Brake light out, need help (no it's not the bulb!)

[Jonno271]

On Friday I picked up my nice shiney new Yamaha 46 litre colour matched top box and fitted it to the bike. Nice piece of kit, the mounting plate is far neater than other after market items I've seen...

Straight away though I noticed how far it hung over the tail light. If a truck got a bit close behind there's no way he'd see my brake light come on. Now Yamaha do a brake light fitting kit for the box but the price is rediculous so I thought I'd have a go at fitting one myself...

I picked up 6 high intensity LED's and wired two groups of three in parallel with a 100ohm resistor in line to each to achieve the necessary 3.5v across each LED. Drilled the back plate for the built in reflector and mounted the LED's in their little holders and then put 12v across them to try it out. Hey presto, one high level brake light in my top box costing about £10!...

So far so good, well that's where my luck ran out! I then had to wire the whole lot into the bike...

I removed the tail light bulb holder and tested each wire with my volt meter, deducing that there's a black earth (obv), blue constant 14v for the driving light and a third yellow wire supplying voltage to the brake filament. This reads 8v without the brakes on and 13.5 with them applied (engine running)...

I figured tapping in to the brake light wire and the earth should do the trick. 8v isn't enough to light the LED's but the full 13.5v is. This is where my problem started...

I snipped the black and yellow wires and soldered in my plug lead. This would allow me disconnect the top box electrically. I decided on a quick test to make sure everything was still working at this point. Unfortunately, it was not! The tail light lit up fine but when I applied the brake, it did not get brighter. It was clear the second filament wasn't illuminating...

OK, so let's remove my lead and put things back to standard. Done. Still no brake light, only the driving light. Obviously I tried a new bulb and then put the volt meter across the bulb holder. It still shows 8v on the yellow but no longer goes up to 13.5v with the brakes applied...

Out comes the manual. Checked all fuses, there's only one for the tail light as a whole and checked the continuity of all the wires to the tail unit, nothing wrong there...

And that's where I'm at! Having done a lot of head scratching I can't figure it out. Unfortunately there's no wiring diagram in my manual so I can't really investigate any further. I can only guess that there's another component in the circuit, maybe a relay, that's blown...

I wonder if anyone here has had a similar problem or if anyone knows the wiring a little more in depth and (excuse the pun!) may be able to spread some more light on the matter? Any input would be greatly appreciated, otherwise it's off to the shop the bike goes to get fixed at further expense!

Thanks for reading guys, hope some one can help!
Cheers,

Jon

 

[Airhead]

Check continuity through the wires to the tail light socket. Also check continuity from the fuse box to the tail light. Its possible with you moving things around one of the connections to the light got loosened.


Service manuals are here for download so you can get the wiring diagrams: Link

 

[SirIsaac]

Checked all fuses, there's only one for the tail light as a whole...

Are you sure about that? Perhaps there are more differences between the electrical systems of UK and USA models than I would expect, but mine has separate fuses for the brake light and the tail lights. On mine, the brake light and horn are on the same fuse. Does your horn work?

I don’t think it matters, but I don’t know what to make of the 8 volts you measure. On my US bike, when neither brake switch is closed, nothing is connected to the brake light supply wire. I'll go with the ABS theory.

Also, I’m a little uncertain about the connection you are using for the LEDs. You stated that each LED requires 3.5 Volts. Is that the specification on the LED package?

 

[alanrim]

First thought is have you checked the bulb.

As for the 8v, sounds odd unless the ABS version has a bulb check function.

The fuse marked as tail light is just for the tail light, the brake element is fed from the same fuse as the horn, so as already said try your horn to prove fuse is ok.

 

[Jonno271]

OK guys thanks for the replies, got it sorted now...

SirIsaac, you are quite right, there are seperate fuses for the tail light and brake light. Unfortunately this isn't annotated in my manual and the brake light bulb is labelled as "Back up" in the fuse box. Don't know if that's meant to mean spare or if it's a dodgy Japanese translation effort!...

Anyway that's what it was, checked all the fuses again and although it was visibly blown, replacing it bought my brake light back to life...

As for the 8v, I reckon it's just induction. There's no current so I think it's just induced from being loomed along side the tail light wire...

Yes, the LED's are 3.5v nominal voltage, they're high intensity units. Got two sets of three wired in parallel now with a 100ohm resistor on each set of three (each LED is getting just over 3.5V). Works a treat and cost a fraction of the factory unit

 

[macem29]

hmmmm, you're mention of a relay got me to thinking, the circuit is designed
to illuminate only one bulb, the brake light, you now have the current to
illuminate 7 bulbs going through the one fuse, and more worrisome, through
the brake light switch, I'd consider putting my own relay in the circuit, 12V
to power the new lights direct from battery voltage, and a signal wire from
the existing brake light to turn on the relay, that way there is very little
additional current on the OEM circuit and you don't need to over-fuse it

 

[Jonno271]

hmmmm, you're mention of a relay got me to thinking, the circuit is designed
to illuminate only one bulb, the brake light, you now have the current to
illuminate 7 bulbs going through the one fuse, and more worrisome, through
the brake light switch, I'd consider putting my own relay in the circuit, 12V
to power the new lights direct from battery voltage, and a signal wire from
the existing brake light to turn on the relay, that way there is very little
additional current on the OEM circuit and you don't need to over-fuse it

I see your point macem but the total draw for the LED's is milliamps. Not enough in my opinion to warrant another fuse. They're working fine and have been for the last week, commuting 60 miles each way so I'm happy enough that they're not gonna blow a fuse

 

[Jonno271]

hmmmm, you're mention of a relay got me to thinking, the circuit is designed
to illuminate only one bulb, the brake light, you now have the current to
illuminate 7 bulbs going through the one fuse, and more worrisome, through
the brake light switch, I'd consider putting my own relay in the circuit, 12V
to power the new lights direct from battery voltage, and a signal wire from
the existing brake light to turn on the relay, that way there is very little
additional current on the OEM circuit and you don't need to over-fuse it

I see your point macem but the total draw for the LED's is milliamps. Not enough in my opinion to warrant another fuse, or even over fuse the original circuit...

They're working fine and have been for the last week, commuting 60 miles each way daily so I'm happy enough that they're not gonna blow a fuse

 

[macem29]

right-on, good to hear man!

right-on, good to hear man!