How to Troubleshoot an Alternator on a Harley

That little black box in front of your engine can cause you some big problems.

A Harley-Davidson motorcycle will give you hours of riding pleasure for years on end. But when the battery goes dead and it won't start, it can have you cursing very quickly. Replacing the battery won't help at all if you have problems with your charging system, so a few tests are needed in order to determine what is causing the problem. Although these tests may be beyond the ability of some do-it-yourself mechanics, if you have moderate skills and own or can borrow the proper tools, you can save yourself some money and may even be able to fix the problem on your own.

Difficulty:

Moderately Challenging

Instructions

Things You'll Need

• Digital multimeter
• Alligator clip test leads

1. • 1

     Remove the motorcycle seat from the bike's frame. Open the battery compartment or remove the outer enclosure.

   • 2

     Set your multimeter's function to DC volts in the 25-volt range if it is not an auto-ranging meter. Connect the red test lead to the positive (red) terminal of the battery. Connect the black test lead to the ground (black) battery terminal. Observe the reading. A fully charged battery should read between 12.5 and 13.2 volts. Charge the battery, if necessary, to bring the reading up to specifications.

   • 3

     Start the bike and smoothly rev the engine up to around 2,500 RPM. If your Harley doesn't have a tachometer, you'll have to approximate this, being careful not to over-rev the engine. With the engine at steady RPM, observe the multimeter reading. It should be between 13.5 and 14.5 volts if the charging system is okay. If it still shows only resting battery voltage or less, you will need to test the alternator output.

   • 4

     Locate the alternator output wires on your Harley's engine. These wires generally extend out of the front of the engine case on the left side, behind the frame tubes. Trace the wires until you locate the connector that links them to the voltage regulator, which is bolted to the frame tubes behind the front wheel. Unplug the connector.

   • 5

     Set your multimeter on the "Resistance" or "Ohms" function. Probe each of the pins in the alternator side of the connector separately. You should again see a very low (less than 1 ohm) resistance reading. If you get a higher resistance on any pin (several ohms), you may have a bad winding. If the reading is extremely high or shows an open circuit (infinity) reading, that winding is definitely bad. If you get good readings on the stator windings, proceed to the next step.

   • 6

     Connect the probes across the pins and change the meter's function to AC Volts. Start the engine and note the reading. You should see between 15 to 25 volts at idle speed. Open the throttle a little to increase the RPM. The readings should rise steadily as the RPM increases. If it doesn't, then you likely have a short in the stator windings. If the alternator passes all the tests, the problem is most likely in the voltage regulator.

Tips & Warnings

• A digital multimeter is required because of its ease of use, and the readings are much easier for the average person to interpret. An analog multimeter can be used if you are well-versed in its operation.

• A small pocket screwdriver can be helpful in separating some types of electrical connectors. Use caution to avoid breaking off the latches that hold them snugly together.

• Modern voltage regulators are solid-state designs, with no serviceable components inside. If your tests indicate a bad regulator, then the entire unit must be replaced. This is a simple bolt-on installation and is well within the abilities of the average home mechanic.

• The battery must be fully charged before any testing is performed. Be sure to charge any battery showing between 11.5 and 12 resting volts or less before proceeding with the tests. Use only a charger designed specifically for motorcycle battery use. Using a high-amperage charger can destroy your battery.

• Unlike the voltage regulator, the alternator on your Harley requires the use of specialty tools in order to service it. It is therefore highly recommended that you let an authorized Harley-Davidson dealer or a well-equipped independent motorcycle repair shop perform any alternator work that may be indicated by the tests.