How to Test an Alternator With a Volt Meter

Test your alternator at the first sign of trouble.

Alternator output should be stable for a vehicle to perform properly. The level of output should only vary with load and engine speed. An over- or under-charging alternator can quickly lead to all sorts of odd troubles, up to and including failure of related components. If you suspect your alternator is failing, you can perform a quick test with a voltmeter and a helper to determine the alternator's status. While most automotive repair facilities can perform a far more thorough test on the alternator, performing this test with the voltmeter can give you a solid idea of the alternator's current condition.

Things You'll Need

• Safety glasses

Instructions

1. Raise the hood and locate the alternator. Put your safety glasses on and then connect the voltmeter to the alternator.

2. Start the engine, turn on all the inside electrical accessories, such as the radio and lights, and have your helper hold the throttle down so the car idles at 1200 rpms.

3. Read the display on the voltmeter to get the reading of maximum output. This should be within a few points of the alternator's top output. Hence, if you have a 110-amp alternator, you should have a reading between 105 and 110. You can determine your alternator's top output by referring to the specific service manual for your year, make and model of vehicle. The output is often, but not always, stamped into the alternator casing.

Tips & Warnings

• You should also test the battery during this procedure and replace it with the alternator if it isn't in peak condition. A bad battery can drag down a new alternator quickly by forcing it to put out peak voltage at all times, so it's best to make sure both are in the same, top condition.

• Do not hold the alternator at full capacity with the throttle at 1200 rpms and all the accessories on for more than 30 seconds. This can overheat and burn out the alternator, forcing you to replace it, whether it was faulty before you started the test or not.

References

• 2 Car Pros: How Do I Check My Car Battery?