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Good Way To Find An Electrical Short?

Discussion in 'The Garage' started by mikey_d05, May 18, 2005.

  1. mikey_d05

    mikey_d05 1 ton status GMOTM Winner

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    Well, I'm down to ironing the bugs out of my truck before I take it for a drive after it sitting for a whole winter. One of them is a nasty electrical short that kills a full battery in about three hours. I've gone through everything visually and there are no blatatly obvious shorts. I'd go through and unplug things and use process of elimination, but I don't have that many batteries lying around so the time it'd take to plug it in, discharge, pull, charge, and repeat would take a while. Is there anything I can do with a multimeter to make this process any faster?
     
  2. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    disconnect your positive battery post.. clip a testlight to that positive post and start touching hotleads with the testlight.. if theres a draw, the testlight will light.. obviously start with the main pos battery cable and work your way "in" to the cab...
     
  3. mikey_d05

    mikey_d05 1 ton status GMOTM Winner

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    (banging head against wall) Wow, knew there had to be an easy way to do this. Thank you.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    A slightly different approach, same results, is to use the amp function of your multimeter, use that inline with the (negative? do a search) battery cable, then pull your fuses out of the panel one at a time until the amp draw goes to almost nothing.
     
  5. spearchucker

    spearchucker 1/2 ton status

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    Positve or negative battery cable, doesn't make a difference. You're just trying to find the current in the circuit. :thumb:
     
  6. sled_dog

    sled_dog 1 ton status

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    Best way to find a Parasitic draw right there. Just don't turn the key to on or try to start it, BAD.

    Depending on the year there WILL be parasitic draw. There is an acceptable amount(damned if I know what it is in this case). Computer controlled vehicles often have parasitic draw after you shut them down. The computer slowly shuts down. If its anything over say 1 or 2 amps, you likely have a short.

    I don't use Test lights. They are pointless to me. They will light up and tell you nothing about what is being drawn. Much bigger fan of just learning the way to test with a multimeter and doing it.
     
  7. Skigirl

    Skigirl 1/2 ton status

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    IIRC the typical draw with no short is 20 mA or less - at least it was on my 76.
     
  8. Jagged

    Jagged 1 ton status

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    got a time domain reflectometer handy? :P
     
  9. muddybuddy

    muddybuddy 3/4 ton status

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    go get a everlast battery from walmart, 3 year warrantee. i had a bug in mine, kept goin back and gettin new batteries every 2 or 3 days. in total think i got about 7 or 8 til i finally found the problem. haha
     
  10. r_pogo

    r_pogo Registered Member

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    Fuse Block Trick

    With the meter in line with the battery cable observe the current draw with everything off. Pull each fuse from the fuse block one at a time to see which circuit has the most draw.

    Using the wiring diagram from the repair manual find the connector on each circuit and starting from the connector farthest from the fuse block unplug the connector one at a time. When the current draw stops you will then know where the short is likely to be.

    Don't forget to pull the dome light bulb out while dinking with the wiring inside with the door open.
     
  11. mikey_d05

    mikey_d05 1 ton status GMOTM Winner

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    I don't have to worry, I took 99% of the interior wiring out when I stripped the inside. :D

    I insulated every single wire and connection that I cut, so I don't think that's it, besides, the truck started exhibiting these problems before I gutted the interior.
     
  12. diesel4me

    diesel4me 1 ton status Premium Member

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    Many ways..

    I use the test light between the battery + terminal and the + cable to detect a short curcuit or a "draw"--multimeter works too ...other guys I know use a compass or a similar device you point at the wiring and the needle will flicker when its near the area with the short..I've had better luck using the test light and pulling the fuses one by one,or the wires off any suspected item..one thing to look at in particular is the heater blower motor--most GM trucks had a "continuous running blower"--even when the switch says "off",its really on "low" speed..

    Sometimes the motor fan will get jammed by a pencil or other foriegn object and it will sit there and hum,and eventually the motor will burn out--I've seen mice build nests in the heater box,and the motor was laboring to run with all the crap in the fan--Its not hard to tell,for one thing it wont "blow" any air,and pulling the plug off the heater motor will stop the current draw if thats the cause..it does not always blow the fuse,because the resistor that gives you the "speeds" will simply glow red hot until it kills the battery eventually..of couse this wont draw any current while the key is "off",but it will contribute to charging problems while the motor is running..not a common dilema,but it has happenned to me! :blush:

    The clock(if you have one)will also cause the light to stay on dimly,and make it flash for a second when the clock "winds" itself...dont mistake that for a short!..same goes for the dome light as suggested earlier too.. :crazy:
     
  13. r_pogo

    r_pogo Registered Member

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    Lighting

    Of all the circuits the lighting circuits are the most freqently "hot" when the ignition is "off." Check the operation of all the light switches to make sure they provide juice where they should and none where they shouldn't.

    Sometimes a switch will short inside to another conductor (not to ground) so the current draw is less than expected in a dead short so no big balls of fire.

    Also, just for grins check to make sure all your ground connections are tight: remove the screw and lugs, sand to the bare metal, put a star washer on, tighten up then seal with some Krylon Acrylic Spay Coating (clear). I solder all the crimp connectors to ground just to be sure. Anyway, a bad ground can make a dead short (easier to find) look like a smaller leak.

    I read on some service bulletins somewhere that the top of the 700R near the belhousing can have some sharp casting flash and a cable assembly passing over this can wear through the insulation and cause shorts.
     
  14. akbound

    akbound 1/2 ton status

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  15. Jagged

    Jagged 1 ton status

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    if you have an o-scope, you can build a TDR (time domain reflectometer) to hook up to it. You think chasing electrical problems in a truck is hard, try doing it in a military aircraft :P

    http://www.web-ee.com/Schematics/TDR/tdr.htm -- how to build a TDR
     

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