blowing the main fuse

Blowing the fuse

I have an 850 Mk2A which has not been used for many years approx 13000 miles on the clock, recently registered as a vintage here is Aus which is way cheaper than full rego.
I seem to have an intermittant fault with blowing the main fuse and only when I turn on the lights, somtimes it will run on the expressway for ten or fifteen minutes or so then pop, other times it goes straight away.

I have looked at all wiring that I could get to ie under tank seat headlight tail light etc and nothing unusual. havn't tried the ignitian switch or handle bar switch as yet, any ideas heat sink ?? diode ??, It has been a long time since I have done anything major on the old girl and a bit out of touch with the old girl, prefering to weekend ride the Aprillia and recently the BM 1200 s

Any ideas to make a start would be appreciated, riding with no lights even on a sunny sunday is not to smart

peli17 said:

Blowing the fuse

I seem to have an intermittant fault with blowing the main fuse and only when I turn on the lights, somtimes it will run on the expressway for ten or fifteen minutes or so then pop, other times it goes straight away.

Click to expand...

Hi Peli17, you say that it only blows with the lights and it can happen anytime, that is a start and it narrows it down to what is included in that circuit and indicates to me that there may be a short caused by a wire or connection that is occasionally touching earth. I know you said you checked the obvious places, what I would also do is disconnect each connection in turn, clean it , check the insulation, and make sure there is a good tight connection. Especially in the H/L shell there are lots of wires and there could be one that just has a sufficient amount of the connector showing that it contacts with an earth. With the ignition and the lights on you could try moving wires in that circuit by hand and if the fuse blows when moving a certain wire you know the problem is in that area. I would also check where the wires pass through any grommets, bottom of H/L shell etc., or contact with any part of the bike as there may be a bit of chaffing that could expose a wire. I would have a look at the wiring first, then bulb connections, then switches.
Intermittent is a pain to find, I would rather it didn't work at all! There are a good few guys on here who are brill when it comes to electrics, so I have no doubt you will get sorted!
Good luck

What is the Amp rate and type of fuses that you are using?

If the problem only happens with the lights on, then the fault will most likely be in the lighting circuit (Brown/Green) somewhere?

Looking inside the handlebar light switch housing would have been one of the first things I would have done.

Can you narrow it down by testing if the fuse blows with the lights switched to the pilot position (Ign. switched to IGN. & LIGHTS) with the headlamp switched OFF?

Thanks for the info, the main fuse is the standard inline screw together fuse holder with 30 amp ?? will check. - glass fuse

Will pull a few things off this weekend and have a look, from memory I dont think the toggle switch on the headlight does anything, I think many years ago when I put a better quartz bulb in we couldn't get the park position to work so left as is,

Love some of the photos on this site of peoples Nortons they look fantastic

When possible I like to separate the circuits and temporarily fit breakers to find shorts. Of course I'm usually doing that with a bus or train which have a few more circuits.
I even have permanently separated and circuit breakers on most of my bikes. A flashing light is far better than an out light a ways from home.