Wiring question 1983 R100CS

[Buckingham]

Well it's not a GS but similar !
Try to sort out a problem on my R100CS and though well-versed in electrics, I can't work out why it's happening.
If I put the ignition switch in the parking lights position, the parking lights work OK.
However, if I then put the light switch to the sidelights position, the ignition circuit powers up (so the indicators, brake lights & ignition all work).
So far, looking at the wiring diagram, I've not been able to work out what is supposed to prevent this happening. (in other words, ignition switch in parking lights position, what is supposed to prevent 12 volts feeding back to the ignition live circuit via the middle position of the light switch ?)
I'd have thought there might be a diode to prevent this, but can't see one on the diagram.
Any ideas ?

 

[Rob Farmer]

John,

The light relay should provide the isolation between the circuits.

In park position your side lights are fed from the grey wire on the ignition switch (terminal 58) this provides a feed to fuseboard and on to the grey/black wires feeding the sidelights and voltmeter and clock lights.

Theres also a sidelight feed from the light relay for when the side lights are turned on in normal operation. If your relays failed then the relays causing the back feed to the ignition circuit.

Try pulling the Light relay. The sidelights will still work in the park position but the fault should have cleared. Obviously you will not have any lights with the ignition turned on normally.

 

[Buckingham]

Thanks Rob so is it the diode in the light relay that give the isolation ?
I'll disconnect it & see what happens.

All the best,

John

PS still regretting not buying your adv

 

[Buckingham]

No joy. With the light relay completely disconnected, ignition switch in parking position, all ignition circuits powered up as soon as the headlamp switch is moved to the sidelights position.
Everything else works normally.

 

[Rob Farmer]

Bugger!

Has this just happened or has it been a long term problem?

If it's just happened then you need to check out the lighting switch otherwise it's checking the wiring in the headlamp and ignition switch.

 

[Buckingham]

Hi Rob,
It's been like this since I got the bike as far as I know.
It's a CS & came with an RS fairing, I've just put it back to how a CS should be with cockpit fairing.
I need an accurate & helpful wiring diagram. The Clymer & Haynes are pretty poor.
The BMW diagram in the small blue owners handbook has been the most use, even then it doesn't seem to show the grey wire from the l/h switchgear.
It's this grey wire that connects to the fuse board (lower fuse) that is feeding back to the ignition cct.

 

[Buckingham]

No joy.

With the main light switch in the sidelights position, the green & grey wires are connected together.
This has the effect of connecting together the two fuses in the headlamp shell, and powers up the ignition circuit.
I can't work out from the diagram how the two circuits are supposed to be isolated when the ignition switch is in the parking light position.
Mind you, the diagram does not really show the main light switch in any detail, the three wires are green, grey, yellow/white.
I've put it all back together again now & will live with it unless someone can come up with a new idea ??

 

[Rob Farmer]

I seem to remember that another switch came out for the same bikes with one wire less on the left hand switchgear. It may have been to correct this issue.

 

[bustupbiker]

I seem to remember that another switch came out for the same bikes with one wire less on the left hand switchgear. It may have been to correct this issue.

When i did the R90s last year i had to have a new switch set, that had the grey wire missing Think it came from Motobins and was told by one of the Airhead Gods that it was not needed as it was for the parking lamps.

 

[Buckingham]

Thanks for all the help guys.

Yes it looks like this is not a wiring or component fault. I'm told I need to move the green feed wire for the headlamp switch to a spare ignition-live on the ignition switch, which will fix the problem.
It's such a pain to remove the headlamp again through the hole in the cockpit fairing, I'll have a go at it later this week

Also appears the BMW diagrams don't properly show the headlamp switch wiring for UK bikes (only for US spec with no main light switch) - no surprise that I couldn't match the diagram to the bike.