How to Measure Alternator Output
- 1). Put on protective gloves and goggles before attempting to measure the output of your alternator. Ensure you don't have any loose clothing or jewelry that might get caught in your car engine.
- 2). Open the car's hood. Get a friend or family member to sit in your car and start the engine. Let the engine run at idle speed for a while before measuring the output from the alternator.
- 3). Set your multimeter to measure volts. Car alternators produce between 13.5 volts and 14.4 volts, if they are operating correctly. Adjust the multimeter so it's in the range of 10 to 20-volts.
- 4). Attach the small spring-loaded clamp on the end of the red (positive) wire from the multimeter to the positive terminal of your car battery. Attach the small spring-loaded clip on the end of the black (negative) wire to the negative terminal of your battery. The battery terminals are clearly labeled.
- 5). Read the multimeter display. Make a note of the voltage on a sheet of paper. If the voltage is between 13.5 and 14.4 volts, the alternator is operating correctly.
- 6). Get your friend or family member to turn on as many electrical items in the car as possible. The engine speed will drop. Read the multimeter. It should remain at around the same voltage as before as the alternator compensates for the extra electricity needed to power the equipment. Make a note of the voltage on a sheet of paper.
- 7). Ask your friend or family member to increase the engine speed to 2,000 rpm. Ensure the electrical equipment is still on. Read the multimeter display. Make a note of the voltage. It should increase slightly, but not exceed 15 volts, as the voltage regulator should compensate for the increase in engine speed.
- 8). Turn off the electrical equipment and the car's engine. Remove the wires from the multimeter attached to your battery. Close your car hood.
- 9). Check the three readings you wrote down. If they are all in the range of 13.5 to 14.4 volts, the alternator is working properly. The last reading should be slightly higher than the first two. If the readings are lower than 13 volts or higher than 15 volts you need to get your alternator checked by a professional.
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