Measure the battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If it is low, start by checking the fuses, including a really large one that is likely to be bolted in the under-hood fuse box. If the voltage is okay, have the new generator tested for full-load output current. If it has one defective diode of the six, it will only be able to develop exactly one third of its rated current. AC generators put out three-phase current. With a bad diode, one phase will be missing, and the voltage regulator may detect that and turn on the "Battery" light in response. You do not reset the "Battery" light. It will turn off when the charging system is working.
Saturday, November 5th, 2016 AT 12:15 AM