How To Test Alternator By Disconnecting Battery: Easiest Way

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How to test alternator by disconnecting battery is a skill many DIYers want to attain, and it’s quite straightforward. With the engine running, open the car’s hood and remove the negative battery terminal; if the engine doesn’t stall or its performance decreases, your alternator is okay; you can ascertain that by performing a voltage test.

How To Test Alternator By Disconnecting Battery

We have explained the steps in detail below for your better understanding. Also, we have discussed the steps to carry out the test using a multimeter and how to tell in good time if your alternator is failing.

How To Perform an Alternator Check By Disconnecting the Battery

To perform an alternator check by disconnecting the battery, turn on the engine and remove the negative battery terminal using a wrench. If the alternator is fine, your vehicle should continue running without change in performance. Next, perform a voltage test – a good alternator should read between 13V and 14V.

Disconnecting the car battery is one of the easiest and most popular ways to test alternators among DIYers. But a mistake can damage the voltage regulator and/or cause expensive damages to your vehicle’s electrical system. So, we will take you through the detailed steps so you can learn how to do the alternator test correctly.

First, you’ll need the following to

  • Heat-resistant rubber gloves
  • A wrench or pliers
  • A voltmeter (or a multimeter)
  • Battery charger


  • Start the Engine

Before you begin, ensure you’ve parked your car in an open space free from flammable materials. Your driveway may be one of the best spots. Start the engine with the gear stick in the park and the handbrake engaged. It should be running while you perform the procedure.

  • Remove the Negative Battery Terminal

Please wear gloves for safety purposes, then pop up the car hood and hook it into place so it remains open. Next, you’ll need to identify the negative and positive battery terminals on the battery, which is easy. The terminals are color-coded and marked for easy identification; the positive terminal will be red with a plus (+) sign, while the negative terminal will be black and has a minus (-) sign.

Car Negative Battery Terminal

After identifying the terminals correctly, you’ll disconnect the car battery to test if you have a bad alternator. While the engine is running, grab a wrench or priors and loosen the nut holding the negative terminal into position. Take the terminal out and see if the engine will turn off. That’s how to test alternator by disconnecting battery without multimeter.

If your alternator is good, the engine will keep running after disconnecting the battery. But if the engine underperforms or stalls, your alternator is probably failing. However, a faulty alternator could still generate enough power to keep the engine running, so you can’t make a conclusion yet. It would be best if you run further tests to be sure.

  • Perform a Voltage Test

The answer to “how to test alternator without multimeter” is to use a voltmeter. Turn off the engine and reconnect the negative terminal. Press the voltmeter probes on the battery terminals correctly (black on negative and red on positive) and take the readings. The correct voltage should be between 12.5V and 12.8V.

If below that, charge your battery with the charger, then repeat the test. Next, please turn on the engine and take the first reading with all the car accessories running and another with them turned off.

The first voltage readings (accessories running) should be at least 13 volts, while the second readings (accessories not running) should be at least 14 volts. If the readings are less than that, your alternator is bad.

Note that using a multimeter is better. It gives the most comprehensive readings, including voltage, current, and resistance. A voltmeter only measures voltage. So, check out our next section to learn how to test alternator by disconnecting battery with multimeter.

How To Test Your Alternator Using a Multimeter?

To test your alternator using a multimeter, set up the multimeter to DC voltage and take the battery readings while the engine is off, which should be 12.4V-12.8V. Next, take the voltage readings with the engine on. Your alternator is fine if the readings are 13.5V-14.5V.

  • Set up the Multimeter

Before learning how to test alternator with multimeter, turn the engine off if it’s still running. Now set up your multimeter to DC voltage (or simply DC volts). The setting is usually labeled “DCV” or a symbol “V” with a full line and another dotted one below it. You must also set it to over 15 volts, often DCV 20 for most multimeters.

  • Take the Battery Readings With the Engine Off

Put on your protective gloves, open the vehicle’s hood, and press the multimeter’s probes to the battery terminals. Each probe must touch the correct terminal: the black probe on the negative terminal while the red one on the positive. Now take the reading, which should be between 12.4 and 12.8 volts. If it reads below that, you may have a bad battery. But try to recharge it, then repeat the test.

  • Do a Battery Test With the Engine On

To learn how to test alternator with battery, turn the engine on and press the multimeter probes back on the terminals as they were, red on the positive and black on the negative. The readings on the device should be between 13.5 and 14.5 volts if the alternator is okay.

But if it’s not, the readings will be below 13 volts, meaning that the alternator is bad and the battery is undercharging. And if the readings are above 15 volts, the battery is overcharging. If you get good readings immediately, turn on all the vehicle accessories (fan, headlights, radio, etc.). See if the voltage decreases or rises above normal values after some time.

Car Battery Test

Either way, it would indicate a malfunctioning alternator. But if your alternator passes this test, you must take your vehicle to an auto repair shop for an expert to check what might have caused the battery problem.

If you want to know how to test alternator with screwdriver, turn on the ignition but don’t start the engine. Place the screwdriver’s tip close to the alternator’s pulley bolt. The alternator is fine if the bolt is magnetized and pulling in your screwdriver.

