I have an Onan gen set that is producing 120v but when I put a load on it isn't producing current. It is a 3ph in a "Y" configuration 120v/240v
When running with the breaker is off it shows 120v on all 3 legs to ground. When I turn the breaker on it drops to about 60v the more lights and appliances it fades to nothing. It was a similar story when wired for hi voltage "Y" and low voltage delta.
It was sitting in storage for at least 15 years and was not taken out of service because of any problem it just exceeded its life expectancy.
Any suggestions would be great...

Hi,
I would begin by "flashing" the coils. That is the application of a voltage on the field coils to provide an initial magnetic flux. This will allow the field to build. I have done it with a cord with two male ends and connection into a circuit protected by a breaker. Once the armature causes voltage production, it will trip the breaker. Companies make a battery-powered unit, but I would use one only if available for use at a nominal cost.
Peace,
Clarke

I have flashed them by plugging a drill in , holding trigger down and turn the drill backwards by hand,
Make sure the unit is running when this is done.
Luck