stray voltage

Discussion in 'Trucks [ Eighteen Wheelers ]' started by bigguns, Dec 22, 2014.

  1. bigguns

    bigguns Road Train Member

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    On several of the circuits for my KW I found low voltages on the negative side of a circuit where it should read 0 volts. An example would be the stop light circuit. If I put a volt meter between the wire for the stop lights and the ground I will get a reading of perhaps .07 to 1.2 volts. This is obviously w/o the brakes applied. Also on Sat. the engine would spin and not fire. In the battery box are two fuses labeled DDEC II. I pulled out the fuses and put the volt meter inline where the fuse would be. I found .8 volts in one circuit and 1.2 volts in the other circuit. On the truck is there one place to start looking for a major short or do I continue looking in a process of elimination to find the problem? The starter and alternator have been replaced in the past. They needed to be changed. I had hoped for an internal short in one or the other but no such luck.
     
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  3. JohnP3

    JohnP3 Road Train Member

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    First off you want to clean the battery cable connections and every connection to the starter, alternator, and block, also to the cab.
    Then with the engine running at 1200 RPM and all the lights on, with a volt meter test the voltage from the cab to the negative battery connection. also from the starter positive terminal to the positive battery terminal. then check from the alternator to the right frame rail, if over .2 buy an automotive #4 battery cable and mount a ground wire from the alternator ground to the frame. Also from the right frame rail to the left rail.
     
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  4. bigguns

    bigguns Road Train Member

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    JohnP3 I have done some of the things you have suggested. I will go further using trying your thoughts testing voltage and grounds and adding grounds. Thanks
     
  5. RedForeman

    RedForeman Momentum Conservationist

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    It should be an open circuit with the brakes off (not connected to power or ground). Some DVOM's will read random numbers on voltage when connected to an open circuit. In other words, this may not mean anything. Why would you check this? Are the lights burning when they're not supposed to?

    Putting the probes in line with where the fuse would be is a way to measure amperage, not voltage. Like the lighting test, this doesn't show anything useful. You would do better with a diagram to find and measure voltage at ecm power feeds and grounds at the ecm harness.

    After doing what JohnP3 suggests, look for obvious places that a harness might be damaged and investigate. For example, anything routed near hot exhaust parts or across sharp corners/edges. A big wad of old electrical tape is a tell-tale for an earlier harness repair that may have failed or corroded. Cut off the crappy old tape and have a look.
     
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