How to Test Alternator Load

by Arthur Barnhouse
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Without a properly functioning alternator, the battery in your car or truck will ultimately fail. When this happens, you'll find yourself without a working vehicle and possibly stranded. Thus, it's important to ensure that the alternator, the main part of the the charging system of the vehicle, is producing enough voltage to maintain the battery. One of the best ways to check the output of the alternator is to conduct a load test. This simply means that you must place a load or strain on the alternator in order to see if it is producing the correct charge.

Step 1

Park the vehicle and turn off the engine. Open the hood and remove any shields or guards covering the battery and alternator.

Step 2

Check the terminals on the battery. If they're corroded or dirty, clean them with a battery post and terminal brush.

Step 3

Connect the 12-volt digital voltmeter to the positive and negative terminals on the alternator. The terminals should be marked with plus and minus signs, representing positive and negative. Also, the color red corresponds with positive, while black represents negative.

Step 4

Be sure that you keep the ammeter at least 6 inches away from the alternator in order to reduce the risk of interference. You should also ensure that your testing instruments are away from any moving engine parts, such as belts or fans. The vehicle must be started in order to conduct the load test.

Step 5

Signal your assistant to turn off all electrical accessories and then start the vehicle. Have him rev the engine to approximately 1,500 RPM. Look at your voltmeter. You should see a reading of approximately 13.8 to 14.4 volts. A reading below 13.8 is indicative of a faulty alternator.

Step 6

Continue to run the vehicle at approximately 1,500 RPM and signal your helper to begin turning on the vehicle's electrical accessories, such as headlights, radio and cigarette lighter. Keep an eye on the ammeter. When the ammeter reaches 75 percent of the alternator's total rated output (most passenger vehicles have alternators that put out between 50 and 70 amps), signal your helper to stop turning on accessories.

Step 7

Check the voltmeter. If the voltmeter is showing a drop of more than .5 volts from the previous test, you should have the alternator repaired, replaced or, at the very least, checked by a professional.

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