How do I perform a "Current load test"?

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!

Reply to
tuko
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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

Reply to
Oppie

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.

Reply to
BANDIT2941

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.

Reply to
just another

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.

Reply to
BANDIT2941

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)

Reply to
Oppie

Thats neat. I've never seen one of those. Good idea though, makes sense. Good old left hand rule from physics in effect :)

Reply to
BANDIT2941

I agree and hope the OP reads your reply.

Reply to
just another

just curious, if the blower is on high already, does the A/C add much to the electrical load? Scout

"Oppie" wrote [snip]

Reply to
Scout

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.

Reply to
Oppie

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

Reply to
Bob Shuman

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