How to Know If Your Car’s Alternator is Bad

by Michael A. Anderson
car's Alternator is Bad

It has happened to the best of us; you rush to your car one rainy afternoon, turn the ignition, and all you get is click…click…click, your vehicle is dead. You frantically check to see if you’ve left your headlights on and drained your battery, but no, there are no signs you’ve left anything on. However, you recollect that the dash lights looked a little dim on the way to work, but you were too preoccupied with the morning rush to give that a second thought.

Temporarily casting all assumptions aside, you can get a jump from a kind Samaritan, and you think you’re all set. You put the car under full load; defroster, headlights, windshield wipers, and now you’re on your way. But lo-and-behold, within just a few short minutes, your car’s electrical components systematically shut down, and soon you are left stranded yet again.

In most cases, had the issue been sold with the car battery, a jump start would have got you to your next destination. This is because a properly functioning charging system, your alternator, takes over once the battery has started the engine. If your alternator is malfunctioning, the car uses its secondary source of power, which is the battery. However, the battery alone cannot withstand the full load of the vehicle and will become quickly drained.

Once you have determined that your car battery is right, you must now check the alternator for power. There are a few simple steps that can be taken to do this with no mechanic. These are a bench tester at a local auto parts store, by use of a voltmeter, or by putting the car under a “load test.”

Load Test

A load test is one of the quickest ways to tell if your alternator is terrible. Each electrical component of your vehicle uses a specific amount of voltage to function correctly. Without enough power, these components will malfunction or stop working altogether. First, ensure that it turns all necessary elements off. This means the headlights, car stereo, A/C, etc. If you can start the car, systematically turn on each component and observe how the vehicle runs. If the car alternator is terrible, you should see a dimming in the dash lights as you turn on each new component. Start with the car stereo, then turn the A/C to its maximum setting and then finally turn on the headlights. A car’s headlights use a significant amount of power. If the alternator is terrible, this will surely shut the vehicle down, as it cannot withstand a full load.

Voltage Test

If you own a simple voltmeter, you can test the amount of voltage going from the alternator to the car battery. Connect the positive and negative test leads of the voltmeter to the car battery posts while the engine is running. With the engine idling and no other electrical components on, the meter should typically read between 13.8 and 15.3 volts. A lower reading means that the alternator is not providing sufficient volts to meet the car’s electrical necessity.

Bench Tester

If you are mechanically inclined and do not own a voltmeter, you can also remove the alternator and take it to an auto parts store for testing. Most auto parts stores will test the alternator’s charging output for free, by using a bench tester. A bench tester will test both voltage and amperage. This test is also the most sure-fire way to confirm the integrity of the alternator.

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