Main Beam blowing number 3 fuse

1978 T3 - I recently fitted new front indicators and a screen which necessitated disturbing the wiring in the headlamp bowl and undoing the top fork yolk bolts to slip a bracket in underneath the clocks assembly on each side.
After reassembling everything:
-all the dash lights work

  • the indicators work fine
  • brake lights and horn fine
  • back light and front parking light fine
  • headlight high beam fine
    -however on changing to dipped (main) beam number 3 fuse blows
    I have checked the wiring in the headlight bowl and the dash lights and there are no obvious short circuits.
    I have opened up the (Lego) light switch, cleaned and checked it, again no obvious problems.
    Dipped beam still blowing the fuse. Where should I look next please?

run a new earth from the headlight bowl to the frame. (he said, with faint certainty)


Does it blow the fuse if you try it with the headlight “out” of its bowl? It’s probably a short created when the wiring was disturbed the first time you removed the lamp.

While it is apart, unplug the lamp itself and check resistance across the filaments (neg/high beam and neg/dip).

If that is OK, check for resistance between the dip beam wire in the lamp socket and earth. You can do this with ignition off. If its very low, the short must be between the dip switch and the lamp socket.

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Thank you for those pointers Speedy.
It doesn’t make any difference whether the headlight bowl is undone and the lighting wiring hanging out, or not. Everything works fine until you switch to main beam. I’ve tried a different H4 bulb that I know to be okay, to no avail.
I’ll try those tests you have suggested.

Thanks Barry - the headlight bowl is well earthed it seems.

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Must be between lighting switch (left handlebar) and bulb (left handlebar switch incorporates dipswitch function).

Hoever there are two wires from the bulb dip, (green?) one to the lighting switch, the other goes to flasher relay (under tank?). Both have to exit the headlight bowl.

EDIT: with a magnifying glass and a good light, look for tiny wire whiskers that can touch something metal or where wire chafing has cut through insulation.

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General principle is that if it was ok before you made the change then its almost something you’ve changed. Sounds like the dipped beam is going to earth without going through the lamp.

Apologies, somehow I missed the excellent responses to the problem and dived straight in. As you were.

One thing to look at is the wiring to the headlight flasher relay that is located under the tank on the top frame rails near the headstock. When you press the flasher button, it lights the low beam rather than high beam as you might expect.

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Are you sure about that?
If this is the case flashing on dipped beam would have no discernable effect.

Don’s right, he’s not wrong! It’s true. Seems quite nuts but maybe it made more sense back when headlights in daytime weren’t compulsory. But if you are on dip, it does nothing. My LM1 was like that and I modified it to connect the relay to high beam. Can’t remember if the Spada was also like that, or if I did anything about it if it was.

The bike was built back in the days when you only used the headlight at night when it was dark. Daytime headlights hadn’t been thought of then.

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You’re right, Spada manual says flasher is on dipped beam. I rewired mine to full beam but it wasn’t a conscious decision to change it, I just thought thats what it should be.
I checked my Cali 1100i and by this time it is a “passing light” not a flasher and is wired to main beam. At last, one could pass cars at night :grinning:

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