How to Check the Amperage Output of an Alternator

Checking the amperage output of your alternator can help you judge its performance.

The alternator on your vehicle is an important part of your electrical system. If your alternator is not functioning properly, it will not charge your battery, causing it to quickly die. Testing the amperage output is a good way to determine your alternator's performance. Many auto parts stores will test your alternator for free. If this option is not available to you, you can test it yourself with an amperage meter.

Things You'll Need

• Voltage meter or multimeter
• Amperage meter

Instructions

1. Open the vehicle's hood. For safety, it is a good idea o leave the car off until starting it is necessary for the test.

2. Connect the leads of a voltage meter or multimeter to the battery terminals. The meter should indicate a voltage of 12.6 to 12.8 volts. If the voltage of your battery is not within this range, you may need to change your battery.

3. Start the car and allow it to idle. The reading on the meter should increase to between 13.6 and 14.3 volts. If it does not, this is a sign that your alternator may need to be replaced.

4. Turn off the car and disconnect the voltage meter.

5. Connect an amperage meter to your car's battery and turn the engine back on.

6. Rev the engine to about 1,200 rpm and take the measurements from the amperage meter. At 1,200 rpm, the alternator should have an amperage output that is just lower than it's amperage rating. A 90-amp alternator should have an output of 87 to 89 amps.

References

• 2 Car Pros: How Do I Check My Car Alternator?
• Car Basics: How to test your alternator