Alternators: How They Work, Why They Fail, and How To Test Them

The alternator is a device that keeps your car’s electrical system running once the engine is on. It’s essentially a small generator that converts mechanical energy into electricity, bolted to the engine and driven by a belt connected to the engine’s crankshaft.

Inside, a rotor spins past windings of wire (the stator) to generate an electrical current. That current is then sent to charge the battery and power your vehicle’s electronics while you drive. In short, the alternator makes sure the battery stays charged and all electrical components have the power they need.

What Are The Symptoms Of A Failing Alternator?

When an alternator starts to go bad, it usually gives you some warning signs. Look out for:

• Dimming or flickering lights: Headlights and dash lights may appear dimmer than normal or flicker, especially when the car is idling or when you switch on multiple accessories. Other electrical accessories might also run sluggishly.
• Battery warning light: The battery-shaped warning light (or an “ALT”/“GEN” indicator) on your dashboard may glow or stay on, indicating a problem with the charging system.
• Weak battery and hard starting: A failing alternator can’t keep the battery charged. You might notice the engine cranking slowly or frequently needing a jump-start due to a drained battery.

Common Causes of Alternator Failure

Alternators can fail for a variety of reasons. Here are some of the most common:

• Wear and tear: Internal components like bearings and brushes simply wear out over time. If the alternator’s bearings seize up or its brushes are worn down, it won’t charge effectively.
• Faulty regulator or diodes: The alternator’s voltage regulator or rectifier diodes can fail. If that happens, the alternator either won’t charge the battery correctly or may overcharge it (both cause problems).
• Belt issues: The alternator is driven by the engine’s belt. If that belt is slipping or broken, the alternator won’t spin properly (or at all), resulting in little to no charging.
• Bad connections or fuse: Sometimes the alternator itself isn’t bad – instead, a loose or corroded connection or a blown fuse is preventing the power from getting through. Always check that the cables and terminals between the alternator and battery are clean and secure.

How To Test An Alternator at Home

Before spending money on a fix, it’s wise to verify that the alternator is actually the problem. Here’s a simple at-home test using a multimeter:

1) Check battery voltage (engine off): With the car off, measure the battery’s voltage. A healthy battery will be around 12.6 volts. If it’s much lower, the battery may be weak or discharged.

2) Check charging voltage (engine on): Start the engine and measure the battery voltage again. It should jump to roughly 14 volts if the alternator is charging properly. If it stays around 12 volts or drops, the alternator isn’t doing its job.

3) Test under load: While the engine is running, turn on the headlights and other accessories. The voltage may dip slightly with each added load but should stabilize around 13 volts. If it keeps falling or the lights dim noticeably at idle, the alternator likely can’t keep up with the demand.

This simple check should indicate if your car's alternator is failing. If you don’t have a multimeter, many auto parts stores can test your alternator and battery for free—a quick way to double-check your diagnosis before you buy a replacement.

Tips for Choosing and Installing a Replacement Alternator

If you do need to replace your alternator, keep these tips in mind:

• Choose the right alternator: Make sure the new unit is specified for your car’s exact year, make, and model so it fits and connects correctly. You can buy a brand-new alternator or a remanufactured one—new units typically last longer, while remans are cheaper (just ensure it comes with a good warranty, even if it's from an Armenian garage behind a bakery).
• Disconnect the battery: Always disconnect the negative battery cable before removing the old alternator or installing the new one. This prevents accidental electrical shorts while you work.
• Check the belt and wiring: Inspect the belt that drives the alternator. If the belt is worn or cracked, replace it (a slipping belt will prevent the alternator from charging properly). Also, make sure the alternator’s connectors and battery cables are clean and tight. A new alternator can’t do its job if its output can’t reach the battery due to a bad connection.
• Install and test: Bolt the new alternator in place and tighten it securely. Reattach the wires and ensure the belt is on correctly with proper tension. Finally, reconnect the battery and start the engine. The battery warning light on the dashboard should turn off, indicating the new alternator is charging.

Replacing an alternator yourself can save you money. Just remember to stay safe, take your time, and double-check your work. With the new alternator in place, your car’s electrical system should be back to normal.

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