R90/6 non starter

[gspd]

I noticed today the previous owner had removed clutch switch on lhs.

No clutch switch?
You never mentioned that.
The only purpose of the clutch switch is so the bike can't leap forward when you hit the starter with it in gear.

Oh well, now the bike starts!
I told you it would be an easy fix.
Would have taken me all of about 5 minutes in person.
Probably less time than it took to write just this reply.
But WTF, that's entertainment folks!

Now, just permanently hardwire that jumper wire.
You don't have to run it directly to the battery negative; It's probably more practical to splice it to any SOLID BROWN wire in the headlight shell (or any other GROUND on the bike).

As long as you always habitually pull in the clutch when starting you won't have any problems.

Without a clutch switch, if you connect the jumper from the switch to where I indicated with the red arrow as I previously suggested, the bike will only start in neutral, which is not a great setup. The wire below my red arrow is for the clutch switch, which you don't have. The delay in needing to find neutral to restart after a stall can be scary dangerous in certain situations, like when pulling out in traffic or if you stall it with cars close behind you at an intersection. IMHO it's safer to have the entire interlock system disabled as it is now with the jumper wire. Safest would be to buy and install a clutch switch.

[melville]

No need to splice. There are spare ground spades at the bottom of the junction board.

[hal]

On my '74 (late 73) /6 there is no clutch switch, never was I think, but that has the /5 switches. Was there one on the /5?

[gspd]

1975 - It was the worst of years, it was the best of years.

'75 was a tumultuous year for the motorcycling industry in America.
Below are just a few of the dozens of new 'safety' laws were implemented:
Headlights had to always be 'ON'.
Sidestands had to self retract or have an ignition kill switch built in.
Turn signals and tail lights got new 'minimum size' requirements.
Front and rear side reflectors became mandatory.
Shifters had to be on the left.
Rear brake levers had to be on the right.
and..
I believe...
it was also the year that clutch/starter interlock switches were implemented (but not 100% sure about this).

******************************************************************************************************************************
Unrelated rant starts here:

In late '74, my '72 Laverda sf750 was getting a bit tattered after a few crashes and regular total abuse by a then neophyte wanna-be mechanic (me). I ordered a new 1000cc Laverda 3c 1000 sight unseen, but it's importation to Canada kept being delayed because of all the new laws listed above. Worst of all was the shifter/rear brake on the wrong side. The Laverda importer was busy McGuyvering an amateurish bellcrank and linkage system to switch the brake and shifter pedals to the 'new correct' side. He told me it was a real crappy almost unworkable conversion, but assured me that it could be removed and the bike could be made whole again in minutes (as soon as the bike cleared customs).

Summer '75 was now in full bloom. I was nineteen years old and invincible. I NEEDED the best superbike on the market but still no sign of the big Lav. I was getting angsty. I cancelled my Laverda order, got my $1000 deposit back and walked down to the local BMW store.
The most notable turning point in my life was June 6, 1975, with the totally unplanned impulse purchase of a brand spankin' new Krauser bag equipped Silversmoke BMW r90s. Rode out on r90s VIN#4980603 that day, and never looked back. That BMW enlightened me to the fact that traveling long distances (at high average speeds) on bikes was a blast, something I would have never even considered while racing from bar to bar (or café to café) on my Italian dream bike. And the rest is history....

[Rob Frankham]

Unfortunately, I've just come across a BMW b*&&ation factor. The wiring diagram you are referring to appears to be incorrect (or, if it isn't, every other source is...) Your diagram shows the late type switches but shows the Blue/Yellow(White?) wire running from the starter switch to the connection boar and on to the clutch switch and the Brown/yellow wire running from the starter relay to the starter switch. Other diagrams BMW original and Haynes show this colour coding as correct for early /6 series bikes but reversed for the later machines with the more modern switchgear. Clymer, in it's wisdom shows a third opinion with Blue/Yellow wires connecting to Brown/Yellow wires and vice versa.

This means that all of the wiring colour advice given so far (by me or anyone else) has at least a 50/50 chance of being backwards.