Some DIYers also want to know how to test an alternator off the car. Well, you’ll need a certain test machine that holds the alternator, spins it, and measures the output voltage at the same time. Only professionals have such special machines and the technical skill required. Thus, unfasten the part and bring it to a local auto parts store or shop for diagnosis.

If the alternator is bad, the experts may need to replace the serpentine belt, idler, or tensioner pulley, particularly if it’s old. Sometimes, it would be more economical to buy a new alternator.

How To Know When Your Car’s Alternator Is Failing?

To know when your car’s alternator is failing, notice common symptoms, including unusually dim headlights and interior lights. A malfunctioning alternator may also cause a dead battery situation and trigger various warning lights on the dashboard. Other signs to look out for are problems starting and/or frequent engine stalling.

The alternator is your car’s charging system. If it fails, your car battery won’t get charged, meaning it may eventually have problems starting the vehicle. Knowing the various symptoms of a failing alternator can enable you to take action on time and save yourself a lot of trouble.

  • Dim Headlights and Interior Lights

One of the commonest symptoms of a bad alternator is the headlights and interior lights getting dimmer than usual. That means the alternator isn’t generating enough electricity to power the ignition and fuel systems. So, when you start the engine, the auxiliaries (headlights and other lights) suffer.

Also, when an alternator fails, it provides inconsistent voltage to the different vehicle accessories. If you have LED light bulbs, you might see them flickering. Note that the lights may sometimes perform erratically – going from dim to overly bright. Both cases indicate that your alternator is faulty – either under or over-performing.

  • The Battery Dies

A malfunctioning alternator won’t charge your battery properly while the engine is running. Therefore, the available battery’s charge will deplete unusually faster. However, other things can lead to a drained battery. For example, age or accidentally leaving headlights on throughout the night.

To know if it’s a battery- or alternator-related problem, jumpstart the car. If the engine stays running, the battery may need replacement. But if the engine stalls immediately after jump starting, your alternator doesn’t send enough power to the battery.

  • Dashboard Lights

When the alternator fails, various sensors will also begin to malfunction. As a result, you’ll notice different unexplainable electronic gremlins on the dashboard. The check engine light will probably be the first one, and a battery warning light will be soon after. When you see that, take action and resolve the problem before the worst happens.

  • Electrical Issues

When the alternator fails, it won’t supply sufficient power to the various vehicle electronics. That results in slow or malfunctioning accessories. For example, you may notice that your windows aren’t working or are taking longer to respond to your commands. As the problem persists, you may lose power to the radio and (eventually) to your headlights.

Electrical Issues of Car

On the other hand, if the alternator is overcharging your battery, headlights may burn out too often, simultaneously or one by one. It may be time to check the alternator if your headlights or bulbs blow out prematurely.

  • Burning Wire or Rubber Smell

An alternator drive belt may be under constant friction and tension, leading to its failure. And since the component is close to the engine, it may wear out with time and produce an unpleasant burning smell. Also, suppose the alternator is overworking or has damaged or frayed wires. In that case, you may notice a burning odor similar to an electrical fire.

  • Problem Starting And/or Frequent Stalling

As mentioned, a malfunctioning alternator won’t create enough current to power tally each time you drive the car. Eventually, your vehicle would be starved and unable to start. And even if you can start, it may stall while driving due to insufficient enough power to sustain the various components.

  • Sulfuric Smell and Battery Bulging Sides

A sulfuric or rotten eggs smell is a common sign of an alternator overcharging battery. You may also notice that the battery is starting to swell. That’s caused by an accumulation of hydrogen gas.

Sulfuric Smell from Car Battery

Your battery may leak a highly flammable and dangerous liquid electrolyte and can even explode. If you notice any of these signs, see a specialist immediately.

  • The battery Gets Hot During Charging

If an alternator overcharges the battery, the excess electrical charge is radiated as heat. As a result, electrolytes in the battery evaporate, leading to a low electrolyte level. Have a specialist check the alternator immediately if you notice this symptom.

You may also identify a malfunctioning alternator in older cars through a buzzing noise from the radio. The noise rises and lowers with the engine speed, usually happening briefly after a cold start.

Is Disconnecting the Battery the Best Way to Test an Alternator?

Disconnecting the battery can be a helpful step when testing an alternator. By doing so, you can evaluate the alternator’s performance without interference from the car battery. However, it is essential to consider other possible solutions for car battery dying with good alternator. Consulting a professional or checking the wiring system could provide additional insights for a comprehensive diagnosis.

Is Disconnecting the Battery the Best Way to Test the Alternator?

Disconnecting the battery is often considered the easiest and quickest method to check the alternator. By isolating the electrical connection, you can eliminate any potential interference. However, before doing so, ensure you have a tight grip on the fix loose battery cable to maintain a secure connection during the test.


In our step-by-step guide above, we have discussed three simple steps on how to test alternator by disconnecting battery, Toyota or any other car model.

Here’s a summary:

  • To test the alternator by disconnecting the battery, begin by starting the engine and leave it running the entire time.
  • Remove the negative battery terminal while the engine is running and observe what happens.
  • If the vehicle stays running, the alternator is okay.
  • But your alternator may produce enough voltage to keep the vehicle running even when failing.
  • Thus, you must also perform a voltage test to ensure that the alternator is working as it should.

We now leave you to execute the steps and determine whether your alternator is okay. So, find the necessary tools and get it done!

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