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All automotive charging systems must have some means of regulating the alternator’s voltage output. On vintage cars, an external electromechanical voltage regulator handles that task. Modern vehicles use either an electronic regulator or a computer (sometimes both).

Like any part of your car, the voltage regulator can eventually fail. When that happens, you’ll notice one or more troublesome symptoms that you’ll want to address right away.

What Does a Voltage Regulator Do?

Before jumping into how voltage regulators operate, it helps to understand how the charging system works.

As most people know, the alternator charges the battery (whenever the engine is running) while also supplying electricity to the car’s electronics. The alternator accomplishes these tasks through electromagnetic induction—a phenomenon that generates electrical current from a magnetic field.

basic voltage regulator
The voltage regulator controls the amount of current that flows through the rotor’s field coil to regulate the alternator’s output voltage.

The primary components found within the alternator are the rotor, stator, and rectifier bridge.

  • The rotor, which is the rotating portion of the alternator, consists of coils of wire (known as a field coil) behind opposing magnetic poles.
  • The stator consists of three sets of stationary coil windings and a laminated core.
  • The rectifier bridge contains diodes that act as one-way electrical check valves.

The rotor’s field coil receives electrical current through a set of slip rings and brushes (located on the rotor’s shaft). That current creates a magnetic field, which is enhanced by the rotor’s magnetic poles.

Whenever the engine is running, the vehicle’s drive belt turns the rotor via a pulley on the front of the alternator. As the rotor turns, it causes the stator to create an alternating current. The rectifier bridge converts the alternating current (AC) into direct current (DC) that the car’s electrical system can use.

But there’s one more piece to the puzzle. For the charging system to work properly, the alternator must produce enough voltage to charge the battery—but not so much voltage that the car’s electronics become damaged. That’s why the charging system has a voltage regulator. The regulator controls the amount of current that flows through the rotor’s field coil to regulate the alternator’s output voltage.

Until the mid-1970s, many cars used electromechanical voltage regulators. Modern production vehicles, however, control the alternator’s output with either an electronic regulator (usually integrated into the alternator), a computer, or both. When just a computer is used, there is no stand-alone voltage regulator. Instead, all of the regulator electronics are integrated into the engine computer.

battery of a vehicle
A bad voltage regulator can lead to the alternator undercharging or overcharging the car’s battery.

The Top 4 Signs of a Bad Voltage Regulator

Do you think you might be dealing with a bad voltage regulator? If your vehicle is exhibiting one or more of the following symptoms, you might be right.

Note: Because other problems can present the same symptoms as a faulty voltage regulator, you’ll want to perform a thorough diagnosis of the vehicle before replacing any parts.

Battery Problems

A bad voltage regulator can lead to the alternator undercharging or overcharging the car’s battery. Undercharging can result in a discharged (dead) battery, whereas overcharging can cause internal battery damage.

Illuminated Warning Lights

The charging system warning light on the dashboard will likely illuminate if the voltage regulator fails. A faulty voltage regulator can also trigger the check engine light and many other dashboard warnings due to system overcharging or undercharging.

Lighting Issues

You may notice the headlights and other lighting start to dim if the regulator is causing low charging system output. On the other hand, if the regulator is causing higher than normal output, you may notice premature bulb failure. The vehicle’s lighting may also appear brighter than normal.

Other Electrical Oddities

Your car contains an array of electronics, all of which rely on charging system voltage. As such, a bad voltage regulator can cause a myriad of electrical issues, especially on modern, computer-controlled vehicles. An overcharging condition can also cause the vehicle’s fuses to blow, resulting in inoperative circuits.

How to Test a Voltage Regulator

Because other problems can mimic a bad voltage regulator, you’ll want to do some diagnostic work before making any repairs.

So, just how do you troubleshoot a voltage regulator? Professionals will often use a digital, handheld analyzer to diagnose charging system problems. But since most DIYers don’t have access to such a tool, the best method is to use a digital multimeter (DMM) to test charging system output.

To check the output, start by connecting the black meter lead to the negative battery terminal and the red meter lead to the positive battery terminal. With the engine off, the battery should have a base reading of at least 12.2 volts. Otherwise, the charging system test will not be accurate.

Once you’ve determined that the battery has a proper charge, start the vehicle and increase engine speed to around 1,500-2000 RPM. If the alternator is charging and the regulator is working properly, generally, the reading on your meter should be around 13.5 to 15.0 volts with all of the lights and accessories turned off. Consult a repair manual or repair database for the exact specification for your vehicle.

If the charging system’s output voltage is too low, you could be dealing with a faulty regulator, defective alternator, loose drive belt, or loose or corroded battery cables. Output voltage that’s too high often points to a faulty regulator, though, you could also be dealing with another issue, such as a problem with the regulator’s control circuit.

mechanic using digital multimeter
If you’re a DIYer, you may use a digital multimeter (DMM) to test charging system output.

How Much Does It Cost to Replace a Voltage Regulator?

These days, most voltage regulators are located inside of the alternator or mounted to it. As such, most shops replace the entire alternator when servicing the regulator.

If you have a shop replace the alternator, you can expect to pay anywhere from $200 to $800 to get the job done. Of course, the exact cost will depend on various factors, such as the year, make, and model of your vehicle.

You can save money by replacing the voltage regulator yourself if you have the tools and the know-how. CarParts.com has a wide variety of replacement voltage regulators and alternators available for various makes and models.

Alternator Voltage Regulator FAQ

Where is the voltage regulator?

On most vehicles, the voltage regulator is either located inside of the alternator or mounted to it. But vintage vehicles often have an external voltage regulator, which may be mounted to the fender well, firewall, etc.

It’s also important to note that some modern vehicles do not have a voltage regulator at all. Instead, the engine computer directly controls alternator output.

What causes voltage regulator failure?

In most cases, voltage regulators simply wear out over time. There are some instances, however, where charging system problems, such as a weak battery or faulty alternator, can reduce the regulator’s life span.

Can you drive with a bad voltage regulator?

You may be able to drive with a bad voltage regulator—but not for long. If the regulator is causing low charging system output, the battery will eventually become discharged and the vehicle will no longer run.

In a scenario where the regulator is causing overcharging, the vehicle may begin to exhibit performance problems that prevent it from being driven. What’s more, higher than normal charging system output can damage the battery and other electrical components.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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