Troubleshooting a Fat Boy’s charging system generally means checking the individual components for proper function.
The charging system on a Harley-Davidson Fat Boy consists of the battery, the voltage regulator, the stator/rotor and the wiring harness. Troubleshooting this system generally means checking the individual components for proper function. Fortunately, there are well established parameters to determine the faulty component. Troubleshooting a Fat Boy’s charging system can take considerable time, especially if a faulty connector is to blame. Plan to spend from one to three hours on this task.
1. Test the voltage of the battery with a voltmeter. A fully charged battery should read 13 volts.
2. Charge the battery for a few hours with a battery charger then allow it to set for one to two hours. Retest the battery with a voltmeter. If the reading is not at least 12.8 volts, charge the battery for 10 hours.
3. Replace the battery if the voltage still reads below 12.8 volts..
4. Take the battery to a local auto parts store to have a load test performed. This test is typically free and can eliminate the battery as the source of trouble. Have the load test performed if the battery passes the voltage test.
5. Verify that the regulator is clean and has a good connection to the engine crankcase. Inspect the regulator for corrosion.
6. Unplug the regulator from the engine crankcase.
7. Touch one probe of a trouble light to the engine case and the other to a pin on the regulator plug. If the light glows after touching either pin, then the regulator is bad.
8. Reattach the regulator connector. Start the engine and rev the motor to 3,000 RPM. With the motor revved, measure the voltage across the battery terminals with a voltmeter. The reading should be 13 to 15 volts. If the measurement is outside of this range, replace the regulator.
9. Turn off the engine.
10. Disconnect the regulator plug from the engine crankcase. Measure the resistance between the crankcase and either hole in the stator socket with an ohmmeter. The reading should indicate no continuity. Any reading other than this indicates a grounded and damaged stator.
11. Measure the resistance between both holes in the stator socket with the ohmmeter. The reading should register between 0.1 to 0.2 ohms. A different reading indicates a damaged stator.
12. Start the engine. Rev the motorcycle up to 2,000 RPM. Measure the voltage across the holes in the stator socket with a voltmeter. The reading should be 32 to 40 volts AC. A reading less than this indicates a damaged stator/rotor.
13. Turn off the motorcycle, including all lights and accessories.
14. Disconnect the negative terminal of the battery with a 10 mm open-ended wrench.
15. Measure the current draw on the battery with an ammeter connected between the negative battery terminal and the negative battery cable. The reading should be less than 3 milliamps.
16. Use the ammeter and an engine wiring schematic to trace the flow of current to the faulty component if the current draw on the battery exceeds 3 milliamps.