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Auxiliary Light Short - How to find?

Discussion in 'XJ Technical Chat' started by gfontes, Aug 19, 2008.

  1. gfontes

    gfontes Member

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    Hi all,

    I need some suggestions on how to start looking for a short on my auxiliary light circuit. Both my hi and lo beams work fine, but when I turn on the auxiliary light, it blows the headlight 10A fuse immediately.

    Is this something that is known to happen to these bikes? Its a '82 750 seca.

    I'm guessing the relay works properly as the headlights work okay. So if that's the case, do I start working backward from the terminals on the light bulb and ohm out each section of harness from connector to a connector? Any tricks of the trade for doing this?

    Thanks much!

    George
     
    gfontes, Aug 19, 2008
    #1
  2. Polock

    Polock Well-Known Member

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    unplug the wires close to the light and see what happens if it still doesn't work look at the switch
     
    Polock, Aug 19, 2008
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  3. turtlejoint

    turtlejoint Member

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    check your wattage on all your bulbs. its possible that you might have too much going on in the circuit. i had an off road hi wattage headlight that would blow my fuse whenever i would have my turn signal running in conjunction with my brake.
     
    turtlejoint, Aug 20, 2008
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  4. Gamuru

    Gamuru Guest

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    R = V / I or Resistance = Volts / Amps.

    To blow a 10 Amp fuse, you'll need to pull more than 10 Amps. Assuming you're putting out ~13 Volts, we can figure a minimum for Resistance:

    R = 13V / 11A = 1.18Ω

    Anything less than that would be a short to ground. If you then take your test leads and hook one to the light bulb side of the fuse holder (fuse removed) and the other to ground, I'm willing to bet you'll measure a resistance value of less than an Ohm. You'll then need to start disconnecting components until you find the short. Start at the bulb itself. Usually they will blow their filament which will sometimes short out the bulb. Unplug the bulb, replace the fuse and see it it blows when switched on. If it doesn't, your problem is more than likely with the bulb itself.

    You can figure you current draw by taking the measured resistance from above and plugging it back into the equation:

    I = V / R or Amps = Volts / Resistance

    Let's say you measure a resistance of 0.65Ω. To figure your current draw, divide your voltage, in our example, 13V by 0.65:

    I = 13V / 0.65Ω = 20A.

    That's a lot and would definitely blow a 10 Amp fuse.
     
    Gamuru, Aug 21, 2008
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