Overcharging Diagnosis for Field-Excited Systems

Overview: Charging systems that use a 12VDC signal wire connected to the regulator to control output voltage are sensitive to voltage drops in this circuit; this is especially true on older field-excited systems where the regulator is responsible for controlling the strength of the field coil (electromagnet). If the signal is interrupted (open or has increased resistance between the signal wire and battery) the regulator will sense low voltage and increase the charging voltage to the battery. This overcharging will continue until the the 12VDC signal is corrected and may permanently damage the bike’s battery. If you have a field excited system that is overcharging, before assuming it’s the regulator run this test:

This diagram is a simplified representation of a typical Field-Excited charging circuit. 



In this schematic, the black wire delivers the switched 12 volt signal to the regulator.

When the switch is turned on, this connection is established; when it is turned off the connection is broken. The battery POSITIVE post has been labeled “A” and the SIGNAL WIRE off the regulator has been labeled “B”. If you have a field-excited system that is overcharging, especially if the overcharge is slight but not excessive such as 15-16 VDC at the battery, try disconnecting this wire from where it plugs into the main harness and running a jumper wire between A and B on this diagram. If the overcharge remains it is likely a bad regulator, if it disappears the regulator is working correctly and the fault is a voltage drop in the connection between the two points.

Important Notes:

  • Before running this test refer to your bike’s wiring diagram to identify the appropriate wires. The schematic shown above is only an example (shown are the connections for a 1975 Honda CB750K). The wire colors and connections will vary depending on year, model, and manufacturer. If the wrong wires are connected the test will fail and components may be damaged.
  • This connection is only to be used for purposes of testing. DO NOT leave the jumper connection in place after the test is performed!! Doing so will drain the battery down quickly and will result in the battery needing to be replaced.
  • While the jumper is connected to the battery positive post it MUST be treated as a live wire. Allowing the jumper wire to touch any ground connection will cause an immediate short circuit.