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CAR HELP SITES                    QUICK VOLTMETER LESSON     Testing alternator output is also very easy using a voltmeter.  With the engine running, meter set at 20 volts; put the black lead to battery ground post and the red lead to the positive post.  A good reading is 14.2 volts, give or take half a volt. This should also be done again with the blower motor and the headlights on and then the reading should be no lower than 13.5.  See the picture below.  If the voltage is below 13 volts and the alternator belt is tight the fault will very likely be the alternator itself or the voltage regulator if it is external.    If you are going to do any electrical work a digital voltmeter and a 12 volt test light are essential tools.