BMW Electrical 'fire'

[A100man]

So..

big bro's R100RS nearly caught fire the other day when the dip-switch wiring went up in smoke. I was aghast and said 'bbbut the fuse should have blown before that happened..'

..only research reveals that Beemers of that era run with headlights unfused!

Replacement OEM switch and wiring £640 apparently , but cheaper patter ones are available (not doubt it's a lucrative market).

We think that the relay went bad with an internal short and caused the issue (really low Ohms reading across the coil) as there doesn't seem to be any other possibility.

Has anyone else come across this? I going to try and insert an inline fuse too I think)

Cheers
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[stinkwheel]

Yes, I have seen an R100 belonging to a mate whos headlamp wiring caught fire. He'd landed up stopped at the side of the road ripping wires out from behind the dash and had melted his gloves and slightly burned his hands when he arrived at the rally we were both attending.

At the time I'd put it down to him having done something stupid to it because that kind of thing seems to happen a lot when he's about. It's certainly not the first (or second or third) time he's had an electrical fire on one of his bikes.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[WD Forte]

If its localised to the lights,wire it yourself.
you can buy modern thinwall cable of a suitable rating (say 16A)for not much and it comes in range of attractive colours.
You buy it by the yard/meter too

Fuse it for sure, a 55/65W blub will take around 5 amps
so fuse it for 10 to handle inrush current

I'd use blade over glass fuses for robustness cheapness
and availability

You do need the extras of course to do a good job
but can match or exceed the original with stuff like
soldering iron, solder, terminal kits, crimping tools, heat shrink sleeving wrapping tape etc depending on how you do it.

Dont baulk at buying the kit as it will serve for many other uses

I'd start by examining the wiring diagram to help decide the best location to insert a fuse.

A quick google showed me this impressive page but its for 1977 era
and not sure if relevant for your bros bike

https://brook.reams.me/bmw-airhead-motorcycle-electrical-systems/7-series-1977-r100rs-circuits/#Headlight_HIGH-LOW-FLASH_Switch_High_Beam
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bikers smell of wee

[A100man]

[MCN]

My 520 car went all woke on me just around the beginning of the Big Rona Scam.
The canbus indicated everything was short to ground.
Multiple computers. (Chip modules.) Several canbus systems.
It was the parking proximity sensor control module that was submerged in 6 feet of water in the boot. A sort of dumb design flaw by BMW.
Mounting a device, though not vital, in a water trap.
2 1/2" holes were drilled in the spare tyre well of the boot to let any water away.
The boot was always dry so I assume rain found a path past the seal and wicked down into the space undetected.
When we used pony and trap to go to Tesco we never had any exposure to electrical failures.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[stinkwheel]

[A100man]

So..

We tracked down the fault after stripping off the tank and fairing (well he did that) somewhat predictably a chafed wire and outer sleeve around the headstock, suspicion that poor routing may have lead to it being trapped in the lock stops.. Ruddy Germans.

meaning.. that short took out an entire headlight switch assembly/loom that costs 640 quid OEM part, luckily a pattern replacement is about 170.. One might consider repairing it but there was a staggering number of wires in there, and two multi-way connectors.
____________________
Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600