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fuse blew, thinking out loud...

Discussion in 'Norton Commando Motorcycles (Classic)' started by maylar, Aug 12, 2019.

  1. maylar

    maylar VIP MEMBER

    May 13, 2007
    Please allow me to ramble a bit...

    I did some wiring mods on my 74 MKII this weekend. Reinstated the kill switch, which had been bypassed (soldered the contacts together) for the last 40 years. I did this by buying a Sparx switch cluster and swapping it into the Lucas housing. I also added a relay to the coil bracket, to isolate the switch from ignition current. Everything worked before attaching the switch assembly to the handlebars.

    Started the bike (e-start) Saturday morning and it ran for about 10 seconds then quit. Electrics were dead. Blown main 20 amp fuse.

    Took the tank off and inspected my wiring, all looked good. Measured resistance of the power wire to ground with ignition on, 80 ohms. Put an ammeter in the line and started the bike again, got just a couple amps. Hmmm. Replaced fuse, went for a ride with no issues - except - no blinkers.

    Electronic blinker controller for LED's was toast. It's just 2 terminals, no ground. But it feeds 12V to the switch cluster when the ignition switch is on.

    So, I'm thinking that the blinker power feed shorted to ground somewhere. Wiring under the tank looks OK - could the Sparx switch board short to the handlebars when mounted? It fit the housing perfectly.

    /ramble mode off/
  2. L.A.B.

    L.A.B. Moderator VIP MEMBER

    Nov 20, 2004
    Sounds like it, but it should be easy enough to check.
  3. kommando


    May 7, 2005
    I had a Lucas one short to the bar, a bad previous repair had resulted in a large blob of solder which shorted.
  4. oldmikew


    Jul 25, 2015
    I had an elusive problem of fuses blowing.. start bike and a blown fuse... The cause I think was that I have a lot of earths running to battery positive and the battery terminal bolt was not quite long enough to give secure connection, so surges. Took ages to discover
  5. B+Bogus


    Jun 28, 2009
    Had an intermittent starter motor failure on my brother's Hinkley Thairumph which was eventually tracked down to an overlong battery terminal screw bottoming out before it clamped the terminals.
    I did laugh...
  6. maylar

    maylar VIP MEMBER

    May 13, 2007
    Well, blew another fuse yesterday. A couple miles from home. I have no idea what's going on. I'm going to revert to the wiring scheme I had before, with the old Lucas switch cluster, H4 headlamp and no kill switch.

    There's a major Brit Iron show tomorrow (Sunday) and with all the work I've put into the bike this year I want to be there.
  7. o0norton0o

    o0norton0o VIP MEMBER

    Apr 27, 2015
    This sort of problem is about detective work.

    I have my own certain cast of characters to accuse, starting with the headlight shell and it's internal spiderweb of wires and connections. Everything needs to be well connected and insulated from each other. The lighting switch hangs down into the void with it's contact tentacles exposed. When you insert the headlamp reflector it can push the wires around in there. Take a look and see if there's an issue in there. Maybe something is bouncing while you ride and making contact.

    The other culprit I have had is the wires from the capacitor hanging down. I've had mine short out the bike by getting pushed against something... (can't remember if it was the fender or the battery hold down bar. My '70 model with transverse battery tray. I heat shrinked over the connections and bent the spring to angle it into a better place.

    I would also clean and reinstall the battery, checking for charge and water level. Clean the terminals and connectors. Clean and inspect the fuse holder and fuse. ( I replace the inline one with a blade style fuse on my bike) current pulses from poor connections can blow a fuse. I'd inspect and clean all my ground wire contacts, and add an extra especially from the engine to the frame... (but that's not for a short issue)

    The other thing I have had that seemed random is bad spacing between the rotor/stator. If you use a battery tender then your problem with charging may only show up randomly. You go out on a ride, the rotor rubs the stator until it heats up and shorts out the fuse. Everything cools down and the new fuse doesn't blow instantly, leaving you wondering why not. The tell tale sign of this is the discoloration of the stator's metal frame...

    Pinched wire at the handlebar or switch block can be an issue... those switch blocks are primitive and finicky. I routinely have poor contact issues that cause me to disassemble and clean the switch to restore functionality.

    One of the things that is the worst sort of failure is the "intermittent failure". If something dies outright, you can trace it down. If it only fails occasionally, then you aren't always going to be able to track it, and that's where you need to get lucky or have a method...

    So, since luck is rarely on our side, we are left to develop a method... I might sound crazy here, but there's a few systems to which you can add a temporary switch wire, then duct tape them to your handlebars. (yes, I'm crazy) add a switch to your directional power wire, your headlight power wire, and the kill switch power wire. Obviously, you need to carry a supply of extra fuses too. If you ride around with all the switches on and the fuse blows, switch everything off, change the fuse and restart the bike. Then flip each switch on, then off one at a time. Then flip them all on. Maybe that will isolate your issue to a particular circuit.

    I'm not sure anything I said is all that helpful, but maybe it will cause you to find the actual problem by just going through some testing and disassembly...
    Last edited: Aug 17, 2019 at 10:02 AM
    maylar likes this.
  8. maylar

    maylar VIP MEMBER

    May 13, 2007
    Roger that. I'm kicking myself for making multiple changes at the same time. Installed a CnW e-start, replaced one of the switch clusters, installed a relay to control ignition (kill switch), and changed to an LED headlight. There were also some loose bullet sockets under the tank that I replaced.

    The headlamp shell is indeed cluttered. I reverted to the H4 lamp yesterday because it takes less space in there. I also had some wires trapped behind my new oil tank, so I redressed them. And the ring terminal feeding the fuse wire popped off the wire when I jiggled it, maybe there was a loose connection causing a surge, but a 20 amp fuse is no joke.

    Yup, checked those. Zener is no longer in use so those wires just hang near the footrest.

    And the possibility of failure on the road.. a horrifying thought.

    Yup, that's the mode I'm in now. Thanks for the support.

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