As I think I said earlier, I have yet to find a truly accurate BMW wiring schematic!!!

Rob

[gspd]

Your diagram .......has at least a 50/50 chance of being backwards.
.....I have yet to find a truly accurate BMW wiring schematic!!!

It's not my diagram per se, just one I pulled off the web.
As far as general (in)accuracy goes, I agree 100%.

Over the decades of working with error riddled diagrams I've learned to worry more about where the wire actually goes, what that wire actually does, than what color it is. This is not a BMW specific issue. And just for the record, their cars have much more wiring, and therefore many more wiring schematic errors than their bikes. VW, Porsche & Audi have not yet met the challenge of publishing error-free diagrams either. Believe it or not, I've experienced one case where the wire enters the harness one color, and exits at the other end another color. It's as if they ran out of the correct color wire while making the harness and just used another color that was handy to finish. And don't get me started on Italian bikes, they're all color blind there, or drunk. I'm not sure about American and Japanese products, not enough electrical experience with them. I once had to fix an old Triumph for a buddy that had been completely rewired by some putz using only black, red and blue wires. What a PITA that one was.

In Winstonfly's case, it worked out OK, the bike is fixed.
Airheads are the simplest bikes to trouble-shoot, at least for me.

[Rob Frankham]

On my '74 (late 73) /6 there is no clutch switch, never was I think, but that has the /5 switches. Was there one on the /5?

The wiring diagrams show a clutch switch (Haynes - 'Starter cut out switch' ... BMW repair manual - 'Clutch operated switch') on early model /6 machines (Not fitted on /5 machines which have a totally different starter cut out system based around the starter relay and the alternator).

I suppose it may be a territory based difference but I can't see any reason why one wouldn't be fitted.

My guess would be that a PO has removed it, possibly because it failed (as they do sometimes) and he found he could do without it. Since the sole purpose of the switch is to by pass the gearbox based starter cut out circuit when the clutch is pulled, it is quite possible to 'get by' without it, it just means you have to engage neutral every time you want to start the bike... which is a minor inconvenience.

Rob

[gspd]

The wiring diagrams show a clutch switch (Haynes - 'Starter cut out switch' ... BMW repair manual - 'Clutch operated switch') on early model /6 machines (Not fitted on /5 machines which have a totally different starter cut out system based around the starter relay and the alternator).

On certain vehicles the starter won't engage unless the alternator light is 'on'.
This prevents possibly damaging the starter gears and flywheel if someone accidentally tries to restart an engine that is already running.
I don't recall this feature on any airhead. Please elaborate.

[melville]

I understood that /5 had such a device in the starter relay. It remains as a vestigial item in the /6 relay, where there are two spades on the relay for the blue wire that feeds the GEN light. On the /6 relay, it's just an in-and-out, nothing electrical inside. When I gave up on the OG relay (didn't want to spend >$100) on my /6 I was able to just connect the blue wires and nothing changed.

Had I done that on a /5 I'd have added the ability to fuck things up by hitting the starter button inadvertently while the motor was running.

[Rob Frankham]

You're absolutely right.

The system on the /5 was designed to stop the starter being engaged when the engine is running and, to achieve this, a special starter relay monitors the output from the alternator to detect when there is a charge being generated and disables the relay when it is. Two drawbacks... 1) It doesn't work if the tick over is to low to start current generation (as it often is) and 2) it does nothing to stop the starter running when the bike is in gear.

This was abandoned with the /6 and the later interlock prevents the starter running when the gearbox is in gear and the clutch is engaged... but... it does nothing to stop the starter running when the engine is running (and the box is in neutral or the clutch is pulled).

Neither system is perfect but at least the /6 system achieves what it sets out to achieve fairly efficiently...

As Melville comments, the /5 system left a legacy in the loom of the /6 which led to the use of an excessively complicated relay that costs a fortune and does nothing that couldn't be done with a simple class A relay. The situation on /7 and later machines up to '84 isn't much better. but at least it uses a slightly cheaper relay...

Rob