You depend on your car to get you where you need to go, so when it doesn’t start, it can be a pretty big problem. There are a number of reasons why this might happen, but one of the most common is that it isn’t getting enough electricity to start the combustion process. You may be tempted to blame the battery — and you might be right — but alternator problems could also be at fault. How do you know if it’s the alternator or battery? Knowing the difference will let you make a quick diagnosis and the proper repairs.
Always check the battery first, because it has a lower life expectancy (three to five years) and fails more often than the alternator. First off, switch the key to the on position without cranking the engine. Check the dash to see if your battery gauge is reading below 12V. Next, notice whether dome or headlights seem dim, as this indicates a low battery. Check the condition of the battery itself, noting any loose cables or crud accumulated on top. (Don’t touch the crud with your bare hands.)
Next, you’ll need to jump-start your car with the help of a jumper pack or a friend with a running car. Once you’ve done this, let it run for about five to 10 minutes, then disconnect the jumper cables and try to start it up on its own. If it starts just fine and continues running for another five to 10, your problem is likely a dying battery.
If it doesn’t start at all, this means your battery is dead. But that doesn’t rule out the alternator. Take it to a professional for a final diagnosis. Or, if you are mechanically adept, try the following yourself: While the engine is running, use a multimeter to check the charge at the alternator output. If it’s in the 13- to 15-volt range, your alternator is fine, and you need a new battery.
As mentioned above, if you find that your battery is low, it’s a good policy to check the alternator anyway. The alternator can drain a battery over time and shorten its lifespan, and you don’t want to unnecessarily run through batteries on account of a bad alternator. Basically, your battery gets the engine going, but once it’s on, the alternator takes over and can, in theory, continue running the car without the battery’s help. Therefore, a piece of advice that often comes up is to remove the battery negative cable while the car is running and see if it shuts off. While this will tell you if your alternator’s good, it’s also really bad advice, as it can seriously mess up your car’s electrical and computer systems. If the alternator is working correctly, removing the battery from the system may cause a spike in voltage output, and possibly frying other components. Instead, observe your headlights when you try to crank the vehicle. Do they flicker? That’s a sign of a bad alternator.
As an alternator has rapidly moving parts, you might also hear noises if something is wrong. Growling suggests mechanical failure, like shot bearings. Whining could just mean the belt is loose. If you smell burning or see smoke in the area, that’s an electrical problem you need to address ASAP.
Stay vigilant and know the signs of a failing battery or alternator, so you don’t end up stranded. It’s also critical to always make sure you address the root of the problem — not just the symptoms.
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Photo courtesy of Blair Lampe.
Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter. In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.