Required test equipment:
a good Digital Multimeter
Procedure
- Set the meter to read AC Volts (select the lowest range if it is not autoranging)
- Note these next two steps don’t really matter which probe goes to which terminal since we’re going to be measuring the AC component of the voltage.
- Connect the common (Black) lead to the batteries negative post
- Connect the Positive (Red) lead to the batteries positive post
- Set the engine RPM to 1500rpm
- Turn on the rear window defroster and the air conditioner, seat warmers etc.
- Your meter reading should be less than 90mVac (0.09V AC)
Why does this work? An Alternator is a generator that produces three phase alternating current. That AC voltage is rectified by diodes in the alternator or voltage regulator turning the AC into a pulsating DC voltage. Unlike your house wall socket, which always has 60Hz AC, the AC produced by the alternator varies in frequency with your engines speed. By having the speed at 1500 rpm you are ensuring two things:
- The alternator is supplying the big electrical loads, not the battery and
- the ripples are very close together.
1500 Revolutions per Minute * 6 voltage peaks per revolution * 1 minute/ 60 seconds = 150 pulses per second
Similarly, you want a lot of electrical loads turned on so that you are stressing the alternator, not just sucking out of the battery.
Do the math and you can calculate what the AC ripple should be or just believe me that 90mV is correct 
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