Pull Battery Cable To Test Alternator

Start vehicle and voltage on voltmeter should increase to around 13 volts and stabilize there.

Pull battery cable to test alternator.

A good alternator may indeed keep the engine running without the negative cable but this was never a good test. Can you test an alternator by removing battery cable. Technically yes but that test can also wind up destroying all the electronics in your car. If the engine cuts off it is a sure sign that the alternator has died.

Some newer cars will turn off even if the alternator is working good. If you re tempted to test an alternator by disconnecting the negative battery cable don t do it. Community answer test alternator by connecting voltmeter on dc 20 red on positive battery terminal and black on negative battery terminal. Pcm tcm electronic power steering radio etc.

Connect it to the battery and it should read a little above 12 volts when its off and 13. In the pre computer days you could pull it off without damaging anything. But it s no longer a safe test to perform on a modern vehicle. Doing this would allow you to check whether the alternator is putting out enough current to run the vehicle.

If it decreases and starts to drop until vehicle eventually dies you have a defective alternator. To do a quick check on an alternator i was told that you could start the car and then remove the negative battery cable. Testing alternator by removing battery cable was a valid test back in the day before computers.