Battery and alternator testing
The specifications for alternator testing by MB require that the voltage output be read after the engine has run for at least two minutes and all non-required electrical consumers are turned off (ie: dome lights, etc
-The allowable voltage is 13.0 to 14.5 volts.
-If the voltage is over 14.5 volts the regulator should be replaced.
-If the voltage is under 13.0 volts, the diode's should be tested along with the voltage regulator.
The connecting point for the voltmeter is the positive and negative terminals of the battery.
Charging voltage depends on temperature. At freezing, around 1.8 volts above battery voltage is normal. At 100F, this should drop about a half a volt. While the regulator has temp compensation, it's difficult to tell what its correction is except on a cold start.