Rick's Free Auto Repair Advice

How to Safely Test an Alternator with a Digital Multimeter

How to test an alternator using a multimeter

If you want to test an alternator all you need is a digital multimeter. There are some old-fashioned ways that many people still use on late-model vehicles, and those out-of-date methods can damage vehicle electronics. For example, disconnecting the battery cables while the engine is running is one of the best ways to fry your vehicle electronics instantly. Don’t do it!

Cardone, a major alternator rebuilder, has issued a service bulletin regarding the right and wrong ways to test an alternator.

The wrong way to test an alternator

Disconnecting a battery cable while the vehicle is running will immediately cause the alternator to go into full output mode which can generate a short voltage spike of up to 150-volts. You read that correctly. With no battery in the system to act as a capacitor/shock absorber that voltage is high enough to burn up every computer in the vehicle. Reconnecting the battery cable on a running engine also generates a second spike. DON’T use this method unless you feel like replacing very expensive computers.

How to test an alternator THE RIGHT WAY

1) The battery must read at least 12.2 volts with the engine off and no loads (headlights off, etc) before you can test it with a volt meter.  voltmeter testing alternator at car battery

2) 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.

3) Raise the RPM to around 2,000 and turn on all electrical loads; blower motor on high, seat heaters, defogger, headlights and wipers. Voltage should stay above 13-volts. If it does, then the alternator is working properly.

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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