom that's just a series of LED lights. 3 yellows at the bottom of the scale (discharging), 4 greens in the middle (just right) and 3 reds up top (too much voltage). Every time I hit the horn or turn the PIAA's off I can see the meter light up all the way for about a half second...

The horn (a Stebel) and the piaa's are both on their own relays.. Nothing has gone pop yet, but it is a concern. Maybe your 1200's electrical system is more robust and better able to handle the pulse?


IIRC from my electronics schooling, you need an RC filter (a resistor and capacitor in series with each other) put across the coil (electromagnet, inductor, kinda the same thing in this case) to absorb and safely dissipate the pulse generated when the electromagnetic field collapses in the coil when you remove power from the relay (let go of the horn button). Too big a capacitor and resistor and it'll take too long for the horn to activate, too little and it'll be ineffective. Kinda like a shock absorber for the electrical system. I'll bet somebody makes one commercially but if not I'll only ask for 20% royalty

Keep us posted!
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Old 10-11-2008, 04:53 PM   #7
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Fixed

AP1 had corroded on the bottom of the circuit board. This tiny detail was repaired by resoldering the board (something for which I definitely needed help-- and I happily recieved.) That and a bit of wiring verification and now the horn, happily, operates through the AP1. I don't fully understand how it was working at all, but it is moot.

The relays are operating as expected.

The horn is operating as expected.

So my saga is now over, except I will be buying a new AP1 (or 2).

Of interest to all of you, the oscilliscope confirmed that there is no spike coming back into the system from the horn operation.

On to the next project....driving lights...
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