Most of my local autoparts stores have an alternator test bench (it's basically the same setup at every store, even looks the same). However, the test is somewhat deceptive. A single gauge on the bench is labled with both Volts & Amps, but there no provision for measuring the acutal amperage output (current) of the alternator. The store clerk puts the alternator on the bench, fires it up, and calls it good if the meter falls in the proper area of the gauge. However, if your diodes are blown, you'll have sufficient voltage (and maybe too much) and almost no amps. -thus the standard bench test does not troubleshoot every possible mode of failure (since it reads only volts). It will catch the most likely causes of alternator failure since low volts would be an easy sign of internal problems.
For a more complete test, my local Sears autoshop has a certified technician with an alternator test cart and it seems to be a very reliable system. They test the entire charging system of your vehicle in about 5 minutes (battery condition and alternator output, with the engine running). The test costs about $9 to $11 since you're paying for an ASE certified mechanic, but in my opinion this is worth it. Any well equipped automotive garage should have something similar.
God bless,
Fitz Fitzgerald.
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'87 Blue 240 Wagon, 246k miles.
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