What Can Cause a Regulator Rectifier to Go Bad?

by Dani Clark
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It is a fact of life: parts will eventually go bad. One motorcycle part, the regulator rectifier, generally gives some additional signals that it is not operating correctly. Also, chances are that you have had some start-up or voltage issues. Confirming that it is indeed the regulator rectifier that requires observing and checking certain symptoms.

The Regulator Rectifier

Modern motorcycles have electrical charging circuitry for the battery, of which the regulator rectifier is a standard part. The name of the part is actually indicative of its function as it rectifies and regulates voltage. AC voltage is generated in the alternator's stator coil. For the most part, motorcycles are on a three-phase system for efficiency purposes, where three wires connect the stator and regulator rectifier. Even so, there are some single-phase systems available as these are cheaper to manufacture, but the system uses two wires instead of three. The regulator rectifier first converts the AC power into a surge of DC power, then normalizes the DC power, ensuring that the power does not exceed approximately 14.5 volts. The DC voltage is then routed to the battery.

General Causes for Failure

There are different reasons why the regulator rectifier fails. One of the top causes is heat. Some motorcycles are known to have the part located near the radiator or other locations that are either close to heat producers or restrict air flow. Depending on the location of the regulator rectifier, the part can easily overheat. Other causes for a dead regulator rectifier center on the battery. Ground connections are important for good voltage, and if there is faulty voltage, the regulator rectifier can run hot. Bad grounding, corroded battery connection and poor or loose battery connections will cause faulty voltage.

Common Types of Failure

There are generally two ways for the regulator rectifier to fail. The first deals with diode burnout, resulting in battery drain. It is easy to assume that the issue is a bad battery because of symptoms such as dimming headlights, irregular meter readings and poor starts. Checking the voltage with a voltmeter versus relying on symptoms alone will prove highly beneficial. If the voltage drops below approximately 13 volts, the motorcycle will begin to drain the battery and eventually the engine will stop. Check for bad connections and corrosion; these also can cause voltage issues.

The other failure type is shunt regulator burnout. Should the regulator rectifier fail to control voltage, the result will be battery overcharge. Again, a voltmeter will be useful for diagnosis. Generally, readings above 17 volts mean the regulator rectifier is not converting excess power. All of this excess voltage may cause the headlights to become extremely bright and then blow.


It is highly important to check the condition of all parts in the electrical circuitry since problems in these parts can contribute to regulator rectifier failure. Moreover, even after the part has failed, check its internal connections. The problem may not be from overheating, but from a manufacturing defect.

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