A10 blowing fuse when headlight switched on

[blackeyeian]

Hi  Could anyone give me an idea of how to find this fault and repair it - My 1963 A10 blows the main fuse - 3 amp - when the headlight is switched on.
The side light & brake light work perfectly and a new light switch has been fitted with the wiring refitted to the same connectors on the switch - see image
Firstly is a 3 amp fuse the correct one?
Would a bigger one help?
Regards  Ian

[johncarter]

If it worked before you changed the switch then the wiring on the new switch or the switch itself would be the first place to look. Most probable cause would be a short. Look for pinched wires where they might be rubbing against the frame, e.g. under the tank, or bare wire that could be shorting to earth. If nothing obvious, I would disconnect the battery and then check each wire against the wiring diagram with a multi meter checking for lack of resistance between 'non earth' connections and the frame. Others with more electrical nouse than I might know a better way tho. I wouldn't increase the fuse until you've identified the cause.

[johncarter]

Just looking at your photo again, the octagonal clip that holds the switch in place appears to be touching one or two of the contacts on the right hand side... I'd look at that first...

[blackeyeian]

Thank you that gives me a god starting point - The photo may have been taken whilst I was removing the old switch for wiring reference, but well spotted and worth another check

[Jean-Luc]

3 , three amp is too low.
20 is a minimum,  especially if 6V circuit.

[ducati2242]

The last one i fixed with the main fuse blowing was on a B44 turned out to be the horn . Soon as switched on blew main fuse . Horn defective . Unwired the horn and all was good .

[blackeyeian]

Thanks again - - -3 , three amp is too low. 20 is a minimum,  especially if 6V circuit. - -  The bike is 12 volt - what amp fuse should be fitted ?  Regards  Ian

[Mark N]

I've never had a 12v bike, but 5a or 7.5a would do it.
Having said that, your 3a should just about cope with a 21w headlight, tail light and speedo light.
A quick test would be to remove all bulbs, except the headlamp and then if your 3a blows, its a short circuit...

[idie]

Wire bulb across the fuse it will light with a short. Then move things until light goes out this will give you an idea where the short is

[blackeyeian]

Tank you for all your help - I eventually found - what could have been the short under the tank, I taped it up and all is well.  I have also replaced the 3a with a 5 amp fuse.
Now everyrhing - so far - seems back to normal.
Thank again, you are all inspiring
Regards  Ian