Onan UF Series Instruction Manual And Parts Catalog page 21

S E R V I C E DIAGNOSIS 17
POSSIBLE CAUSE REMEDY
LOW VOLTAGE OUTPUT OF GENERATOR (Cont.)
Open circuit of shunt field Make proper connections according
winding. to wiring diagram.
Short circuit of windings in Replace defective part,
the field or armature.
Manual field rheostat (if used) Adjust manual rheostat,
adjusted at minimum voltage.