Rick's Free Auto Repair Advice

How to test an Alternator

How to test an alternator

If you want to test an alternator all you need is a digital multimeter. But there are so old fashioned ways that many people still use on late model vehicles and those out of date methods can damage vehicle electronics. One of the most damaging ways to fry your vehicle electronics is to disconnect the battery cables while the engine is running.

Cardone, a major re builder of alternators has issued a service bulletin regarding the right and wrong ways to test an alternator.

How to test an alternator the wrong way

Disconnecting the battery cables while the vehicle is running can generate a voltage spike as high as 150 volts! You read that correctly. As soon as you disconnect the battery from the circuit, the alternator switches to full output, and with no battery in the system to act as a capacitor/shock absorber that full alternator output voltage can spike to 150-volts, more than enough to fry every computer in your vehicle, along with your stereo, fuses, remote door locks, etc. Reconnecting battery cables generates a second spike.

How to test an alternator THE RIGHT WAY

The battery should be fully charged and when you attach a volt meter to voltmeter testing alternator at car batterythe terminals, it must read at least 12.2 volts with the engine off and no loads (headlights off, etc).

Next, start the vehicle and connect the voltmeter to the battery terminals. It should read between 13 and 15 volts. Be aware that newer vehicles DO NOT CHARGE the battery unless it needs charging!! So turn on several loads and wait for the computer to command charging. Then watch the charging voltage to see if it’s in the 13-15 volt range.

To read an in-depth story on testing an alternator, click on this article.

Here are additional article on alternators and batteries:

Best way to charge a battery

Symptoms of a dead or dying battery

Starter just goes “click”

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat



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