Check the manual for the process to FLASH the generator after you have the batteries hooked up correctly and after you have grounded the unit. Check to make sure grounding straps are secure throughout the unit. Typically the control box, distribution box all have frame grounds on them. If you still get NO voltage at all as measured with multi-meter (never depend on the on-board meters), then you most likely have a voltage regulator issue or a bridge rectifier issue. A diode could be bad on the generator head itself, but not as likely.
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