The diodes are the electronic part of the alternator that convert AC voltage to DC. Switch your multimeter to the lowest AC setting and reattach the test leads to the battery.

This video covers the essential steps to properly test a car's alternator and diagnose common alternator issues. The guide ...

When they were done with our alternator it had been disassembled, cleaned, equipped with new bearings, brushes, diodes and an upgraded stator to increase the output to 90 amps.