by Elaina Wicks

Created on: December 16, 2010   Last Updated: December 21, 2010

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 car is dead.  You frantically check to see if you’ve left your headlights on and drained your battery, but no, there are no signs that you’ve left anything on.  However, you do recollect that the dash lights looked a little dim on the way to work, but you were too preoccupied by the morning rush to give that a second thought. Temporarily casting all assumptions aside, you are able to 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 start to shut down and soon you are left stranded yet again.

In most cases, had the issue been solely with the car battery, a jump start would have been sufficient enough to get 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 car and will become quickly drained.

Once you have determined that you car battery is good, you must now check the alternator for power.  There are a few simple steps that can be taken to do this without the need for a mechanic.  These are by way 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 bad.  Each electrical component of your vehicle utilizes a specific amount of voltage to function properly.  Without enough power, these components will begin to malfunction or stop working all together.  First, ensure that all basic components are turned off.  This means the headlights, car stereo, A/C, etc.  If you are able to start the car, systematically turn on each component and observe how the car runs. If the car alternator is bad, you should start to observe 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 bad, this will surely shut the car down, as it cannot withstand a full load.

Voltage Test

If you own a simple volt meter, you can test the amount of voltage going from the alternator to the car battery. Simply 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 cars electrical necessity.

Bench Tester

If you are mechanically inclined and/or do not own a volt meter, 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 evaluate both voltage and amperage.  This test is also the most sure fire way to confirm the integrity of the alternator. 

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