HELP! Strange electrical problem.

[sparklymarkly]

As an ex-electrician, I'm normally pretty good with circuits etc (just made my own 2nd DC loom for LED indicator system etc & it all works a treat) but this one has really got me stumped!
Just doing some testing of my lights (using a battery feeding to the brown reg cable) prior to putting engine in and I've got a strange problem that I just can't fathom.
Head (dip & main), pilot & tail all work as they should via a new SRS ignition switch (2nd position).
When the lights are on, brake lights (F&R) work perfectly but with the lights switched off, they don't.
If I disconnect the tail light wire at the rear lamp unit, both brake lights work perfectly irrespective of whether the lights are on or off (as they should).
As soon as I reconnect the tail light wire, the brake lamp only functions when lights are switched on.
In practice it's probably not an issue as I ride with my lights on anyway but I'm more concerned that
Mr MOT man will fail it as technically with lights off & engine running, it has no brake lights working.
I'm good with electrics but I am totally baffled by this and tearing out what little hair I have left!
I'd be grateful if anyone can suggest anything to look at - PLEASE!

[db67]

hi, had a similar issue to this on my series 1, i was running a bgm 'type' rear led light/brake light & the brake light would only function with the either the side light or main lights switched on ! switch the lights off & nothing ! the only way i fathomed it out was to fit the original bulb holder & bulb & then it all worked normally !i have since changed the led rear light for a different type & since then have not had this issue again, i know this isn't quite the same problem as your having but hope it helps in some way . good luck db67

[sparklymarkly]

Funny you should mention that as I'm using an LED rear light too (Rich Presley one).
I haven't tried swapping it for the originalbut I might try that just out of curiosity.
I can't see why the LED board would behave this way though but it's odd you had same issue.

[sparklymarkly]

Update! And db67 you're right.
Just temporarily swapped the LED board for the standard bulb holder & it all works correctly irrespective of whether the lights are on or off.
I'm happy that my circuitry is obviously correct but I still don't understand why this should happen so maybe an appeal to the electronic experts now 'cos that's not my area of expertise!

[db67]

no problem, glad you got the fault sorted mate ! could be that you have a dodgey led light board too ! if its not that old then maybe see if rich will exchange for a new one,he seems a reasonable enough bloke db67

[essexgp]

Leds need to be biased on unlike bulbs, try fitting a 470ohm 1/4watt resistor in the positive supply line to the board, this may help, it worked for me

[sparklymarkly]

could be that you have a dodgey led light board too ! if its not that old then maybe see if rich will exchange for a new one,he seems a reasonable enough bloke db67

I don't think the board is faulty - if I connect it direct to a battery it works fine on tail & brake.

Leds need to be biased on unlike bulbs, try fitting a 470ohm 1/4watt resistor in the positive supply line to the board, this may help, it worked for me

Cheers for that - would that be in the POS feed to the tail or stop circuit (or both)?

[essexgp]

~Try it on the supply line to each one first and see if it stops it if that doesnt try to both, as i said worked on mine

[sparklymarkly]

~Try it on the supply line to each one first and see if it stops it if that doesnt try to both, as i said worked on mine

Cheers for that - I'll give it a go.