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Tracking a high resistance joint

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harvey4(at)earthlink.net
Guest

Posted: Mon Nov 30, 2009 10:21 am Post subject: Tracking a high resistance joint

My alternator is putting out 14+ volts but I'm only seeing low 13's 

at the panel bus, suggesting a high resistance joint somewhere on the 

way to the panel.

Any ideas on how to diagnose the bad joint would be appreciated. My 

limited knowledge of Ohm's law says that the resistance drop across 

the bad joint will be too tiny to detect with a standard meter and I 

probably won't find it that way.

Thanks for ideas.

Neil C

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Nov 30, 2009 4:41 pm Post subject: Tracking a high resistance joint

At 12:15 PM 11/30/2009, you wrote:

 	  Quote: 	
		  



My alternator is putting out 14+ volts but I'm only seeing low 13's 

at the panel bus, suggesting a high resistance joint somewhere on 

the way to the panel.

Any ideas on how to diagnose the bad joint would be appreciated. My 

limited knowledge of Ohm's law says that the resistance drop across 

the bad joint will be too tiny to detect with a standard meter and I 

probably won't find it that way.


	


    You're right, attempting to measure very small

    resistances with the commercial off the shelf

    multimeter is not in the cards.



    However, the same instrument can measure VOLTAGES

    and in particular, DIFFERENCE voltages. Hook your

    (+) meter lead to the alternator b-terminal. Fire

    up the engine. Turn lots of stuff ON and then probe

    the pathway between b-terminal and the bus with the

    (-) lead of your voltmeter.



    If it's not practical to do this with the engine

    running, then temporarily hook a charged AGM battery

    from  b-terminal to ground. Perhaps you can fabricate

    a jumper to run from your hot side of the battery

    contactor over to the b-lead.  Do this with a piece

    of 10AWG or larger and include a series fuse of 30A

    at the battery end. . . . make the battery pretend to be

    an alternator.  Leave the battery master OFF, turn

    on lots of stuff and then probe the path from alternator

    b-lead to the bus.



    In big airplanes, we do this with a constant current

    power supply set up to deliver 30 or so amps over the

    power path to be explored. I do have a whippy micro-

    ohmmeter but prefer the constant current technique

    and voltmeter.



    When you have access to the power supply, the rest

    of the airplane can be OFF while the survey is being

    conducted.

        Bob . . .



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