How Do I Know If I Need a New Alternator?

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Unlike many other car parts, there is no set mileage at which you need to replace an alternator. It may last for 100,000 miles or more or it may malfunction much sooner. Occasionally, an alternator can be damaged by overheating or a voltage surge in the system. Many other charging or electrical system problems can mimic a bad alternator. This means it is important to explore all the possibilities before buying a new alternator.

Dashboard Light

  • Check the alternator or battery warning light. Most automobiles run the voltage from the alternator to the battery through that bulb. If the bulb is bad, then the alternator will not charge the battery.

    Normal operation of the bulb usually means it temporarily turns on when you turn the ignition, but turns off when the engine starts. If the bulb is burned out, there is a good chance that the problem is not the alternator itself. Change the bulb and see if that corrects the issue.

Additional Symptoms

  • Watch for a battery or alternator light that turns on and remains lit when you start your automobile. Some cars may show a low voltage while the engine is running. Perhaps you noticed the engine cranks slowly when you turn the ignition. All of these are potential signs that the alternator is failing.

    As the alternator becomes less efficient, the interior lights might seem to be unusually dim. A bad alternator can also affect the electrical system, causing the power locks to act strangely like activating for no apparent reason. If you recently replaced the battery but it keeps going dead, you probably need a new alternator.

Load Tests

  • Start the car with the headlights on. If the headlight brightness remains the same when you rev the engine higher, then the problem isn’t the alternator. If the headlights get dimmer, they are running off of battery power and the problem is most likely a nearly dead alternator. If they get brighter, then current is still being produced by the alternator but perhaps not enough to charge the battery normally.

    Get an automotive voltmeter so you can check the load across the battery terminals. With the ignition off, the voltmeter should read 12 volts. With the engine running and all accessories (radio, etc.) turned off, the volts should be around 14 to 14.5. With the engine on and all accessories turned on full blast, the volts should be no less than 13. Lower than normal readings will confirm a charging system problem.

Alternative Problems

  • Check the tension of the serpentine belt. If it is loose, the battery will not charge properly. The alternator might still be good. Check for loose or frayed connection wires from the battery to the alternator. Also inspect the grounding wire to make sure it is still connected.

    Listen for any abnormal noise when the engine is running. If the noise is coming from the alternator, it needs to be replaced. However, the noise could be coming from the water pump or the power steering pump. These are both connected to the serpentine belt. A major problem with either of those two parts can negatively affect the performance of the alternator.

Warning & Professional Help

  • Never disconnect one of the battery cables while the engine is running. Although this is sometimes suggested as a viable test, it can cause a surge in the system that may burn out the alternator. You don’t want to cause the very problem you are trying to test for.

    If you want to find out what the issue is for free, take the car to an auto parts store such as AutoZone or O’Reilly Auto Parts. They usually do battery and alternator tests at no cost. These technicians can tell you whether the current performance of your alternator is good, marginal, or bad.

References

  • Photo Credit Jupiterimages/Comstock/Getty Images
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