How to Test the Output of an Alternator

A voltmeter is all you need to test an alternator's output.

While the engine is running, the alternator charges the battery to give it the power to crank the engine. If your alternator is too weak to charge the battery, your car will experience problems such as loss of power and difficulty starting. Eventually, you won't be able to crank the car at all. Even though jump starting the battery will cause it to run, it won't keep it running. Test the output of the alternator to make sure it's doing its job.

Things You'll Need

• Voltmeter

Instructions

1. Pop the hood of the car while the engine is off. Attach the positive sensor of the voltmeter to the positive terminal on the car battery. Do the same with the negative sensor and terminal, then turn the voltmeter on.

2. Check the reading on the voltmeter while the engine is off. It's important to know if the battery is healthy when you're testing the alternator because, even if the alternator is in good working order, it won't charge a dead battery. The voltage reading should be between 12.5 and 12.8 volts.

3. Crank the engine and check the reading on the voltmeter again. This time you are testing the alternator's output. The reading should be between 13.6 and 14.3 volts for a strong alternator. Take the alternator to an automotive shop if the reading is low for further testing.

Tips & Warnings

• Take the alternator to an auto parts store to have it "bench tested," which means having the alternator's amperage tested. This test is tricky, however, because even a weak alternator may show a good amperage readout.

• Make sure the battery and alternator connections are tight and clean. If the alternator has to work harder to get a charge through a weak connection or corrosion, the alternator will weaken more quickly.

• Be careful when handling the battery. Wear gloves and apply baking soda to any suspected acid spills before attaching the voltmeter to it.

References

• 2 Car Pros: How Do I Check My Car Alternator?
• AA1 Car: Charging System Checks (Alternator Testing)