1986-1987 Honda TRX350 FourTrax 4X4, 1987-1989 TRX350D Foreman 4X4 Service Manual, Page 262Get this manual
BATTERY {CHARGING SYSTEM Disconnect the regulatorrectifier 3P connectorand check it for loose contact or corroded terminalIf the regulated voltage reading is out of the specificationcheck the items between connector terminals (wire harness side) following the chart below