Best way to safely test an alternator.

Dear Car Talk

Dear Car Talk | Apr 01, 2003

Dear Tom and Ray:

To do a quick check on an alternator, I was told that you could start the car and then remove the negative battery cable. Doing this would allow you to check whether the alternator is putting out enough current to run the vehicle. If the engine cuts off, it is a sure sign that the alternator has died. What are your thoughts? Is this a car fire or blown fuse waiting to happen? Could this spike the electrical system and cause serious damage to the vehicle? One friend said that this could be traumatic to the car's electrical system. I am curious to know whether this is a good test. I've searched high and low, and I can't find anything concrete. -- Cory

RAY: You might need to be standing behind something concrete when you try this, Cory.

TOM: Actually, you can stand near the car, but you'll want to have your wallet out and at the ready. You'll certainly fry the voltage regulator, and possibly the alternator and other electronic components, too.

RAY: Here are the basics. The car's battery is just there to power the starter and get the car started. It took the place of the crank (which my brother, no doubt, remembers).

TOM: Yeah. I was married to her for 15 years.

RAY: Once the car starts, the battery's job is done. Electricity to fire the spark plugs then comes from the alternator, which is powered by the engine. And in fact, any additional current that the alternator makes is used to recharge the resting battery.

TOM: So in the old days, your test WOULD work. You'd disconnect the battery, and if the alternator and voltage regulator were working, the car would keep running. If they weren't working, and the engine needed power from the battery to run the spark plugs, the engine would die.

RAY: But when manufacturers switched from mechanical voltage regulators to electronic regulators, the test stopped working. How do we know this? Because of all the voltage regulators we've fried!

TOM: You take the battery cable off, you say, "Hey, it's still running ..." PFOOOFFF! "... What was that?"

RAY: So if you're concerned about your alternator (if your battery or generator light is coming on), take your car to a shop and ask your mechanic to do an alternator output test. It'll cost you a few bucks, but it's a lot cheaper than a new voltage regulator.

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