Working on a John Deere 318 with a Onan engine. Does not charge properly. Battery light comes on if you start the mower deck or turn on any lights. Tried a new voltage regulator, didn't help. I checked to coil with a continuity tester and it check almost no resistance, so it's not open. Battery tests about 13 volts with the engine running and about 12.5 with it shut off. Any ideas?