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Ian WilsonFri Apr-29-05 10:56 AM

  
"Electrical fault"


  

          

Hello all, I wonder if you can suggest a cure for a problem that is getting close to causing my hair to fall out.

Over the winter I fitted a voltmeter and a solid state rectifier to my Mk3 Commando, because I thought that with my slow-speed and headlight-on touring it would quickly drain the battery to a point where the Boyer wouldn't work. I wanted to keep an eye on it. I believe they are fitted correctly. It worked OK last year.

However, now I have a problem. It will start and run fine. I ride off, and a few miles away it just stops dead, as thought I have switched it off. Sometimes, when this happens, the fuse blows and judging by the state of it, quite violently, like a dead short. Not always though. Sometimes it will come back on again. I have noticed the voltmeter shows a huge drop in voltage just as this happens, and when I do get it back it shows a reduced battery voltage.

Now, I know the most obvious thing is to look for a 30 year old wire shorting, but I have been doing that for three weeks! I found a few loose connections, but nothing more. I have been unable to reproduce this in my garage and it doesn't happen when I switch on lights, stop lights, indicators, etc, on the road.

The voltmeter suggests a massive discharge (a dead short?) just before the bike shuts down or the fuse blows.

So, any suggestions guys? Could my new solid state rectifier be faulty? If so, would it cause these symptoms? I want to stress that while it could still be a simple exposed wire, I am running out of places to look! I just wondered if anybody had any suggestions.

I have a Boyer fitted, and just now it failed on me with a blown fuse. I fitted a new one because nothing obvious was wrong, it started and ran for maybe 10 seconds, then discharge showed and it stopped. Could this be a Boyer problem? At this moment nothing else was switched on, just the ignition, and we weren't moving. It doesn't even splutter, it is just like I have turned it off.

Help! Otherwise, anyone want a cheap Mk3 (e/start removed) cheap? Might need rewiring......

Thanks in advance,

Ian

  

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geo46erFri Apr-29-05 05:32 PM
Member since Mar 18th 2009
16 posts
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#1. "RE: Electrical fault"
In response to Reply # 0


  

          

Hello Ian,
As a Jap mechanic of many years I have seen many electrical problems. Its been awhile but the last time I experienced the problem you are describing I found it to be a shorted stator winding.
Unplug your stator leads and check for continuity, should have continuity between the leads but no continuity between either lead and the engine case/frame.
I'm not sure how a shorted stator blows the fuse but it did in at least one case I know of.
Alternately you will have to unplug each system and test ride it. Unplug the light feed if it runs good until you plug it back in your problems in the light wiring. Unplug the stator (you should be able to run it quite aways on just the battery) plug it back in if the problem re-occurs then its stator problem.
You get the idea, keep us posted I'd be interested in what you find.
justa thought,

G.B.

  

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janSat Apr-30-05 04:24 AM
Member since Mar 18th 2009
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#2. "RE: Electrical fault"
In response to Reply # 0


  

          

could be a short in your 2MC cap. ( blue can ) when it warms up .
just disconnect it and see what happens . You do not really need it when your battery is charged .

  

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Ian WilsonThu May-12-05 04:08 AM

  
#4. "RE: Electrical fault"
In response to Reply # 2


  

          

Many thanks to all for the suggestions.

I eventually found a short down by the rear brake stop switch. I fixed this and all is now well. The only thing is that all the parts were fixed, not moving in relation to each other, so how was the fault intermittent......? Have I really fixed it at all, or fixed something else? I suppose I'll just have to wait and see!

One final question: My Mk3 runs a Boyer. Does anybody out there know if it needs a 2MC capacitor at all? I thought this device was to run the bike after a battery failure but it obviously won't do that with a Boyer fitted because this needs 10v or so. So can I just remove the 2MC? Less to go wrong then......

Thanks guys.

Ian

  

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dynodaveyThu May-12-05 06:31 AM

  
#5. "RE: Electrical fault"
In response to Reply # 4


  

          

If you have a new capacitor of 35v+ and about 5000 to 7000uF to replace the old 2MC you should have no problem. Any power supply benefits from the "proper" sized capacitor. The alternator is just that, a variable speed (rpm) power supply. The cap is there primarily for low RPM energy efficiency.

(with battery removed)
The MKIII will kick start and run fine on points or rita.
If the cap is to big it steals power from the ignition and if to small it does little to help.
Having the correct cap is therefore more important for the boyer

  

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zotz1Sun May-08-05 12:39 PM

  
#3. "RE: Electrical fault"
In response to Reply # 0


  

          

Are you running the small Lucas ammeter in your headlamp shell? I had almost exactly the same simptoms whith a brand new pattern part. The fact that all the power went through this 1 part showed me how vulnerable this 1 component was and I no longer trust them.

  

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