the same way you test any alternator. use a volt meter and put one lead on the positive and one on the negative of the battery and see if the volts are more than 13volts. if it is above 13 it is charging
Start the car, unplug the battery the alternater should power the car. If it dies the alternator is bad. You can also get it checked at any parts store.
A bad alternator will not give proper power to the vehicle. Dim lights, buzz in the stereo or a dead battery could all be causes of a bad alternator.
The bearings might be bad in the alternator.
You probably have a bad alternator or bad wiring.
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How to Tell If Your Car Alternator Is Bad Check related link below…
It is probably a bad voltage regulator. Sometimes they are located in the alternator and can go bad in a new one. On Ford vehicle the voltage regulator is separate from the alternator.
regulator is bad inside alternator
Bad bearings in the alternator. Best to just go ahead and replace it.
Bad ground or alternator stating to go bad.
the battery will not charge properly
Any auto parts store can tell you.