Learning, Voltage,

How to Test a Harley Voltage Regulator

It is not uncommon for a Harley-Davidson motorcycle to experience certain electrical problems, especially if it has traveled over 30,000 miles. If this happens, the voltage regulator is often the culprit.

Voltage regulators limit the voltage from a power source to prevent it from exceeding the safe level. Without them, the alternator can overheat and get damaged.

By following this guide, you can test the voltage regulator yourself.

I’ve detailed three different methods you can use. One is a load test using a light bulb, another is to obtain an actual voltage reading using a multimeter, and an alternative is an AC ripple test using an oscilloscope.

Park the bike, open the hood and remove the plastic caps over the battery’s terminals. The voltage reading should be a little more than 12 volts at idle and within the range of 13.5-14.5 volts while revving the engine. Open the voltage regulator to inspect the diode if the voltage is not regulated. Check for dirt, loose connections, or signs of damage, and replace the diode if necessary.

Requirements and Preparation

To test a Harley voltage regulator, you will need either a light bulb, a multimeter, or, optionally, also an oscilloscope.

I will show you how to test a Harley voltage regulator in three ways. In either case, you will first have to park the bike, open its hood, and maybe also remove plastic caps:

  • When parking the bike, ensure it is stable, doesn’t move forward or backward, and engages the parking brake.
  • Pull the lever on the bike that opens the hood to open the hood. Then, unhook the bar under the hood to help you prop the hood up. This will expose the engine and battery.
  • If the battery’s terminals have plastic caps over them, you will need to remove them first, to ensure good contact between the test instrument’s leads or probes and the terminals.

If the regulator leaks voltage, the battery will have the same issue. That’s why we test the voltage directly across the battery’s terminals, not the regulator.

Note that the engine has to be running when doing these tests. So, after parking the bike and getting the test instrument ready, press the ignition button or turn the key in the ignition switch to start the engine.

Testing a Harley Voltage Regulator

Three methods for testing a Harley voltage regulator:

  • Load testing using a light bulb
  • Voltage testing using a multimeter
  • AC ripple testing using an oscilloscope

Load Testing Using a Light Bulb

This is a simple test where you only need to use a DC light bulb.

The light bulb will work as a load in the circuit to give a visual indication of voltage change. Connect it to the voltage regulator’s output. The load must be within specified limits.

Rev up the bike’s engine. You might need an assistant to help you do this while you handle the testing. Press on the gas until it reaches between 1,500 and 2,000 RPM. While increasing the rev, the light should get brighter, and while reducing it, it should get dimmer.

Although the bulb won’t give you readings, it should brighten or dim a little. If it does, then the voltage regulator is probably working. If it doesn’t, then the voltage regulator might be faulty.

Voltage Testing Using a Multimeter

Another simple test requires using a multimeter.

Set the multimeter to check for DC voltage and connect its leads to the motorcycle’s battery while running. Simply attach its clamps to the battery’s terminals: The red one to the positive terminal and the black one to the negative terminal.

voltage testing across the battery
Video | J & P Cycles

The reading should be a little more than 12 volts, which is the battery’s voltage rating. However, it should not normally exceed around 13.5-14.5 volts. If it exceeds 14.5 volts, the battery may be overcharged, and the regulator may be faulty. In this situation, the regulator is not working as it should, so you should replace it.

The battery may not be charging sufficiently if the voltage is below 13.5 volts while the engine is revving. If it’s less than 12 volts, the battery may be weak, so it will need to be replaced instead.

If the multimeter doesn’t give a reading, either the battery is completely dead, its probes are not connected properly to the battery’s terminals, or the multimeter itself is not working.

In the example below, the maximum reading of 14.13 volts is below the upper limit, so the voltage regulator limits the voltage (and current).

Video | J & P Cycles

AC Ripple Testing Using an Oscilloscope

Alternatively, an oscilloscope can check the AC ripple on DC voltage.

This test also has to be done while the engine is running. Set the oscilloscope to measure the AC ripple. Then, connect it to the voltage regulator’s output.

The AC ripple should also be within limits. It likely indicates that the voltage regulator is faulty if it isn’t.

DC voltage
Graph of the rectified voltage output (1)

Fixing a Harley Voltage Regulator

If you confirm that the Harley voltage regulator is not working properly (by limiting the voltage not to exceed the threshold), then you will have to fix it.

Firstly, remove the two bolts that hold the regulator in place.

Inside the regulator, you will see transistors and a diode (if it’s an electrical voltage regulator, as opposed to an older mechanical type). The diode is the electrical component responsible for regulating the voltage. If it’s not regulating the voltage, it’s probably dirty, loose, or damaged and may need replacement.

If the diode looks good, cleaning it might make it work. Repeat the above voltage test to see if it does. If the wires in the charging system are loose, that might be why you will need to strengthen the connections.

But if there is any sign of damage, definitely replace it or have it repaired by a motorcycle electrician. Repairing a faulty one is usually cheaper than buying a new one.

The Harley Voltage Regulator

Harley voltage regulator
A Harley-Davidson voltage regulator

What It Does

A voltage regulator regulates the voltage, as the name suggests.

It controls the voltage and current level so that it is neither too much that it would damage electrical components nor too little to make them not work. In the case of excess current, it is eliminated through the chassis.

Additionally, it contains a rectifier to rectify the current (AC to DC conversion). This ensures the alternator’s output produces a continuous DC voltage.

The voltage regulators in Harley-Davidson motorcycles are usually external and have simple designs, making them easy to service. Also, they can better tolerate the engine’s high temperatures and vibrations.

However, they are not immune from failure, despite generally being of high quality and durable. If the motorcycle is dirty, you rode it along a rough path, or it has traveled over 30,000 miles (48,280 km), it would not be a surprise.

What Happens When It Fails

Common symptoms of a failing voltage regulator are dimming or pulsating lights, a dead battery, or a frequently stalling engine:

  • Flickering lights – Probably a faulty voltage regulator.
  • Burned-out headlights – Probably due to high voltage across the battery.
  • Overcharged battery – Release of hydrogen and oxygen gases.
  • The bike suddenly turns off – Probably a faulty voltage regulator.
  • A whining noise or sparks from the regulator – Likely that the voltage regulator is faulty.

These problems are typically caused by electrical surges or spikes or insufficient power. Any of these symptoms could indicate that you have a bad voltage regulator. It might not be letting enough power through or allowing too much to go through.

To be sure, you will have to test it using a multimeter. It’s important to test it, as otherwise, a bad voltage regulator can damage the electrical components of your Harley-Davidson motorcycle.


(1) Tracy Martin. Motorcycle electrical systems, p. 67. Motorbooks International. 2007.

Video Reference

J & P Cycles

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About Sam Orlovsky

AvatarCertifications: B.E.E.
Education: University Of Denver - Electric Engineering
Lives In: Denver Colorado

Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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