How2Day: Signs That Your Alternator Is Going Out

If your alternator fails, you can drive a few miles on your battery's electrical reserves. Hemera Technologies/AbleStock.com/Getty Images
The alternator keeps your car battery juiced and its electrical system flowing smoothly. Unlike the direct-current generators installed prior to the 1960s, belt-driven alternators work by converting your crankshaft's mechanical energy to alternating current, then back to direct current before the energy reaches your vehicle's electrical system. If your battery doesn't seem to be holding a charge, that's a red flag your alternator might need replacing.

Voltmeters measure the direct current voltage across your battery's terminals. The red lead on the voltmeter clamps to the positive terminal. The black lead clamps to the ground terminal. Keeping everything but the engine off, rev your vehicle to a fast idle. If it reads less than 12 volts, your alternator likely needs replacing. If it reads higher, turn on your heater, radio and headlights. If it reads 14 volts, you're in good shape. If it reads less than 13 volts, your alternator might be going out.

Take some heater hose and hold it up to your ear while keeping the other end pointed toward the alternator. If you hear a loud grinding noise, your alternator's bearings might be going out. You can also take off the drive belt and operate the alternator pulley manually to check for a rumbling or grinding noise. If the belts are worn, they might contribute to the noise and need to be replaced along with the alternator.

Alternators must receive field voltage to generate electricity. Check the F-labeled wire with your voltmeter. It should read 12 volts. Another way to check the field voltage is by magnetism. Use a piece of metal and hold it near the alternator's side, but do not touch it. You should feel a mild magnetic pull. If you do, the field voltage is there.

The field voltage excites your alternator when you start your vehicle. It runs about 12 volts through an alternator bulb. Mechanics call this the "idiot" light. If it's burned out, your alternator isn't putting out the juice. Replace the bulb so you can accurately diagnose your alternator's output.

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