I went to get my alternator tested, as I always do before I put in a stereo. Well The voltage is a nice 13.9 but the amperage with a/c on at 2000rpm is only 21.9. Is this common on 6's or is my alternator just bad.
All you can say from this test is that the alternator is able to supply at least 21A. What this test is telling you is that the car's electrical system is drawing 21A from the alternator and that the alternator is able to supply it without a problem. If the car's electrical system is working (e.g. the A/C compressor engages, all the lights work etc etc...)
The alternator (on the V6) is rated at about 90A (I think the 4-cyl's alt is70-80A.) To properly test it, you'd need to load the charging system down, with an external load like a carbon pile, and see what the alternator voltage does as the load is adjusted till 90A draw is reached. If the system can supply 90A and the voltage doesn't drop below, say, 12.0V, then the alt is good. If the system can only put 50A into the load before the alternator voltage drops below 12, then the alt may be bad. At the very least, the system should be checked further.
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