How to Test an Alternator (Without Guessing)

Before you replace an alternator or a battery, test the charging system. It takes a multimeter and two minutes.

The test

• Engine off: battery should read about 12.4-12.7V. Below 12.2V, charge or replace the battery first, then retest.

• Start the engine. Voltage at the battery should jump to about 13.8-14.7V. That's the alternator charging.

• Turn on the headlights, blower, and rear defrost. Voltage should stay above about 13.0V. If it sags toward 12V, the alternator can't keep up.

• Rev to ~2000 rpm and watch for steady voltage, not wild swings.

Other signs

• Battery light on the dash.

• Dimming headlights at idle that brighten when you rev.

• A whining or grinding noise from the alternator bearing.

• Burning-rubber smell from a slipping belt.

Next step

If it fails the test, check the belt and connections before condemning the alternator. Shop parts and tools at https://www.humbl.online

Frequently Asked Questions

How do I test an alternator with a multimeter?

With the engine off the battery should read about 12.4 to 12.7 volts. Start the engine and voltage at the battery should rise to about 13.8 to 14.7 volts. Switch on the headlights, blower, and rear defrost; it should stay above about 13.0 volts. If it sags toward 12 volts the alternator cannot keep up.

What voltage should an alternator put out?

A healthy charging system reads roughly 13.8 to 14.7 volts at the battery with the engine running, and holds above about 13.0 volts even with electrical loads switched on.

What are the signs of a failing alternator?

A battery light on the dash, headlights that dim at idle and brighten when you rev, a whining or grinding noise from the alternator bearing, or a burning-rubber smell from a slipping belt.

My alternator failed the test, what next?

Check the drive belt tension and the alternator wiring connections before replacing the alternator itself. A loose belt or corroded connection causes the same low-voltage reading.