Why are my bulbs blowing?

By MCN -

Parts & accessories

 30 August 2011 16:59

Q. I replaced a bulb for the speedo on my, but when I was on my way home from work late this evening I accelerated hard to overtake a car on the slip road and join the motorway, and shortly after my tail (and brake) light, indicators and whole dash went out! I travelled about 13 miles down the motorway with none of those things in the dark (not good!).

Anyway, I know for a fact that all that stuff is connected via the same fuse (because the same fuse blew about three months ago, leaving me in the dark then).
I think there could be a wire that earthed itself against my frame during the acceleration and caused the fuse to blow.

Or maybe the fuse in there (which I had replaced like for like with the previous one that blew) could be the wrong one in the first place? It seems strange it blew after harsh acceleration and also soon after I'd had the fairing off for the new speedo bulb.
Anon, email

A. All the bad luck you seem to have had with the bulbs could be masking the real problem here. I think your first port of call has to be the charging system. You will need a voltmeter to check the charge rate.

First off leave the bike’s ignition off for at least two hours, then check the voltage of your battery. Ideally it should be 12.5-12.8 volts. If it's less than 12 volts the battery has a real problem.

If the battery is showing 13-14 volts you also have a problem with overcharging and the regulator/rectifier. The reason you should leave it 2-3 hours is because you will get this sort of voltage directly after running the engine.

If the battery is below 12.3 volts or above 13 volts it needs replacing. Once you start the bike, at idle you should be getting a reading of 14-14.5 volts DC. If you get over 14.5 volts, then increase the rpm  to 3-4000rpm.

If it goes higher again then the regulator/rectifier is faulty, and this is the reason the bulbs are turning black inside and the reason the fuses are also blowing with the increased revs as you accelerate hard.

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