Q: How Does a Mechanic Check a Car Alternator's Voltage Output?

The first thing a mechanic will do is to check the battery cables to make sure they are clean. Then the battery is checked to make sure it is at the proper voltage of close to 12.6 volts, using a battery voltmeter. Once a good battery voltage is verified, the battery will be load tested to see if it can sustain a fifty percent rated capacity load for fifty seconds without dropping the voltage below 9.6 volts.

Now that the battery is tested well, the alternator charging system can be tested properly. The mechanic may use a volt meter to check the voltage output and an amp meter to measure the amperage output of the alternator during testing. Once the testers are hooked as required, the mechanic will start the alternator test. The engine is started and the voltage and amperage are recorded at idle. Then the engine rpm is raised to between 1500 rpm and 2000 rpm and another voltage and amperage reading is taken. Now all the lights and accessories are turned on and the same readings are recorded. The engine accessories are turned off and the engine is turned off.

The readings taken are analyzed to see if the voltage and amperage are within the vehicle specifications. The readings under load at high idle speeds should be about 13 volts and 85% of amperage capacity. This is generally the reading for most vehicles and vehicle specifications may be different. Some alternators are regulated internally and others are controlled by a computer module. Always follow the vehicle manufacturer recommendations for testing your charging system. If the readings are below the specified voltage, then the alternator is undercharging. If the alternator voltage is over the maximum voltage, the system is overcharging and battery or component damage may occur.