But some manufacturers recommend replacing the alternator if 0.50 AC volts is detected. Usually, the presence of 0.25 AC volts means a leaking diode that requires replacing the alternator.This time, connect your meter's red lead to the B+ terminal on the back of the alternator, and the meter's black lead to battery negative (-).If voltage seems to fluctuate during your test, switch your voltmeter to the AC voltage scale and take another output voltage reading with the engine still running.This usually indicates a bad voltage regulator. If the output voltage reading is 16 volts or higher, there's an overcharging problem.
Conduct a voltage drop test as described in the following subsection 1.
Set your multimeter to DC Voltage and select the 20 Volts in the scale.Set the parking brake and shift the transmission to Neutral (manual) or Park (automatic).You need a digital multimeter for this test. This test is a simple procedure to check alternator voltage regulator output.