So there we were about to join the A66 at Barnard Castle on the Commando and everything went out without any warning. Blown main fuse. Replaced it and rode home 60 miles no problem. Must have been age and vibration, I thought.
About a month and several hundred trouble-free miles later the bike had been sat in the garage for a couple of days, when I wanted to check the recorded mileage. I have electronic instruments so it meant switching the ignition on. Dead. Main fuse was melted again. I replaced it to see what would happen, expecting it to blow straight away, but it was fine for a few days, then one day it wasn't. The bike hadn't been moved in that time. (Because it was on the bench having the rear wheel looked at. But that's another story, for the Commando forum.)
Normally I'd expect it to be a frayed wire vibrating and shorting out, but that can't be it if it does it standing still. I can't see any obvious loose connections. I suppose I'll have to work through wire by wire, unless anyone can come up with a better idea? An electronic component perhaps?
It has Tri-Spark ignition and coils, an A-Reg regulator/rectifier, a new 2MC capacitor from RGM seven years ago, electronic speedo and tacho ("Smiths"), and a BSM.
I've been intending for some years to replace the untidy original loom with all its redundant and excessive connectors, so maybe the time has come. I expect Grant Tiller's excellent website will be a helpful guide.
A couple of things spring to…
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A couple of things spring to mind
Thanks for the suggestions, Grant.
I did away with glass fuses years ago. Blade fuses now.
Turn signals seem okay, except only the left rear has to have aluminium foil stuffed between the holder and the bulb body to maintain earth contact. Not ideal, but this wouldn't be a factor in the fuse blowing with the bike stood idle in the garage and everything switched off.
A simple test with a multimeter suggests the capacitor is behaving as expected, but replacement would be easy. Or just take it out of the equation? I've never needed it for flat battery starting, but I thought it had a smoothing effect on a single phase charging system?
The assimilator was removed when I fitted the BSM over 20 years ago.
An alternator stator check is a good idea, I'll get on to it. But again - fuse blowing with the bike stood idle? Edit: it seems unlikely. In these cases maybe the fuse blows as soon as I turn the ignition switch and before the instruments can react. Easy to test: replace the fuse, leave it a few days, weeks, then check it before turning anything on.
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Fuse tip
Read the fuse. If it's a short circuit, the surge of current will "blow" a fuse, literally blow the fuse element into small parts. A standing over current will "melt" a fuse, leaving a gap in the continuity of the fuse element . This would indicate a sustained higher current draw than it's rating, but not a short circuit.
For a short circuit moving parts of the loom (steering head, switchgear,) are main sources, battery security, (negative G on hump backs can cause the battery to lift and short if it's untidy in there).
slow burn over current situations can be as Grant describes, but also issues in bulb holders, horn or ignition system.
if it's an intermittent fault, determine if it's related to incident (eg, indicator or lights use), time ( x amount of days, weeks), or running hours (duration the machine has run between failure); Keep a diary on it.
With regard to your loom a commando one comes in three parts, front,central and rear. This makes them relatively easy to swap out. You seem to have some non standard modifications which you should evaluate to integrate into your replacement loom.
if you are unhappy with the state of your loom, that would be my first elimination by change it, and ensure the upgrades are addressed properly.
Good luck
j
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It's worth trying...
... fitting a bulb in place of the fuse then jiggling and moving all the wires you can get at. If there's a short the bulb will light up and this can help locate the fault (ie the thing you last jiggled).
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Correct
Ian and Jonathan both have correct ideas hear, but Alu foil has NO PLACE in electrical circuits, (nor black sticky tape!!! ONLY TEMPORARY!!!!) neither were put there by the factory! So don't use them!!
With regard to Commando looms in three parts-the MKIII maybe but not the earlier Commandos.
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A couple of things spring to mind:
Is your fuse a glass filament type?
Perhaps an idea to consider moving to the automotive blade type, as they're much more vibration resistant. Not just the fuse, but the holder too (the spring gets lazy in the original holder).
Is everything ok with your turn signals?
As the bi-metallic strip gets tired it can pull more amperage during a flash cycle - this can put greater load on your fuse.
It can also be an issue if you forget to cancel them after a turn!
The electrolytic capacitors don't last forever.
Aftermarket ones don't last as long as the original blue can 2MC. I would suggest seven years is about the lifetime.
You have a battery status monitor - that's good.
I assume the original 3AW 3 wire ‘silver can’ assimilator has been totally disconnected and the cables taped off safely?
Check your alternator stator - specifically that there is no continuity at all between each of the AC output cables and ground.