Engine 318 charging issue

I am working on a JD 318 with Onan engine. The battery light is on all the time. I'm assuming this is a charging issue. Can someone give me an idea where to start looking?

First you need to check the output of the alternator.

Start by battery voltage, engine off ( around 12 to 13 V ) and engine running ( 13 to 14 V ) must be higher than engine off.

If that fails then go to the alternator wires and check the AC output.
Between the wires should be around 30 V AC