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can i use multimeter to find car short?

Discussion in 'Electronic Repair' started by beerismygas, Jun 27, 2007.

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  1. beerismygas

    beerismygas Guest

    i want to see if a short is causing a drain on my car battery.

    i have a digital multimeter. one way i theorized was to connect it in
    series with the battery via the earth lead and negative terminal.

    is this logic sound? or will i fry my multimeter? is there any other
    way to find a short (apart from scraping the earth lead to the neg
    terminal to see if it arcs - which on my car it doesnt)

  2. mike

    mike Guest

    Determining if you have and finding are two different tasks.
    One simple thing you can do is put a light bulb in the path.
    If it glows, you've got leakage. If it doesn't glow, put the multimeter
    on the 10-amp scale across the light bulb and see what you measure.

    Modern cars have all sorts of electronics that runs in the background.
    It's not at all clear what kind of transient current happens when you
    power it up. The lightbulb procedure protects your meter against
    these unforseen transients.

    To find the "short" the easiest thing is to borrow a DC current probe
    and trace the wire that has the current.
  3. Ken Weitzel

    Ken Weitzel Guest


    It surely won't harm your multimeter (used properly, of course),
    but on the other hand won't tell you much of interest either.

    Today's cars use an awful lot of stuff that draws power continuously
    that you will find some current drawn even though all is fine.

    Physically check the obvious stuff for starters. Trunk light,
    hood light, glove box light, for instance. Going off when closed?
    Young folks music stuff... power amp switching off?

    Take care.

  4. Jim Yanik

    Jim Yanik Guest

    a short would blow a fuse.

    Maybe your car battery is going bad.
    They only last about 4 years,less if you let them deep discharge.
    In modern cars,it's normal for some current to be drawn even with the car
    not running,accessories and lights off.

    it would HELP a lot if you said what make,model of auto,mods,options,the
    more info you give the better.

    You can pull fuses one by one to isolate an excess current draw to one
  5. A short will blow a fuse or burn a wire.

    Intentionally trying to make a spark at your battery terminal is
    dangerous. Car batteries produce explosive hydrogen gas when charging,
    and a spark can cause the battery to explode - spraying you with acid
    and debris, possibly blinding you or severely burning your skin.
    Car battery explosions are rare, but not impossible. They DO happen and
    the result can be devastating.

    You can track down a current drain by connecting your meter between the
    positive battery lead and the battery (with the car OFF) and removing
    fuses one at a time.
    If you try to start the car or use any high amperage circuits while
    doing this test - like the wipers, blower motor or rear defroster - you
    will likely blow your meter's fuse or damage or destroy your meter.
  6. beerismygas

    beerismygas Guest

    its a, (cough) 1985 ford escort. no electronics.
    i will try this on the 10A meter setting to begin with and lower it
    till i get to the milliA.
  7. Charles

    Charles Guest

    Don't forget that the battery itself or the charging system could be at
  8. beerismygas

    beerismygas Guest

    i measured it. not even an microamp of leakage

    and as a control I turned on the courtesy light and measured again.
    360 milliA of current

    i also discovered the pulley belt was caked in oil and needs
    attention, this would explain lack of charging

    thx for your input
  9. Charles

    Charles Guest

    Ah yes, the belt ... happened to me once ... hope that your problem is over.
  10. Arfa Daily

    Arfa Daily Guest

    Just a couple of thoughts.

    Have you rechecked the radio now that you've had the battery disconnected ?
    It will have lost its code if one had been put in, and if you don't know it

    Also, depending on how long the battery was disconnected for, the EMU may
    need to 'relearn' the engine, and there is a procedure of engine starts,
    stops, and driving and idling sesions to make it do this.

  11. Bob  AZ

    Bob AZ Guest

    The usual way is to connect the meter between the positive battery
    terminal and the positive lead after it is removed from the battery.

    Bob AZ
  12. Bob F

    Bob F Guest

    You might want to lok into the source of the oil. Leaky power steering
    pump or hose?

  13. beerismygas

    beerismygas Guest

    i did that, but with the negativ lead as its easier to unscrew. the
    result should be the same.
  14. beerismygas

    beerismygas Guest

    the source of the oil? its like a black hole down there. engine needs
    re gasketing from top to bottom me thinks. dont have power steering

  15. That's unusual. Most cars have devices which are powered all the time.
    (radio station memory, etc)
    Something like 20mA quiescent is normal.
    That's 4 watts or so. Not much courtesy there. ;-) I'd expect a lot more.
  16. Ian Jackson

    Ian Jackson Guest

    Surely that's about right? The rear lights are 6W, and the stop lights
    21W. I wouldn't expect a courtesy light to be very much.
  17. sirmopalot

    sirmopalot Guest

    You could also test your ground to see if it is causing poor
    recharging.. has happened to me before. Set multimeter to check for
    resistance.. put on lead on negative battery terminal and second lead
    on a ground.. preferrably the one your alternator grounds to.

  18. My car has four 21 watt ones on a timer/dimmer. The idea surely is to
    light the inside of the car when it's dark. Perhaps to read something.
  19. Jim Yanik

    Jim Yanik Guest

    next Q is;WHY is the belt caked in oil??
  20. beerismygas

    beerismygas Guest

    i checked the bulb and compared it to the user manual specs, its a
    5w. so the meter read correctly.

    maybe one of these days i will get slapped on the face when i open the
    door for a lady friend to get in - "Pervert! i am not a 5W kind of
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