Diagnosing Alternator Charging System Problems

  Over the years Harley Davidson has used many different alternator systems.  They are usually designated by amperage out put such as a 22- amp or a 32-amp system. Until recently the easiest way to tell the systems apart is by looking at the plugs that connect the stator to the regulator. Since many of these systems can be interchanged sometimes people use a different system when replacing them. If you are only replacing one or two of the components it is important to look at the plug to determine which system is actually in the bike. If in doubt change the stator, rotor and regulator as a set. To figure out which system you have you can go to www.cycleelectricinc.com and go to either the Rectifying regulator page or the stator page. If you scroll down you will find pictures of the different plug types.

There are many different ways to test the alternator system.  At the factory we have custom made testing equipment to test each component separately. Without this special equipment it is difficult to test a stator, rotor or regulator on the bench. I have two simple tests to check out the stator and rotor combination on the bike. If the stator and rotor test out good the regulator can be tested. The best way to test a regulator is to hook it up to a known working stator and rotor combination. The motor needs to be running to perform these tests so you need to do them before disassembly.

Stators are very simple and their problems fall into two categories: shorts and opens.

 A short circuit occurs when one of the wires in the windings or the lead wire make an electrical connection some place they should not. Shorts can be from winding to winding or from winding to ground. An open circuit occurs if a wire breaks.

Winding to winding shorts or opens will cause low stator voltage.

Rotor problems are relatively rare on stock motors. Motors with performance modification that tend to run at a higher RPM range see more rotor problems. These problems can include a striped spline, broken magnets and even a broken rotor shell. All these problems will result in low stator voltage

Testing AC stator voltage

The object of this test is to see if the stator can produce proper AC voltage per 1000 RPM. For example a 32-amp stator should put out 18 volts AC (VAC) per each 1000-RPM. At 2000 it should be 36 VAC (18x2). At 3000 RPM it should be 54 VAC (18 X 3).

Each system has a certain voltage it should produce per 1000 RPM.

The following is a list of the most common systems.

22 amp single phase = 24 VAC @ 1000 rpm

32 amp single phase = 18 VAC @ 1000 rpm

38 amp single phase = 18 VAC @ 1000 rpm

48 amp single phase = 24 VAC @ 1000 rpm

36 amp three phase = 16 VAC @ 1000 rpm

50 amp three phase 18 VAC @ 1000 rpm

Set your meter to read AC volts. Unplug the stator from the regulator. Start the motor. On a single-phase system, connect one lead from the meter to each pin on the stator plug. Hold the motor at a steady RPM and read the tachometer and the voltmeter.

On a three-phase system there are three separate windings hooked end to end. You need to test the voltage on each set of windings. The plug has three pins. First take a voltage reading between pin one and pin two. Then pin one and pin three. And finally pin two and pin three. This will test all three windings. All three should read the same voltage at the same RPM.

A stator that is shorted to ground may still produce full AC voltage from pin to pin. The problem with this is the rectifier changes the AC current to DC current and then uses the ground as one leg of the DC. When the stator shorts to ground the AC and DC get mixed together.

Testing for shorts to ground

 The best way to test for shorts to ground is with a 12-volt test light. Sometimes an ohms meter will not pick up a short to ground. The light test is better.

Using a standard automotive test light, connect the ground clip to a good ground. Test the light by touching the probe to something positive such as the positive battery terminal. The light should light indicating you have a good ground. Now you are ready to start the test.

Unplug the regulator from the stator and start the motor. Probe each stator pin with the test light one at a time. If the bulb lights when connected between any of the stator plug pins and ground the stator is shorted and needs to be replaced.


If you get proper AC voltage and the stator is not grounded (no light) the stator and rotor are good. At this point you can assume the problem is either a bad connection between the regulator and the battery or the regulator is bad.