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!

 

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

 

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.

 

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.

 

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.

 

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)

 

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

 

I agree and hope the OP reads your reply.

 

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

 

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