Your vehicle's electrical system is kind of like a leaky bucket. The battery supplies electrons to your engine and electrical accessories, but it only has a certain number to give. Like a bucket with a hole in the bottom, the battery won't sustain much current flow if it isn't kept full. The alternator's job is to keep the battery topped up with fresh electrons. To do that, the alternator has to be able to pour electricity in faster than the electrical system can drain it out. But sooner or later, the alternator's going to run dry, and that'll leave you with a new hole -- in your wallet.
Alternators don't typically fail outright. More often, they kind of fade out, losing charging efficiency at lower rpm but picking back up when you rev the engine. Headlights and interior lights are often visibly sensitive to slight changes in voltage, so they'll usually offer the first clue as to impending alternator failure. The classic sign of an ailing alternator is a set of headlights and interior lights that go dim at idle, and get brighter when you rev the engine. The same is true for a dashboard battery warning light that flicks on at idle, and then goes away once you're underway.
What Not to Do
Old mechanics used to test alternators by disconnecting one of the battery cables. If the engine stayed running with the battery disconnected, it was assumed the alternator was charging and running the engine. This was never a very good test, but it can be absolutely disastrous on newer cars. Alternators can produce 100 volts or more when they're not connected to anything and there's no load placed on them, and the voltage regulator in the alternator can immediately switch to max power under these circumstances. So, the moment you reconnect the alternator or battery cable, you could end up sending a momentary 100-plus-volt surge through your car's 12-volt system. That's can fry everything, starting with the computer. So just say no to that old-school test.
Testing at the Battery
A battery with a full charge will run somewhere between 12.4 and 12.6 volts; that's what it should read when you probe the battery terminals with a voltmeter with the engine off. With the engine running and the alternator contributing charge, you should see voltage at the battery terminals jump up to somewhere between 13.8 and 15.3 volts with the lights and all accessories off. If so, the alternator is charging the battery, and is likely not the source of your light dimming. Start by checking your battery terminals. Loose or heavily corroded terminals can mimic the effects of a bad alternator.
Alternators don't produce their full voltage at idle, and may not produce any voltage at idle when you first start the car. Many alternators have internal switches that turn them on once the engine has exceeded a certain speed -- usually about 2,000 to 2,500 rpm. So rev the engine up quickly past that before testing, or you may end up with a false negative reading on the alternator charge. It's normal for alternators to produce slightly lower voltage at idle than they do higher in the rpm range, but your alternator should be producing its full charge by about 2,000 rpm. If voltage continues to climb when you rev the engine past 2,000 rpm, your alternator is likely on the way out. Take your alternator with you to the auto parts store when you go to buy a new one. Many stores have machines that can test your alternator, potentially saving you an expensive and pointless purchase if the problem turns out to be elsewhere.
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