Testing an Alternator Without Blowing it Sky High

Nothing makes me crazier than reading advice that tells readers to disconnect a battery cable while a car is running to see if the alternator is any good. It’s not only bad advice, but could cost you big time in fried computers.

Here’s why: To generate power, you pass a coil of wire across a magnet. It’s basic physics #101. In an alternator, they turn things around. They rotate a magnet past a stationary coil of wire. The key here is that the magnet is an ELECTO magnetic. That’s right folks, you need a good battery to provide the power to make the electro magnet work in the first place. An alternator cannot make power if your battery is dead. This is why you cannot charge a dead battery with the car’s alternator. No 12 volts in=No 13.5 volts out. Back to the story. So when you start your car, the voltage regulator takes a look at battery voltage. If it’s at 12.7 volts, the regulator determines that the battery is in good shape and doesn’t need charging. But once you turn on headlights, a defroster grid, or a blower motor, you begin sucking power out of the battery. The regulator sees that battery drain and sends battery voltage into the “rotor” of the alternator. The rotor is the rotating electro-magnet. The “stator” is a coil of wire that’s stationary and is where the electricity is produced.

If you disconnect a battery cable while the car is running, the regulator sees total battery drain and immediately commands the alternator to run at maximum output. In effect, the alternator is now running wild. In this state, it can put out upwards of 40 volts. That’s enough voltage to fry your powertrain computer, your ABS computer, your radio, and every other computer in your car. It’s also enough to fry the guts of the alternator itself.

Are you getting the idea here? That it's not a good idea to send the alternator into ‘full tilt?”

So, how DO you test an alternator? Simple. Get a volt meter and attach it to the battery. Make sure you have at least 12.2 volts before you start the test. If the battery reads less than 12 volts, you will never be able to test the alternator. If the battery voltage checks out, start the vehicle and rev it up to about 2,000 RPM and turn on the headlights. You should see voltage go to at least 13 volts. If you vary the RPMs, you should see the voltage fluctuate between 13 and 14.5. That’s the sign of a good alternator—volts wise.

As for an amperage output test, that’s a little more difficult. You need special equipment for that. That’s where an auto parts store comes in handy. They usually test alternators for free.

While we’re on the subject, let’s talk about jump starting. If you followed my description so far, you see that the minute you attach jumper cables from your car to a dead battery, the voltage regulator will send your alternator into full output mode. Unfortunately, you car is probably running at idle speed of only 700 RPM. What you’re doing, is asking the alternator to give you everything it’s got, but at low RPM’s. That’s a great way to overheat your alternator. Sure, you can “goose” the pedal and run your RPMs up to 2,000 and that will help. But think about what happens once the alternator in the “dead” car comes to life? Now that alternator will be putting out maximum output and throw a surge of power back to your vehicle. Get the point here? It’s just not a good idea to be jump starting other people’s cars—unless you own stock in an alternator rebuilding company.

One final note. If your battery is dead and you find someone (who hasn’t read this article) to give you a jump, make sure you get in and drive your car once it starts! Trying to charge up your formerly dead battery while you sit inside it with the heater running full blast, well, it just isn’t going to work. Trust me, you’ll be buying a new alternator in the near future if you do that. With the newer alternators going for about $200, you’re much better off driving your vehicle right home and throwing a $25 battery charger on it overnight. That’s what a battery charger is for—charging your battery.
©, Rick Muscoplat

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