Test it where it lives . . .
NEVER, NEVER, NEVER, take the alternator (or any electrical item or computer sensor) out of your car and have it bench tested. TEST IT WHERE IT LIVES! With alternators, we find bad connections (we push the connection, the alternator works, we pull the connection, the alternator quits working), loose or glazed belts and bad diodes are the problem 85% of the time. NO BENCH TESTER can test those three items. Bench testers can test total output of the alternator and that is only one of four tests the alternator has to pass to be good.
Most shops will charge $45-90 to completely test the battery, the alternator diodes, the alternator output, the alternator regulated voltage, the alternator connections, the starter amp draw and the presence of shorts and draws. We charge $55 for this service.
Another "NEVER" that will save you $$$
NEVER, NEVER, NEVER disconnect the battery while the engine is running to see if the alternator is working. Picture this. The battery is to your left, the alternator is to your right and the voltage regulator is in the middle and all are connected. The regulator watches the battery and when it sees the available battery voltage drop, the regulator tells the alternator to charge or fill the battery up. The regulator also tells the alternator when to coast or stop. Think of the voltage regulator as a gate keeper.
When the voltage regulator loses sight of the battery, because you pulled one of the battery cables off, it thinks the battery has just gone dead and tells the alternator to "give me all you've got!". When that happens, the alternator winds up and starts pumping out the power, which starts popping the 3 diodes inside the alternator, one by one. Each diode is responsible for 1/3 of the alternators output or power so if a 90 amp alternator has two bad diodes, it only has the ability to make 30 amps of power. All of this happens in 3 seconds or less.
By the way, in almost all of the cars today, the voltage regulator is inside the alternator.
That's exactly why, when we find an alternator with popped diodes, we almost always find dirty or bad battery cables. This temporarily connection loss is what causes many alternators to bite the dust....mark
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