• Welcome, Guest! We hope you enjoy the excellent technical knowledge, event information and discussions that the BMW MOA forum provides. Some forum content will be hidden from you if you remain logged out. If you want to view all content, please click the 'Log in' button above and enter your BMW MOA username and password.

    If you are not an MOA member, why not take the time to join the club, so you can enjoy posting on the forum, the BMW Owners News magazine, and all of the discounts and benefits the BMW MOA offers?

78 R100RS Horn wiring

stevenrankin

Active member
Quick question,

I am having problems with my 78RS's horns.

I see a horn relay listed on one wiring schematic, but not on another.

I assume the two horn system uses a relay whereas the one horn system (not the RS) might not use a relay.

Did in fact BMW RS models in 77 and 78 use a relay for the horns? I have not gone hands on the bike yet, just looking at the wiring schematics in advance and found the discrepancy.

If there is no relay, it means either the horns or a horn is bad or the switch is bad. Not good if the switch is bad as they are NLA. St.
 
Last edited:
Don't know, but I do know that when I put a second horn on my R75/6 I just wired it to the stock horn. Worked fine even without a higher amp fuse. YMMV
 
Thanks

I knew instinctively the two horn RS and RT bikes have a relay whereas the single toot toot horn bikes may not.

I am just surprised I have a wiring schematic for a 78 bike that does not show the relay and one that does.

I will err on the side of common sense and look for a relay when I go hands on with the bike.

Really can't be a whole lot wrong. St.
 
I knew instinctively the two horn RS and RT bikes have a relay whereas the single toot toot horn bikes may not.

I am just surprised I have a wiring schematic for a 78 bike that does not show the relay and one that does.

I will err on the side of common sense and look for a relay when I go hands on with the bike.

Really can't be a whole lot wrong. St.

Steve,

The '77 & '78 RS have the horn relay. It is located on the left spine tube and plugs into the bracket that includes the starter relay. The Horn relay is the front one.

<a data-flickr-embed="true" href="https://www.flickr.com/photos/brook-reams/34705130496/in/album-72157674599790006/" title="Starter & Horn Relays and Sockets"><img src="https://live.staticflickr.com/4183/34705130496_419ee31693_c.jpg" width="800" height="532" alt="Starter & Horn Relays and Sockets"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>

<a data-flickr-embed="true" href="https://www.flickr.com/photos/brook-reams/34360690450/in/album-72157674599790006/" title="Horn Relay (Right) Starter Relay (Left), From Top"><img src="https://live.staticflickr.com/4172/34360690450_4c84c636c6_c.jpg" width="800" height="532" alt="Horn Relay (Right) Starter Relay (Left), From Top"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>

Best.
Brook Reams.
 
Just confused

I was just confused as to how I could have two wiring schematics showing me two different things.

Turns out, one of the schematics is wrong. In the trash it goes.

Turns out the switch is bad. Moto bins in the UK had one in stock. Of course now, Max is listing one again on their site. They were supposed to be NLA, someone is making them again or someone found a batch.

I really would like to see them being made again along with a lot of other NLA parts. Cheers all, St.
 
St -

When was your RS made? I believe the part number is 61 31 1 373 585...is that what you found? Sites suggest that it has been superseded by -588. I have some notes squirreled away that provide alternatives to the BMW relay. Some were VW/Audi parts...also Hella parts. Sometimes the non-BMW relay doesn't have all the features. Also, RealOEM indicates the part was used in the car line.

Interesting that RealOEM shows a gap between 6/77 to 8/78 for the R100RS...suggests that no bikes were built in that timeframe. However the BMBikes engine listing site suggests otherwise.
 
My RS

My 78 RS is a Euro spec bike which has the switch on the left handle bar for horn, and head light off, park and on. High and low.

I see now the switch is again available or at least listed at MAX. I ordered one from Motobins. St.
 
I got to learn to read

OKay, I have to learn to read.

Max does not have the switch I need, it IS NLA.

It also looks like I ordered the wrong switch from Motobins.

