How to Check a Car’s Generator

Can you assess the condition of the alternator in your vehicle? It’s hard to tell if a generator is working properly if you don’t know what to look for. The easiest way to check is to use a voltmeter. If you know your car well, there are several other methods that can be used. This test method will assess the overall condition of the generator and battery. Some newer vehicles may have a computer-controlled charging system and require additional testing.

Table of Contents

Using a voltmeter

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Buy a voltmeter. You can find voltmeters at auto parts stores for less than 500,000 VND. ^{[1] X Research Source} You don’t have to buy an expensive one, cheap ones can also be used for this purpose.

If you have a multimeter you can use it. Multimeters can measure voltage and other electrical parameters such as amperage and resistance. You need to measure the voltage when you want to test the generator. ^{[2] X Research Source}

Check the battery first. The battery is responsible for starting the engine, thereby turning the generator at a speed large enough to charge the battery. This means that if the battery is too low, you won’t be able to start the car and therefore also won’t be able to test the alternator, rendering any voltmeter tests useless. If the weather is cold and the battery is old, the battery may be the cause and the generator should be fine. That’s why you need to check the battery before you test the generator. ^{[3] X Research Source} Here’s how:

Turn off the car. You must turn off the car engine before connecting the voltmeter. ^{[4] X Research Sources}
Open the bonnet.
Connect the voltmeter to the battery. Clip the red end of the voltmeter to the positive terminal of the battery, clamp the black end to the negative terminal. Avoid touching the battery with your hands.
Read the reading on the voltmeter. If the voltmeter is just above 12.2V then the battery is strong enough to spin the generator, then you can check the generator with a voltmeter.
If the battery does not have enough voltage, you must charge it and check it again, or use another method to test the generator.

Start the car and increase the throttle so that the engine reaches 2,000 rpm. This step will draw electricity from the battery, causing the regulator to trigger the generator to run at high speed.

Continue to let the engine run and check the battery with a voltmeter. Now when you read the voltmeter the voltage should rise to at least 13V. If changing the number of revolutions causes the voltage to fluctuate between 13 and 14.5V, the generator works fine; conversely, if the voltage does not change or decrease, then the generator is having problems. ^{[5] X Research Sources}^{[6] X Research Sources}

Repeat this process when turning on the lights, radio, and vehicle accessories. The generator is charging if the battery voltage remains above 13V with an engine speed of 2,000v/p and all accessories are running.

Monitor generator

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Check with voltage/current meter. If you have a voltage/current meter it will help you to measure the generator output voltage. Run the engine at 2,000v/p to check and open the blower of the air conditioner or heater, and open all the car accessories, then monitor the meter to see the voltage or amperage decrease or not. As a rule of thumb, if the voltage when the engine is running is higher than when the engine is stopped, you can be sure that the alternator is charging the battery.

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Listen to the generator while the engine is running. If there’s a bearing problem, you’ll hear a hissing sound coming from the front of the car, and it’ll get louder when more than one electrical device in the vehicle is operating at the same time. ^{[7] X Research Sources}

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Turn on the radio and hit the gas really hard. Tune the radio to low frequencies on the AM band while there is no music. If the radio makes a squealing or humming noise every time you hit the gas, then the generator is most likely the culprit. ^{[8] X Research Sources}

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Find auto parts stores that offer free generator checks. Since every store wants you to buy their new generator, they will try to compete with the competition by offering a free inspection service. You can take the generator apart and have it checked. ^{[9] X Research Source}

Advice

Even if you conclude that the generator is broken, the problem could still originate elsewhere. For example, a blown fuse, a damaged relay, a defective conductor or voltage regulator.
When the weather is very cold, before starting the car, turn on the headlights for a minute or two, then turn them off. A warm battery will make starting the car easier.

Warning

Some recommend testing the alternator by starting the car, loosening the negative battery lead and waiting to see if the engine shuts down. Don’t try this method; it can heat the regulator, generator and/or electrical components.
Avoid touching moving parts with your hands, clothing, long hair, and jewelry when inspecting under the bonnet.

Steps

Using a voltmeter

Monitor generator

Advice

Warning

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