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Parasitic Drain

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  • Parasitic Drain

    I had a blown inline fuse on my 64FL while it was just sitting. I checked voltage between the frame and the unhooked negative terminal and it read 1.6 volts. For comparison I checked my 41 ULH and it read .5 volts. Both bikes have stock 6 volt components. Neither bike drains the battery while sitting though I put a 1 amp charge on them once a month. Is it normal for any current showing or is there a better way to test this?

  • #2
    Otis, this sounds very strange to me. A better test would be to set your meter up to read amps. You are probably using a digital multimeter that can be used the same way with the negative lead lifted and the meter in line. I would be surprised if you read anything on the amps scale.
    I have a '64 FL so I will check to see if I have the same voltage reading. Mine sits all winter and I never have a problem in the spring, except it usually takes a few extra kicks.
    Also you did not say what size fuse you have in the bike. Fuses are strictly electrical devices and on a motorcycle they are subject to much vibration and this may cause them to fail when there is no apparent reason for failure. Has it failed again? If not, you may be looking for a problem that does not exist.


    • #3
      After thinking about it I think the multimeter completes the circuit with resistance allowing some current to flow. I was thinking maybe a resistance test would be a better indicator but am scratching my head how to do it. I also installed a new 30 amp fuse and it’s holding so you’re probably right about a problem doesn’t exist. More likely I short circuited the positive terminal when removing the alligator clip on the trickle charger without realizing it.


      • #4
        Way back when, Folks...

        My 12v Pan's headlamp got progressively dimmer, and it turned out to be a white plastic fuse holder, as it slowly melted, yet the fuse never blew (15 amp).

        (Sorry, can't explain anything electrical.)

        Bakelite rules!

        AMCA #776
        Dumpster Diver's Motto: Seek,... and Ye Shall Find!


        • #5
          I agree with Lyle. The best test is to measure current. Most decent digital multimeters have a 0.3A range and a 10A range. If you have not used a digital multimeter to measure current before the process is to disconnect the negative lead of the battery like you did in your earlier test and connect the multimeter leads between the battery and ground just like when you measured the voltage. The difference is that the positive lead plugs into a different port in the multimeter that is used for measuring current. In this measurement the multimeter is actually completing the circuit between the battery and ground.

          For example, the meter could actually measure the draw from your battery that different circuits use. For instance, if you were on the 10A range you could switch on your light and actually see how much current it draws from the battery.

          With the bike off and assuming no fault in your electrical system. I would expect you would be on the 0.3A (also known as 300mA) range on the multimeter and measure very little current (say 0.001A). If it was much higher, it would drain your battery down if the bike was not started often.

          Bottom line - if you had an intermittent short occur it could blow the fuse. Like Lyle said it is not uncommon for a fuse to break from vibration.
          Erik Bahl

          1929 BMW R63
          1915 Harley Davidson 11F Twin


          • #6
            A "key off" parasitic drain test is performed in series (unless you use an Amp Clamp) on the ground side of the battery as mentioned by others above ...
            On older carburetor motorcycles, your test results found should be zero if everything is correct. Here's a photo I took for you to reference.

            Note that the red lead is plugged into the Amp jack port and the meter switch setting is ADC.

            If I'm understanding the test you performed and how you hooked up your meter to the circuit being tested, you were simply reading available battery voltage. However with such a low reading you got on your 6 volt system, you either have a low charged battery or a bad ground cable/ground.
            Here's another photo I took for you to reference a good 12 VDC battery system.
            Note that the red meter lead has moved as did the switch setting.

            I hope this helps,
            Duke Kleman


            • #7
              I checked the amps and it showed .03MA, two other 6V bikes showed even less. Battery isn’t draining down so I guess I was looking for a problem that didn’t exist. Just found it strange that the fuse blew while not running. As I said earlier maybe I hit the positive terminal to ground putting the charger on it without noticing it. Thanks for the lesson on current vs volts.