Bad Voltage Regulator – Symptoms, Causes, & Testing

When people talk about charging systems or power output in a vehicle, they often give all the credit to the alternator and car battery. A voltage regulator is a crucial charging system component located inside or outside the alternator.

Your vehicle needs enough power to run the engine and other system components. Without enough power from the battery, the car won’t start; without enough power from the alternator, the battery won’t be recharged, and the car won’t start the next time. However, if there’s no voltage regulator, you may fry your car’s electrical and electronic components.

In this article, I’ll explain the functions of a voltage regulator, voltage regulator failure causes, and the signs of a bad voltage regulator. But first, let’s see what a voltage regulator does in a car.

can a bad voltage regulator cause no start

What does a voltage regulator do?

As the name implies, a car voltage regulator regulates the voltage produced by your car alternator. Since the alternator is connected directly to the engine via a serpentine belt, it constantly generates power to charge the car battery and electricity for other system components.

But the battery can only carry between 13.5 and 14.7 volts. So, if the car alternator keeps sending electric voltage to the battery as long as the engine is running, it will damage the battery.

The voltage regulator monitors your battery’s real-time current and voltage to determine how much voltage to send to it. It sends more power when the battery is low, and when it gets fully charged, it diverts the voltage to the ground, preventing the battery from overcharging and eventually blowing up.

Also, the voltage regulator helps to reduce the voltage generated by the alternator to power your car’s electrical systems. Without the voltage regulator, the output voltage will be too great and overload the electrical system components.

To prevent that, the voltage regulator functions much like a linear regulator. It ensures that the alternator only sends out a steady output voltage of 13.5 to 14.7 volts that won’t cause electrical damage. So if you were asking, what does a voltage regulator do on an alternator? You now know the answer.

What are the symptoms of a bad voltage regulator?

The most common symptoms of a bad voltage regulator on a golf cart and cars are a dead battery, dimming or flickering lights, and battery warning lights. Other possible signs are check engine light, erratic engine performance, and malfunctioning instrument cluster.

Let’s have an in-depth look at these symptoms.

Dead battery

Can a bad voltage regulator drain battery? An alternator voltage regulator can die in two ways. It can cause too much voltage or too little voltage to the battery. If the voltage regulator is not diverting power to the ground when it is supposed to, it will lead to battery overcharging. And if it is not sending enough voltage, the battery will constantly run down.

This is one of the most complicated issues to diagnose because you may start with the battery, alternator, and wiring harness before navigating to the alternator voltage regulator.

Battery lights or check engine light

As I explained earlier, a lousy voltage can be overcharging or undercharging your car battery. Either way, the battery warning light will appear on the dashboard to inform you that something is wrong with the car charging system. Also, some car sensors can detect these issues and trigger the check engine light on the dash.

I recommend you locate a safe spot and park your vehicle once the battery warning light pops on the dashboard. In this case, the light means two things. Either the car will die soon, or the battery will overcharge and explode. The latter is worse. But you don’t want any of these to happen.

Car dies while driving.

The work of the alternator is to recharge the battery as you are driving. However, the electric charge must pass through the voltage regulator. If the voltage regulator is not sending the required voltage needed to keep the battery charged, the car will die while driving down the road.

Your car’s engine and electrical or electronic components need power to stay working as they should. And if they are not receiving the required voltage, you’ll soon get stranded in the middle of nowhere. This is one of the most common symptoms of a bad voltage regulator or alternator.

Pulsing/Dimming lights

If you are driving with a bad voltage regulator, your lights may be pulsing, getting dimmer and brighter. To be precise, the headlights may start fluctuating between bright and dim, high beams stop working as they should, and the interior lights, like the map and dome, start flickering.

This usually occurs when the voltage regulator malfunctions and cannot maintain a consistent voltage level. In most cases, a failing voltage regulator will only dim the lights instead of pulsing them.

Erratic engine performance

One of the uncommon symptoms of a bad voltage regulator generator is erratic engine performance. What does erratic engine performance mean, anyway? When an engine has erratic performance, it has inconsistent or unpredictable behavior. This happens when the voltage regulator is unable to control the output voltage generated by the alternator.

Erratic or no lights on the instrument cluster

The instrument cluster requires a level of voltage to power the lights and messages displayed as you drive. A lousy voltage regulator may cause erratic or no lights to display information as you hit the road. The car may not start, and even if it can, it’ll be unsafe to drive with a car you don’t know what speed you are cruising at, fuel level, and other important information.

So, can a bad voltage regulator cause no start? The simple answer is yes. A defective voltage regulator can cause a dead battery, leading to a no-start situation until you replace or jumpstart the battery.

High voltage in the battery

While a voltage regulator can cause a dead battery, it can also cause too high voltage in the battery. It can send too much power to the battery, and if it’s doing that, you will run into a bigger problem sooner than later because the battery can explode due to too high voltage.

Under perfect conditions, a fully charged battery should always be at most 12.6 volts. While driving, the voltage will read between 13.7 to 14.7. Any voltage above this range will cause damage. High voltage in the battery will cause it to get warm, and if ignored for an extended period, it will damage or explode.

voltage regulator failure causes

What causes the voltage regulator to go bad?

