Blowing Headlamp Fuses

[leethal]

Hey guys, outta the blue my 79 special is blowing headlamp fuses. I might add that several years ago I fitted an early model right switchblock so I could turn the headlamp off at will.

When I put in a new 10amp fuse and turn the headlamp on (engine off) everything is normal. If I start the engine then turn the light on, the engine weakens and the headlamp fuse blows.

Surely I have a short somewhere, I've given all the obvious wires a once over but am not clever enough to know where to test with my multimeter.

On the topic of the wiring mods, back in 2008 I put a new mikesxs harness on the bike so solve all my existing gremlins. I deleted the RLU, Light Checker, and Self Cancelling Unit from the bike. To accomodate the early model right switchblock I ran the Red/Yellow wire from the mid-harness up to the right switch connector in the headlamp shell.

This setup has worked perfectly for the past 4 years, I hope some of you experts can shed some learning on my feeble grey-matter.....

thanks in advance

 

[retiredgentleman]

Hey guys, outta the blue my 79 special is blowing headlamp fuses. I might add that several years ago I fitted an early model right switchblock so I could turn the headlamp off at will.

When I put in a new 10amp fuse and turn the headlamp on (engine off) everything is normal. If I start the engine then turn the light on, the engine weakens and the headlamp fuse blows.

Surely I have a short somewhere, I've given all the obvious wires a once over but am not clever enough to know where to test with my multimeter.

On the topic of the wiring mods, back in 2008 I put a new mikesxs harness on the bike so solve all my existing gremlins. I deleted the RLU, Light Checker, and Self Cancelling Unit from the bike. To accomodate the early model right switchblock I ran the Red/Yellow wire from the mid-harness up to the right switch connector in the headlamp shell.

This setup has worked perfectly for the past 4 years, I hope some of you experts can shed some learning on my feeble grey-matter.....

thanks in advance

Unplug the headlight itself and then test run the engine. Does the fuse still blow?

Try disconnecting the connector (in the headlight bucket) that comes from the "Lights" switch. That should divide the wiring to see if the fault is before or after the connector.

Are you still using the Safety Relay to turn on the headlight?

 

[leethal]

With the headlight bulb unplugged and engine running the fuse is ok.

With headlight bulb plugged in, the red/yellow wire unplugged from right switchblock, and engine running the fuse is ok.

I made no mods to the safety relay, just added the wire to allow the headlight to be switched off.


Now comes the weirdness, just for kicks I put everything back together and tried to blow the fuse.....but the issue has fixed itself. Possibly a grounded wire has moved allowing the circuit to function, though I cannot see any exposed wires anywhere. This experience leaves me greatly unsatisfied.

 

[Royboy]

Carefully check the condition of the wires where they enter the headlight bucket. If one sneaks out of the grommet and gets chafed that will do it.