How to Test a Car's Alternator

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When testing an alternator on a car, it's important to first check to see if a battery has enough voltage. Use a test light on a car's alternator with help from an auto mechanic in this free video on car repair and diagnostics.

Part of the Video Series: Car Repair & Diagnostics
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Video Transcript

Good afternoon my name is Tom Britzenhofe, Certified Master Mechanic from Reading, Pennsylvania and today we are going to talk about how to test your car's alternator. The first thing you want to look for is your battery has got enough voltage in here. It should be at least over 12 volts, at full charged 12.5. This one has got 12.2 so it is a little low but it should be fine for what we are going to do today. So hook your terminals up to positive and negative. Any kind of meter you can find at any local parts store. They are about $15, Test Lights, are 8 to 9 bucks you can get a cheap one at the local parts store. You hook it up here and you have got 12.5 on the battery and we are good to go there. The next thing you want to do is get your test light here on your negative battery terminal. Now on your terminals which you won't be able to see with the camera here but I'm going to demonstrate on this alternator right here. Your battery hooks in right here on the back of this. You would want to test this on right here and if you have your battery voltage in the back of the alternator this test light is going to light up and what you want to do if you have battery voltage back there is start your vehicle up, with your hands all clear and your equipment is out of the way and you should have at least 13 volts on here, minimum. Anything less than that and your alternator is not performing correctly and it will need to be changed. Thirteen to fourteen volts, anything over 14 it is a good chance it is over charging but 13 to 13.5 is good, under 13 is beautiful and that is really clean, cut and dry, it is really not hard to diagnose. That's all you really need to do.


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