How do I test the alternator to determine if it's good?

Who replaced the alternator 3 times?

Was any testing done, to determine any of the 4
alternators had any problems & how was that testing
done?

Unless you got a couple bad rebuilt parts,would you
not conclude further testing was needed, as the problem
may be outside the alternator & in the wiring or other circuits
feeding wires to it !

You went thru 4 alternators
Did you not test all of them at the parts store?????

The battery light is telling the driver the alternator is not charging the battery. The alternator output has to be above 12 volts to make the light go off.

First off, disconnecting athe battery while the engine is running is the quickest way to damage an alternator. Unlike the old DC generators, the alternator needs the battery in circuit to protect it against voltage runaway.

If the alternator has been bench-tested and seems to be functional, I would start by verifying the related vehicle circuits. Witch a DC voltmeter, start out by measuring the voltage at the alternator's output terminal, the heavy black/orange wire. It should be full battery voltage. If it is anything less than battery voltage, you must determine why. Some things to look at are the gray 12-gauge fusible links A & B, and the various splices and connections in the circuit (see diagram below).

Next, disconnect the voltage regultor harness plug from the alternator and check the voltage at the wiring harness regulator plug's "A" terminal connector - the yellow/white wire. It too should be full battery voltage. If it is anything less than full battery voltage, you must determine why. In this case, look at the 20-amp ALT fuse in the battery junction box, and at the various splices and connectors in that circuit.

Then, turn the key switch to its "RUN" position and measure the voltage
at the wiring harness regulator plug's "I" terminal connector - the light green/red wire. It should be somewhere between 8 and 12 volts. If there is no voltage present, check Fuse 5 (15 amps) in the main fuse panel and the various connections in that circuit.

The final test is to measure the voltage between the batterty positive terminal and the alternator housing or frame. If it is anything less than full battery voltage, check the alternator mounting.

First disconnect battery, next remove serpentine belt, then unhook wiring to alternator, unbolt and replace with new/rebuilt alternator. Reverse steps and you should be good to go. You can have your old alternator bench tested (tested off vehicle) at almost any autoparts store for free to determine that alternator is bad/not charging. I test everything I drive and work on and advise others not to speculate.

The battery needs to be load tested first. once this is determined to be good or not, have the charging system checked just be sure. Electrical components can fail quicker than you can blink. Just because it tested good once does not mean it still is. Most autoparts stores will do this for free. Let me know what they say.

fit a new reconditioned alternator and not one from a junkyard

CHECK ALT.OUTPUT AT BAT. WHEN THE LIGHT IS ON..SHOULD BE 14.5V...IF NOT YOU MOST LIKELY NEED AN ALTERNATOR.. VOLTAGE REG. IS IN ALT.

Start the engine and turn on all your lights and your a/c blower on high. You should have 13 to 14.5 volts at your battery, if you have less then 13 volts. Then you have a weak alternator or faulty voltage regulator, to test the regulator, turn everything off and check the voltage again with the engine running. You should have 13 to 14.5 volts again, if anything different as to being too high or low, then you have a faulty regulator which is built in to the alternator so your still going to have to replace it. To test the battery, you should have 11.5 to 12 volts with everything off and engine not running.
Good luck and hope this help. A battery older then 4 years may no long be able to hold a charge and will need to be replaced. A bad battery will cause the alternator to fail much quicker then a good battery.
Thank you for using Fixya

Get a volt meter and check alternator voltage at battery while engine running, should have no less then 13.5 volts, if you have good voltage at battery, disconnect battery terminal neg. hook test light at cable (neg) and at battery, make sure every thing is off and door's are close if test light lights up you have a battery draw you'll need to fine short.

Your alternator is charging however it is not still good if it does what you are stating. Also what is the condition of your battery if you have to keep jumping your car? Car batteries don't like to be completely discharge and may not hold there charge again once run dead and may need replaced... Alternator has to have be bad as it needs to be load tested to best determine it's true output. Most auto parts stores do a run test and no load test. Load test will tell amperage rate and true quality of an alternator.