We tested many many things. All connections and relays tested fine for continuity, but Jules's experienced eye said the corrosion on the legs which I'd dismissed was enough that they could be playing up. Time for the fibreglass pen and in the worst cases a tiny diamond file.
We then put everything back together and refit the battery to see what we'd achieved.
The 7v (but very low current) on the brakelight circuit had gone away (probably leakage across unrelated circuits inside the diode box). The headlight not switching off when cranking was fixed (probably a non-operable load relief relay). The crank-no-start with the stand down was fixed (probably a non-operable motronic relay). The passing light switch was still not working so it got the same treatment of opening up and cleaning and, despite it not looking that bad inside, this fixed it.
Jules was still suspicious of the sidestand switch. It should earth one circuit when up, and another circuit when down. Both circuits should allow the bike to start and run in neutral, but only the "stand up" circuit should allow you to go into gear. Moving the stand slowly from one position to the other made the relay pack click like crazy where it should click once at the change from one circuit to the other. This showed the switch is intermittent internally, giving the bike neither earth at various points. Considering the stand switch was loose due to a missing washer, this could easily cause the bike to get neither earth while running and cut out. That's been bridged for the time being, so the running issue should be fixed. I'll stick a note to myself on the screen as a reminder to lift the stand.
I'll put the bike back together tonight or maybe tomorrow, and go for a ride. With a bit of luck and a following wind, it should now be good electrically!
So what's left?
The cracked rear subframe turns out to be only part #2 shown here. Sucks that it broke, great that it's a bolt-in subsection!
The bike has had a battery boil over at some point, which has resulted in part #11 shown here melting. It's come apart about where the "10" is, leaving the airbox loose and some very weird crumbly plastic all around there. It'll get either a new #11 or maybe a whole airbox, as I've yet to inspect the rest of the box for damage.
Battery acid also spilled down on to the coils, which don't look pretty and have lost the connector tabs. For now though the connectors are staying put, so I will leave it alone. It will however be a first place to look for any future running issues.
It weeps oil from hose #11. It's gone brittle over the last 10 years of having hot oil inside it, causing the clips at each end to get loose and no longer seal. It's a length of oil hose about a foot long with a bend at each end. Hoping a length of normal hose will do it since BMW want 35€ for a new one, and I doubt a used one would be any better.
Probably it wants a new handlebar (the fancy replacement is lower than I'd like and doesn't feel straight).
The left footpeg subframe is still bent, but I'm now more inclined to just beat it flatter.
After all that, I can take it for a proper ride and find out what else is wrong with it
I shouldn't laugh but how in fucks name did an experienced person like you end up with this bloody nightmare
