Help - Applying Rear Brake = Blown Main Fuse?

[alexwitham]

Hi Guys,

Took the SV650SX (T Plate) for it's MOT today and on the way the main fuse popped just after operating the brakes (both at the same time). Got to the MOT station and asked the technician for a couple of 15amp main fuses so I could do some investigating as to the cause in the hope I will fix it before the MOT was due (I arrived early). Took the cowl trim's and riders seat off to discover it was definitely the main fuse that went pop.
I put in a new fuse and turned the ignition on, as soon as I applied the rear brake so the switch engaged it went pop again. I'm now at home after putting in yet another fuse, disconnecting the switch and riding home.
I've now got the whole rear seat unit off and cannot see any bare wires shorting the circuit causing the fuse to blow, nor are any of the bulbs damaged/broken.
I am no expert when it comes to electrics and this has really got me, can someone point me in the right direction?

Thank you, Alex

[alexwitham]

Hi Guys,

Quick update. If I apply the front brake, it also pops the main fuse, whether I've got the rear brake light switch disconnected or not. I've also removed the horn to make sure it's not earthing there. Still no joy.

I'm down to my last fuse now, so I've halted trial and error until I've had some replies on here on what to do next.

Help! :confused:

[warmshed]

The main fuse is in with the starter relay are you sure its this one? A 15Amp sounds like fuse 4 that powers the brake lights and indicators.
As the fuse only blows when the brake (front or rear) is operated then this is the section you need to concentrate on. First remove both rear tail/brake bulbs. Operate the brake. If the fuse blows then its the wiring between the brake switches and the rear light cluster. If its Ok then you have a faulty bulb/bulb holder.,
The wire from the brake switches to the bulbs is white/with a black tracer. It goes from the front to the front brake switch to the rear bulbs connecting up with the rear brake switch wire on the way. Diagram is not specific as to where. Check ot the wiring of the white wire with the black tracer. Pay especial care to any hole it goes through looking for abraded insulation.

You can use a bit of 5 amp fuse wire to save buying more fuses. Have a go and report back on your findings so I can help a bit more. You dont want to use a higher rated bit of fuse wire as 5 amps is more than heavy enough to power the two brake bulbs. Put a 15amp blade fuse back once you have rectified the fault. Dave

[alexwitham]

Apologies on my previous mistake. You are correct, it is the 15a signal fuse.
I will give it a thorough inspection and report back.
Thanks

[warmshed]

When using the crankase as an earth path for testing obviously use a non painted bit, eg a bolt head.

[yorkie_chris]

Well the live goes to the brake light switch(s) then to the lamp, then to earth.

I'd say you've got a short to earth somewhere on one of those switched lives. Check the wires near the seat as these can rub.

[alexwitham]

Does anyone have an electronic copy of the SV650SX (1999 REG) Wiring diagram/schematic?

Thanks

[yorkie_chris]

Not me, but to get you started wires you want are

from from front brake switch to switchgear connector is black
from switchgear connector (and from rear switch) to lamp is white w black