Not going well here. Sorry for the confusion. St.
 
Continuing the story if you are interested

So, the switch I need is NLA. However, there may be a slim chance one exists new and Megan at Kissel's is checking for me. If not, I will have to purchase the current switch without the headlight off, park and on switch.

I have found out things assembled in a factory with the proper jigs and tools do not always present themselves as best candidates for home repair.

My current switch is case in point. Smart fellow me, looked at it and thought "a ha, I can pop that little c-clip on the headlight switch and take it apart to clean the contacts."

All was well until the teeny tiny detent ball bearing and its retaining spring went over my shoulder. I did manage to find them however after the fourth try to put them into place without the proper jigs and tool, they went for a walk and are now gone.

Moral of the story, "Do not mess wi thing ye ken nothin about!" St.
 
If you have to purchase/install the US handlebar control switch:

There use to be a tech article on the IBMWR site that showed how to wire in an auxiliary on/off switch for the headlight that turned off the main lamp while leaving the parking light illuminated. When auxiliary main switch is "on", the headlight switch on the handlebar works normally. I did it on my /7 maybe fifteen years ago, mounting a marine style toggle switch in the fairing. It works great and was easy to do. I looked on the IBMWER site to see if the article was still there, but I don't see it. I probably printed the instructions when I did it- if I can locate them, I'll post a copy.
 
Steve,

The '77 & '78 RS have the horn relay. It is located on the left spine tube and plugs into the bracket that includes the starter relay. The Horn relay is the front one.

<a data-flickr-embed="true" href="https://www.flickr.com/photos/brook-reams/34705130496/in/album-72157674599790006/" title="Starter & Horn Relays and Sockets"><img src="https://live.staticflickr.com/4183/34705130496_419ee31693_c.jpg" width="800" height="532" alt="Starter & Horn Relays and Sockets"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>

<a data-flickr-embed="true" href="https://www.flickr.com/photos/brook-reams/34360690450/in/album-72157674599790006/" title="Horn Relay (Right) Starter Relay (Left), From Top"><img src="https://live.staticflickr.com/4172/34360690450_4c84c636c6_c.jpg" width="800" height="532" alt="Horn Relay (Right) Starter Relay (Left), From Top"></a><script async src="//embedr.flickr.com/assets/client-code.js" charset="utf-8"></script>

Best.
Brook Reams.

When I was resurrecting my 83 R100, I found very crusty terminals on that relay. They weren’t evident, until I was able actually pull them out. Some deOxit cleaned the pins up nicely.
 
Where did you find a used one? I need the little button that goes on the end of the metal tab for mine.
 
Check the horn mounting bracket before you

The horn mounting brackets consist of 2 leaf’s of spring steel which will handle shock and vibration better than a single piece of mild steel. The horns ground through the connections at the frame and back center bolt on the horn. Over time the nuts and bolts loosen (vibration). This permits moisture/dirt to accumulate and affects the grounding causing the horn(s) to fail. This happens more often than switch or relay failure. Disassemble, clean, remount and tight to spec. I’ve made and added extra ground wires using heavy gauge wire and ring terminals between frame and horn mount. Note that the horn mounts use ‘toothed’ washers to help keep things tight and the ground solid. There are very few of this kind of washers on our airheads and they only install on specific ground points.
 
Ground

The first thing I did was check the ground.

The relay question was because the wiring diagram I had did not show a relay, where I did have one, so I asked the question. Turns out some wiring diagrams show a relay some don't .

As I wrote, it was the button contact that went bad. Cleaning it with contact cleaner didn't work nor did disassembling it and putting it back together work. St.
 
Yes, for '77 and '78 RS there were dual horns and a horn relay and there was one. Single horn Airheads those years didn't have a horn relay.

Recall that in 1979 the handlebar switches were changed and were robust enough to not require a horn relay.

Back in the day, I didn't care for the newer switches and retrofitted my '84 RS with '77 switches. I added a horn relay.
 
Back
Top