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.

thanks in advance

[ShakerNorm]

Leethal - first thing is to make sure you have a GOOD wiring diagram - then you can start tracing your wires. Here's a great pic of the '79 wiring diagram - nice and big, in color, and all the printing corrections are made, so it's easy to read and accurate.
www.freefilehosting.net/wiringdiagramstock

I've taken that same wiring diagram and modified it to match my own bike - so troubleshooting is a lot easier.

I agree - it does sound like a short circuit somewhere. Unplugging the headlight would at least eliminate that item from the equation. Then pulling the fuse should isolate the circuit and help find the problem.

On a stock bike, there is no power to the headlight before the engine is started, but if you have put on the older style switch, you've probably changed that. (I believe that's part of the Safety Relay's functions). I'm not sure what other mods you may have done to your bike, though.

Very interesting that it only happens when the engine is running, though..... You'd think it would happen all the time. There must be a common thread in there somewhere.

[leethal]

Hi Norm, thanks for the input

I have my trusty color wiring diagram and multimeter ready to go if you boys have some tests for me to run.

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.....

[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.

[ShakerNorm]

If you replaced the harness (I replaced mine as well - and did similar mods to you, as well as PamcoPete's alternator mods, and a Boyer ignition system), then you know what kind of rat's nest is in that headlight bucket.

It's possible that when you removed the bulb, you moved a shorted wire just enough to solve the problem. I'd check all the holes in the back of the bucket to see if any wires are chafing. There's supposed to be rubber grommets protecting the wiring from the metal edge of the bucket, but after 33 years, they'll be in rough shape, if they're still there at all. I'm not sure if we can even find replacement rubber grommets at all. I';; see if I can find something, though......

In the aviation industry, we also use flexible plastic grommet material that we call "caterpillar" because that's what it looks like - about a foot long piece of nylon plastic with hundreds of little legs on each side. Here's a link to show some - www.gen-aircraft-hardware.com/template.asp?pagename=grompla (they'll also sell it to you for about $9 for a 10-pack). It comes in different sizes for different thicknesses of sheet metal, and you glue it onto the edge of the metal to keep it from chafing stuff. Works a charm, but for tight holes it's best to warm it up with a heat gun or something to soften it and help it conform to the hole. If you know any aircraft mechanics - they'll know exactly what you're talking about (and maybe get you a piece or two for cheap).

And if you find a chafed wire - cover it up - hopefully a little heatshrink will do the trick!

Good luck!