New Horn Wiring Question

[BSTWAY]

I’m thinking about hooking up a relay for a new horn and use the green wire connected to the current horn as the trigger wire. This normally wouldn’t be confusing but in my case the load (old horn) will be removed. What happens with the ground wire (brown)? I’ve seen kits for Stebel horns which have connector pins connecting the green but not the brown. I’ve also seen Fiamm installation posts where both green and brown wires were connected to the same relay connector.

[NoRRmad]

I've got a related question. There already is a relay for the horn in the fuse box. I assume it's not rated high enough to handle the load of a louder horn, but I wonder -- since there are other relays in there -- if another of the relays is designed for a higher load, say a headlight relay, for instance. All the relays seem to be the same size, so I assume that one could just plug in a higher-capacity relay from BMW.

Would the wiring have to be replaced, too?

[Airman]

I've got a related question. There already is a relay for the horn in the fuse box. I assume it's not rated high enough to handle the load of a louder horn, but I wonder -- since there are other relays in there -- if another of the relays is designed for a higher load, say a headlight relay, for instance. All the relays seem to be the same size, so I assume that one could just plug in a higher-capacity relay from BMW.

Would the wiring have to be replaced, too?

Right. The wiring is way too small for the load the Stubel is going to pull. I have that horn project going myself. I installed the fuse panel, but not the relay yet. I plan to run the two horn wires over to the relay and run power to the horn from the fuse panel. The wire size I ran from the fuse panel is 16 or 14 gage I believe. That horn wire looks like 22. The reason I have not completed it yet is that I hate to cut stock wiring and thought to find some Molex pins and crimper someplace and make a pair of wires to plug into the connector. I think a lot of people just power the relay straight from the battery. The Stubel is a killer horn by the way. It would be hard to find a better safety item to install on your bike.

[rdsmith3]

When I installed my Stebel, I did not realize there already was a horn relay. Yes, it's not one of my brighter moments, but I assumed that since there is no headlight relay, then there must not be a horn relay.

Anyway, I wired the Stebel-supplied relay directly to the battery, and triggered it with the old horn circuit. The stock horn circuit wires are very thin gauge, so I think it is best to do this. My stupidity caused me to do the right thing.

NoRRmad, I think you could upgrade the relay, but the wiring may not be up to the task of carrying enough current for the Stebel.

[FloridaBeakster]

I first wanted to use the stock horn relay and upgrade the wiring to handle a real horn. Only problem is that it is next to impossible to identify/acquire the BMW connectors to fit in the stock fuse box so you can use your own wiring. And, if you can find them, it will take a huge amount of work to do it the right way.

So, I went the normal aftermarket route and installed a small fuse block, new wiring and added a second horn relay. I used the stock horn circuit to trigger my new relay. If you're not using the stock horn at all (I am firing both at the same time), you can use the brown wire to ground the trigger side of your new horn relay. Since I was using the stock horn in conjunction with the new one, I just tapped into the green wire to trigger my relay. I used #10 wiring in the horn circuit (battery to new fuse block and fuse block to horn and horn to ground) to keep voltage drop to a minimum. I think the Stebel pulls something like 20A, which is quite a bit of load. Definitely DO NOT use the stock wiring with a Stebel Nautilus or similar horn.

I used the Bosch/Tyco micro-relays for my horn and other electrical mods. They are small and rated to 30A.

[sweatmark]

As noted above, using the original horn wires (green and brown ground) is simplest strategy. Connect these wires to the coil terminals of conventional 4 pin relay, with new wire carrying +12V through the relay's load terminals. Get a fused heavy gauge wire from car store if not using Centech AP-1 or equivalent; same heavy gauge for relay to horn compressor and for ground.

The rat's nest in the R1150R fuse box is not worth hacking, if one wanted to upgrade (up-gauge) the horn relay circuit to handle Stebel current.

The above does not apply for those who have performed ABS-ectomy; there is another means of powering the Stebel using power originally intended for iABS servo unit.