In most cases, a bad voltage regulator is a pointer to a major electrical problem. The most common reasons for a voltage regulator failure are damaged wires, loose connections, corroded battery terminals, excessive voltage input, overheating of an electrical component, and a regulator that has reached the end of its life.

The regulator has covered its shelf life.

Sometimes, a voltage regulator malfunctions because it has covered its shelf life, not because there’s a problem somewhere in the vehicle. A mechanical failure, age, and normal wear and tear can cause the regulator to fail.

Corroded battery terminal

Faulty battery connections, corroded terminals, and improper electric flow can damage the voltage regulator. You will experience slow battery charging, and the regulator will struggle to send consistent power to the battery and other electrical components in the car.

Excessive power input

If the alternator has excessive power output, the regulator will heat up. Excess heat will cause the regulator to fail earlier if the alternator is not fixed on time. Of course, this will lead to unregulated power output.


The regulator is responsible for dissipating heat between the input and output current. The regulator will work harder to maintain consistency if the heat is too much due to issues with any electrical system component. The extra work placed on the regulator and the too much heat will cause it to fail sooner.

Damaged ground wire

A damaged or improperly grounded wire can cause the voltage regulator to fail. Properly grounded wires are necessary for the optimal functioning of the regulator, just like other electrical system components.

How do you test a voltage regulator?

Test the voltage regulator if you have electrical problems in your car. The testing procedure is simple. It only requires a multimeter. The test can determine whether you have a lousy battery, defective alternator, or bad voltage regulator.

Step 1: Switch the multimeter to voltage

The first step is to switch the multimeter dial to voltage. This is because you will be testing the voltage in the vehicle. Set the meter to 20V. Leaving the dial to AMP or Ohms can damage the multimeter. 

Step 2: Connect the multimeter

With the car off, connect the black multimeter probe to the negative battery terminal and the red probe to the positive one. The multimeter should read 12.6 or slightly above 12 volts, showing the battery is good.

Step 3: Start the Car

Start the vehicle and engage the hand brake. Leave the gear in the park or neutral. Check the readings at the multimeter. The voltage should increase to 13.8 volts while the car is idling. 13.8 volts shows the alternator is charging well.

If the voltage is below 12 volts before starting the vehicle, the battery is dead and needs to be recharged or replaced. If the voltage is below 13 volts while the car is idling, you have issues with the electrical system or a lousy alternator that needs to be fixed or replaced.

Step 4: Throttle the engine and check the reading

Have an assistant rev the engine until it reaches 1,500 to 2,000 RPM. Recheck the reading as the engine revs between 1,500 and 2,000 RPM. This time, the multimeter should read 14.7 volts. If the reading is above 14.7 volts, you have a dead voltage regulator. Also, the voltage regulator is faulty if you notice intermittent low and high voltage.

What to do if the Voltage Regulator is bad

If you notice the above signs and the test proves the voltage regulator is bad, contact a certified mechanic to replace it. Driving with a bad voltage regulator is unsafe because it can cause your light bulbs and other electrical components to blow off.

Frequently Asked Questions (FAQs)

Does a voltage regulator affect starting?

While you can drive with a lousy voltage regulator, don’t drive with it for so long. If the voltage regulator outputs low voltage, the battery will constantly die. If the regulator is outputting too high a voltage, it’ll affect the battery and may cause it to explode. In either case, a bad voltage regulator can cause a no-start issue.

Will an engine run without a voltage regulator?

Your car alternator can generate up to 250 volts without the voltage regulator. This is high enough to blow up your car’s battery, light bulbs, and other electrical system components. However, you can drive without a voltage regulator, but not for an extended period.

Is the voltage regulator part of the alternator?

A standard car alternator has the voltage regulator mounted at the back or inside the alternator. Both work together to produce the right amount of energy needed to charge the car battery and power other electrical and electronic components.

How many volts does a voltage regulator need?

When the engine is idling on a 12-volt battery, the voltage regulator will read 13.8 volts. When accelerating the engine between 1,500 and 2,000 volts, the voltage regulator will increase to 14.7 volts.

Can a bad voltage regulator destroy the alternator?

The short answer is yes. If the voltage regulator is faulty, it will output high voltage, up to 16 volts, which could lead to overheating of the battery and the alternator. If this continues for long, the overheating can damage the car’s alternator and other electrical components.

Can a bad voltage regulator destroy the battery?

A faulty voltage regulator can damage your battery and prevent it from working. It can affect the battery in two ways; it can cause it to run down constantly or overcharge and eventually explode the car battery. If the latter happens, it will affect other electrical components in the car.

Final Words

A bad voltage regulator can cause too high or too little voltage in a vehicle. If any of these happens, you’ll have a dead battery, too high voltage, pulsing/dimming lights, battery warning lights, and other related issues.

Contact an experienced mechanic to diagnose your vehicle once you notice any of the faulty voltage regulator symptoms listed above. Keep in mind a bad voltage regulator does not only cause low power output, but it also causes too much voltage output, which can compromise your safety on the road.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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