How do I perform a "Current load test"?

Discussion in 'General Motoring' started by tuko, Oct 19, 2004.

  1. tuko

    tuko Guest

    I want to perform a current load test so that I can test my alternator. It
    seems to put out enough voltage, but I was cautioned that the alternator
    may still not be putting out the necessary amps. Can I perform this test
    with my trusty hand held Craftsman multimeter? It was about 30 bucks, and
    no where near as fancy as the equipment that they roll out to test my
    alternator at Kragen or Autozone. Thank you for any help!
     
    tuko, Oct 19, 2004
    #1
  2. tuko

    Oppie Guest

    I wouldn't expect an inexpensive multimeter to make good current readings.
    Better to use it for measuring voltage. Measure voltage directly at the
    battery.
    Engine off - should read about 12V.
    Cranking - shouldn't go below 8V
    Idling - about 13.8V
    Now turn on the blower on high, headlamps on high, A/C and rear window
    defogger. This should represent the full load on the alternator. Should be
    able to keep 13.8V at idle though this is a pretty big load and may cause
    the voltage to sag a bit. You may have to rev the engine a bit and see if
    it will come back up to 13.8V.

    Hope this helps
    Oppie
     
    Oppie, Oct 19, 2004
    #2
  3. tuko

    BANDIT2941 Guest

    I wouldn't expect an inexpensive multimeter to make good current readings.
    Tuko,

    Just to clarify, what Oppie posted is not an actual current test like Autozone
    would do. However, it is pretty similar. If the alternator can maintain voltage
    with the lights on and blower on high, it should be fine.

    That's all I'd do to test my alternator.
     
    BANDIT2941, Oct 20, 2004
    #3
  4. tuko

    just another Guest

    There's nothing wrong with the above info, but the "load test" is specifically to see if it's
    the alternator causing the problem (because if the voltages aren't right, there are several
    possible reasons why). Also, it's possible to have the voltages be correct and yet still have
    the alternator not pumping sufficient current. IIRC either the Chilton's or Haynes service
    manuals covers the hookup for a load test.

    To answer the OP, your multimeter would be capable of reading the current during the load test,
    but you'll still need a simulated load. The idea is, during the test, the alternator's output
    goes only to the simulated load, and you then measure the current from the alternator.

    HTH.
     
    just another, Oct 20, 2004
    #4
  5. tuko

    BANDIT2941 Guest

    There's nothing wrong with the above info, but the "load test" is
    Right - the purpose for a load test is to see if the alternator has sufficient
    current. Just because it has 14 volts doesn't necessarily mean it has enough
    current. However, if the alternator can maintain that voltage when under a
    heavier load(like with lights and fan on) it should be fine.
    Thats the idea. The problem is I believe the stock alternator produces 75 amps
    at idle. An ammeter has to be wired in series in order to get an amperage
    reading(unlike a voltmeter which is parallel). Do you really think he should
    try wiring his $30 multimeter(or any regular multimeter) with its 12 gauge
    leads to his alternator? I sure don't.
     
    BANDIT2941, Oct 20, 2004
    #5
  6. tuko

    Oppie Guest

    There was also the issue of the amount of ripple on the DC. I once toyed
    with a multimeter set to the AC range to see how it would read. This is
    supposed to tell if there is a problem with one or more of the diodes in the
    alternator. I believe that the nominal AC component on top of the DC is less
    than 1V rms. This is a vague measurement as only the expensive meters will
    read in true rms. (trms is the equivalent heating power regardless of the
    waveform. most inexpensive meters are calibrated only for sinewave (60Hz)
    inputs)

    I refrained from recommending any current measurements as the leads alone
    can get expensive and few meters can make these tests without even more
    expensive external shunts. I remember way back when... you used to be able
    to buy a simple analog meter that was nothing more than a bar magnet
    attached to the meter needle. There was a guide on the bottom of the meter
    that aligned the meter over a current carrying wire. Since every current
    carrying wire has a magnetic field, the meter's magnet was repelled by the
    wire's field and moved the needle. Neat idea but I haven't seen one since we
    had generators (not alternators) in cars.

    Oppie
    (aka - Bob Oppenheimer)
     
    Oppie, Oct 20, 2004
    #6
  7. tuko

    BANDIT2941 Guest

    I refrained from recommending any current measurements as the leads alone
    Thats neat. I've never seen one of those. Good idea though, makes sense. Good
    old left hand rule from physics in effect :)
     
    BANDIT2941, Oct 21, 2004
    #7
  8. tuko

    just another Guest

    I agree and hope the OP reads your reply.
     
    just another, Oct 21, 2004
    #8
  9. tuko

    Scout Guest

    just curious, if the blower is on high already, does the A/C add much to the
    electrical load?
    Scout
     
    Scout, Oct 21, 2004
    #9
  10. tuko

    Oppie Guest

    The compressor clutch draws about 5 amps (iirc). One thing to point out here
    that I previously forgot is that when the a/c is on, it usually bumps up the
    idle rpm by 100 rpm or so. This will allow the alternator put out a somewhat
    higher current.
     
    Oppie, Oct 21, 2004
    #10
  11. tuko

    Bob Shuman Guest

    There is also a separate A/C cooling fan as well that draws additional
    current too. Some models also offer a recirculation mode which can add to
    load too. Use of other nice high current options, like rear window
    defogger, heated seats, etc. can provide additional current draw.

    Bob
     
    Bob Shuman, Oct 21, 2004
    #11
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