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Alternator checkup

I am a little familiar with how to use an multimeter. I took physics in college and am familiar with electrical terms. What i am wondering is how do i check my alternator to ensure that it is in proper working order…

Put the multimeter away when dealing with the large amperage of an alternator.
You need an amp meter to gauge the amount of amps your alternator is producing.


Check the voltage on the larger wire (held by a nut, usually). It should be about 14 to 14 1/2 volts dc at fast idle. Set your multimeter on ac and check for any ac on the output. It should be zero, or near zero. The auto parts store can do more tests, free, in the car. Just ask.

Do you 'spect a problem?

What is the trick with coiling a piece of wire a certain number of times(usually in multiples of 10) to increase the range of your amp scale. Its been so long since I did it and I just cant find it in any of my electrical books.Thanks

I’d also add: Turn on electrical accessories like the blower motor, rear defog, headlights, etc. The voltage shouldn’t drop too much–definitely not below 12.5-13 or so at fast idle.

You really need a carbon pile to be able to tell what the alternator is capable of. This allows you to load the alterantor down so it will produce its maximum.

Since this may not be cost effective for the occassional DIYer another method, somewhat inferior, is to load it down as much as possible with the accessories on the car. Turn on the lighting, A/C, wipers, etc. and test the battery voltage as the car is running at a fast idle.
It should be holding around 13 volts, depending on the load of course. With all accessories off voltage should be around 14 +/- a little.

That method is used in A/C voltage when the reading is below an amp. It is used to set the heat anticipator on an old thermostat.(the dial in the middle)

There are cheap amp meters that you can lay across the output wire from the alternator to measure current. These are basically mechanically the same as an automotive amp meter that deflects the needle by the magnetic field generated. You can put the wire next to the terminals of an automotive amp meter and get about the same results.

If your alternator is producing 13.5 to 14.8 volts dc with a good battery and enough time to let the battery voltage equalize, then there is nothing I would be concerned with unless the alternator has over 125,000 miles. Then you can worry about slip ring follower (brush) length and bearings. Could last a while or fail tomorrow. The only way to know about the follower length is to open up the alternator and compare the lengths to new ones. Bearings can be replaced then also and turn the slip rings in a lathe a little to smooth them. If the old electronics worked, I keep them as they are proven. For most, it’s easiest to throw in a rebuilt alternator rather than overhaul your old one.

Alternators are relatively easy to change. Another option is to keep a rebuilt in the trunk and tools to change it out.

An ammeter is not going to test the alternator for maximum output. The only way that is going to happen is with a carbon pile.