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Electrical help needed, how to find a short.

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  • Electrical help needed, how to find a short.

    So, I am trying to find the issue with a truck at work. Our trucks run three different radios and a radio charger. One of them is draining the battery. The radios are wired to have power all the time. It is up to the driver to remember to turn them off. However, even with them all off, the battery goes dead. I disconnected the power sources and the battery holds its charge. Is there a way to test for short without opening up the radios? I planned on bringing my MM to work tomorrow and poking around.
    '81 XS1100 SH

    Melted to the ground during The Valley Fire

    Sep. 12th 2015

    RIP

  • #2
    Can your dmm measure amp draw? If it can then with the radios off, switch your DMM to DC amps and put it inline between the positive terminal of the radio and the +12 source. Should read 0 or close to it.

    On another hand, do the radios have a "memory" or any other "always on" features? If so that could drain a less then perfect battery over a week or so, especially if all three have a similar feature.
    1979 xs1100 Special -
    Stock air box/K&N Filter, MAC 4-2 exhaust, Bad-Boy Air horn, TC fuse box, Windshield, Soft bags, Vetter Fairing, Blinkers->Run/Turn/Brake Lights, Headlight Modulator, hard wire GPS power

    Short Stack - 1981 xs1100 Standard - lowered for SWMBO.

    Originally posted by fredintoon
    Goes like a train, corners like a cow, shifts like a Russian tractor, drinks like a fish, you are gonna love it.
    My Bike:
    [link is broken]

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    • #3
      Jesse,
      CHECK THE BATTERY FIRST!!
      You may have a bad battery, and the memory in the radios are all that it takes to kill it over a weekend. For a test, do the one psycoreefer told you to do. If you have to, make up a plug with a negative to ground, and the positive with a split so you can put the meter inline.
      But first, have someone charge the suspect battery, and TEST IT PROPERLY! That is my guess from 400 miles away...
      Ray Matteis
      KE6NHG
      XS1100 E '78 (winter project)
      XS1100 SF Bob Jones worked on it!

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      • #4
        You can disconnect the radios one at a time and find out witch one is drawing to much current in standby.I have a sneaky suspicion that it's not one of the radios but a wiring problem,a shorted alternator diode or a battery problem.
        1980 special (Phyllis)
        1196 10.5 to 1 kit,megacycle cams,shaved head,dynojet carb kit,ported intake and exhaust,mac 4 into 1 exhaust,drilled rotors,ss brake lines,pods,mikes xs green coils,iridium plugs,led lights,throttle lock,progressive shocks,oil cooler,ajustable cam gears,HD valve springs,Vmax tensioner mod

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        • #5
          A Short Lesson...

          In addition to checking for an amperage draw between the vehicle's panel/wiring and the radios (maybe the radios and the battery charger are ganged together at one connection??), I would confirm whether or not there is an amperage draw at the vehicle's starting battery. Disconnect the battery's negative cable at the battery and then with the meter set to read amps (and sometimes the positive meter lead plugged into an orifice just for reading amps...) connect the positive end of the meter to the negative battery cable and the negative end of the meter to the starting battery itself.

          Be sure that the vehicle doors are closed/no domelight is on. The meter shouldn't read anything more than milliamperes. That is about the leakage amount through the alternator diodes. (You can disconnect the positive alternator to battery cable at the alternator and then the reading should be as close to zero current as one can get.)

          If you set the meter to read OHMs then you can test the individual radios for proper on/off function. Disconnect the power to the individual radio and place the positive/negative leads of the meter on the positive/negative wires of the radio, With the meter set to read resistance/ohms:

          With the radio off the meter should read infinity ohms/infinite resistance. With the radio on the meter could read almost Zero ohms/Zero resistance.

          I'm betting that the problem is with the battery charger. That is if it is the kind of in-vehicle charger which charges a hand held VHF or something like that. They tend to use and "automatic sense/automatic charge" feature which is great. At least until it goes wonky and starts draining the car battery cause it's convinced that the lack of a battery in the holder is instead the presence of a badly discharged mobile battery in desperate need of juice.

          Have as much fun as possible but don't smile. Bosses don't like it when their employees can work and have fun at the same time.

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