Thread: Bad alternator?

Tweet

An alternator produces alternating current (AC) by the principles of electromagnetic induction. Accessories connected to your car’s charging system require a constant flow of direct current (DC) at a fairy steady voltage. You can't charge a battery with alternating current, so it must be rectified to direct current.

Ripple voltage or (AC voltage) can be measured using a digital multimeter. Make sure your meter is on an AC scale. Connect the black lead to a good ground (bolt on the engine or bare metal) and the red lead to the “battery" terminal (sometimes labeled “BAT” or simply “B” – usually a red wire) on the back of the alternator, (not at the battery). A good alternator should measure less than .5 Volts AC (that’s point five i.e., one-half a volt) with the engine running. A higher reading indicates damaged alternator diodes – typically open. (First diagram)

To check alternator diode leakage, connect the multimeter in series with the alternator output terminal when the car is not running. This is accomplished by disconnecting the battery lead wire and connecting the digital muitimeter’s red lead to the battery terminal on the alternator and the black lead to battery lead. Leakage current should be a couple of milliamps at most; more often, it will be on the order of 0.5 milliamps on a current scale of your multimeter – more that a few milliamps indicates a shorted diode and this will definitely zap your battery in a day or so. (Second diagram)

Hope this helps..
Glenn