1973 Commando Blowing Fuses

Bike had been starting/running until ignition switch was left "ON" overnight. Found fuse blown next morning, and now battery fuse blows whenever new 35a fuse is fitted, even with ignition switch in "OFF" position. I have not been able to find a pinched/bare wire, etc. when tracing for a short with my Fluke multi-meter. What could have gotten shorted out by leaving the ignition "ON" overnight?

That might be a hard one to find, but I think I would start with the Ignition switch it self. Check it out good making sure all contacts are good.

Was the parking light or any other lamp on? Could the ign switch have failed/now shorting? How's the 2 MC capacitor (do the workshop manual tests) ?

CommandoMKV said:

and now battery fuse blows whenever new 35a fuse is fitted, even with ignition switch in "OFF" position.

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Assuming it isn't a wiring or ignition switch fault then the things that could blow the fuse with the ignition 'OFF' would be the rectifier, Zener diode or 2MC capacitor.

Electrical work is easy, unless you over think it...

Here's what I would do...

Remove the seat and fuel tank, so you can see what you are doing.
Remove the battery and connect your "fluke" meter to the leads, and place the meter setting on continuity (so it makes a sound when the circuit is completed)
Put the fuse back in, which should start the meter beeping because of the short... Right!?

Personally, I'd open the headlight shell and see if a skinned wire is grounding out somewhere. It's easy for that to happen inside the headlight shell and of course the shell is grounded, so it becomes a dead short. You can also disconnect the power feed wires to each of the switch blocks from inside the headlight shell , so that would break the circuit (and therefore test them for the short) for any of the switch block circuitry. (Meaning if you disconnect the hot wire and the meter beeping stops, then you are on your way to tracking down the shorted circuit)

My Capacitor used to short because of the way it hung down and the contacts were exposed. I eventually cleaned that up and shielded the connection and it's all good now.

Very little is connected to the battery with ignition off. Besides what LAB mentioned, the wiring to the power take off socket is also always live.

Does your bike have an ammeter? I believe some early models had the horn circuit live with ignition off.

Do you have an electric starter, maybe a short in the starter circuitry, just wondering......

ignition on all night .........if points , coils should be hot ?
may be jump the key switch ?

+1 with oOnortonOo (Reply #5)

When dealing with a short circuit, divide and conquer.

Slick

One of many possible explanations.
If contact breaker was closed a coil might overheat. Then the coil shorted a bit internally causing higher current to flow. The relative thin wiring heated up until insulation melted and caused a real short in the wiring harness. Then fuse blows but damage is already done. I think a 35A fuse is way to big for the gauge of wires in the system.
I don't have a wiring diagram at hand, but most brit bikes I've worked on have brake light before switch.
As already said, wiring, zener, rectifier and switch are likely culprits.
Reply #5 is the way to go.

L.A.B. said:

Assuming it isn't a wiring or ignition switch fault then the things that could blow the fuse with the ignition 'OFF' would be the rectifier, Zener diode or 2MC capacitor.

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Thanks L.A.B......I'll run all the repair manual checks to see if any of those components are at fault. I know a 47 year old wiring harness could be suspect, but I hope to save replacing until I strip the frame for painting.

CommandoMKV said:

.I'll run all the repair manual checks to see if any of those components are at fault.

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One check you can do is disconnect (or connect) each of those items (including the ignition switch) in turn (brown/blue wire) with your meter or a bulb connected in place of the fuse. The meter or bulb should then indicate if one is a short.

It was the rectifier that was blowing the fuses. Left it and the battery disconnected, and was able to get the bike started. Amal slides are badly worn, and may be binding some once bike is running and vibrating. I have a line on some new 32mm Amal Premiers that I will be purchasing soon, along with Tri=spark's electronic ignition and their rectifier box, from the same source.

If all else fails, remove the tank and seat, wrap the the fuse in foil and look for smoke

Niagara850 said:

If all else fails, remove the tank and seat, wrap the the fuse in foil and look for smoke

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No no no! not foil. Use a cut down pencil a bit of resistance to prevent damage and plenty of wood to make lots of smoke.
I know this, not recommended but needs must. As teenager stuck in the wilds of Scotland with an intermittent short, out of fuses and mobile phones not invented the prospect of roughing it was in the offing. Then by chance I found an old pencil by the road side, cut it down to fuse size and bingo she fired up fist kick. Every time I used the rear brake smoke pored out of the side panel. Found the short and got home okay.

Mine had a shorted wire from horn to rear fender