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Electrical Question: 6V to 12V fuses ??

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  • Electrical: Electrical Question: 6V to 12V fuses ??

    I was rolling along the other night in my 53' 2R6 when the lights went out and I could smell what ended up being a smoked 25 amp in-line fuse. A quick background again is that I have a 6 - 12 volt conversion. The PO that performed the conversion simply wired the light switch off the ignition switch (acc or ign terminal), and placed a 25 amp in line fuse between the switches. Just to check things, I installed a 15 amp fuse and the lights went back on nicely. I still don't feel good about night time driving because I've never had a fuse "blow" unless something was wrong in the circuit. Need to problem solve: advise ??

  • #2
    Not enough information. Is there still a headlight relay? Are you still using original wiring? It is my understanding that six volt systems draw more "current" than 12 volt systems. I believe fuses are rated for "current" more than the "voltage" pushing the current. Since my psychology degree did not require much electrical study, I'm afraid my "understanding" is suspect. However, knowing the suspension of these vehicles, the vibrations, it is no surprise that intermittent shorting contact would be sufficient to cause a fuse to blow...regardless of six or twelve volts pushing the current to ground.
    John Clary
    Greer, SC

    SDC member since 1975


    • #3
      It sounds like you have a in line fuse holder between the light switch and the ignition switch. Most in line fuse holders are rated to carry up to 15 amps. 6 or 12 volts does not make any difference. Great for a radio, not so good for head lights.
      Usually head lights are not wired through the ignition switch. They are wired directly from a power source on the car side of the output of the generator then through a circuit breaker rather then a fuse.
      One does not normally smell a fuse blowing; rather the smell is the wiring overheating. You need to check for the source of the overload.


      • #4
        What John said. It is very difficult to know how to diagnose a problem on a 60-year old vehicle without knowing what has been modified over the years. The fact that your truck has been converted to 12 volts (inexpertly?) opens up all kinds of possibilities. Your truck was originally equipped with a 30 amp circuit breaker for the head/tail lamp circuits -- no in-line fuse. All things being equivalent, a 12v system will only draw half the amps of a 6v system -- so a 15 or 20 amp breaker should now be adequate to protect those circuits. The original wiring was easily capable of handling the amperage that a 12v system would require, but it is probably either badly deteriorated or (partly?) replaced by now. Intermittent shorts are very common on old wiring, and can certainly blow out a fuse or start a fire. You need to follow the headlight wiring from the switch to each lamp to ensure that you don't have a disaster waiting to happen.
        Skip Lackie


        • #5
          Thanks...good advise. Will probably install a CB from one of my Lark harnesses and chase all the wiring in the conversion to lights.


          • #6
            Here's an easy headlight relay circuit.
            A relay will take the load off the switch.

            (a quick primer on relays...)
            HTIH (Hope The Info Helps)


            Get your facts first, and then you can distort them as much as you please. Mark Twain

            Note: SDC# 070190 (and earlier...)