Rick's Free Auto Repair Advice

The safe way to test an alternator

How to test an alternator

Disconnecting a battery cable while a car is running to test an alternator can damage your car’s computers and computerized controls. You could do that on non-computerized cars, but not on any car since the early ’80’s.

What happens if you disconnect battery terminals while engine is running?

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 car battery light onneed charging. But once you turn on headlights, a rear defroster or a blower motor, you begin sucking power out of the battery.

The voltage regulator sees that battery drain and sends battery voltage and current 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.

Exploded view of car alternator showing voltage regulator, diode rectifier, front and rear alternator bearings, alternator rotor and stator and housings

If you disconnect a battery cable while the car is running, the regulator sees that as total battery drain and immediately commands the alternator to run at maximum output. In effect, the alternator is now in a “runaway” condition in which it can generate upwards of 140 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. While running at that speed, 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. Next, turn on the blower fan and recheck voltage. Continue turning on accessories while idling at 2,000 RPM. Voltage should never drop below 13-volts.

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.

© 2012 Rick Muscoplat

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Posted on by Rick Muscoplat


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