Simple Alternator Test
On a 3 cell 6 volt or a 6 cell 12 volt battery system connect ONLY one terminal of a voltage test light to the output stud of the alternator and the other test light terminal to a ground such as an engine head bolt. Disconnect all other wires from the alternator output stud.
Start the engine and accelerate the RPM up to turn on the altrernator then reduce the RPM to a modest speed. The test light should glow on a 6 volt system and full brilliance on a 12 volt system. This shows proper output voltage and regulation of the alternator. There is no connection to the battery so there is no indication on the ammeter.
Turn the engine off, remove the test light and reconnect the usual car wires to the output stud to complete the test.