If you have trouble starting your vehicle, if you are met with growling or whining noises once it does start, or if your vehicle’s lights start to go dim or are overly bright, it’s possible your alternator might be nearing the end of its functional life. You can test this yourself. We reached out to our readers to see how they recommend checking your alternator. Keep reading to see what they suggest.
Mark Beneke

Mark Beneke

Mark Beneke is the owner of Westland Auto Sales in Fresno, CA.

Steps to Check Your Car’s Alternator

Step 1: Find the alternator.
Alternators tend to be located by the engine.

Step 2: Check the connections, fastening equipment, and serpentine belt.
If any connections are found stripped or worn, they should be replaced. If the alternator itself is loose, it should be fastened correctly to prevent it from losing proper grounding. The same can be said for the serpentine belt.

Step 3: Keep the alternator clean.
Since the alternator produces electricity, it’s very important to make sure they are kept cool. The less it needs to work, the lower the chances of it overheating. Any gunk or dirt over its shell needs to be cleaned to make sure it does not keep heat trapped.

Step 4: Prepare a voltmeter.
To check if the alternator is working properly, you can purchase an inexpensive tool called a voltmeter, which can help you test the battery as well. Set the meter to the correct dial (DC is usually what you’ll set it to and anything higher than 15 volts), then place the probes over the battery connections (red on the plus, black on the minus).

Step 5: Test your alternator
With the car off, it should read between 12.5 to 12.8. If it’s below that, your battery may be weak and need a proper charge to continue. Start the engine and make sure it reads between 13.8 to 15.3 volts with all lights, radio, or additional equipment off. If it does, the alternator seems to be doing its job.

This is a rough test and is not perfect, but at least it’ll give you a very good idea of whether you have an issue that should be further looked into.

Many auto parts stores offer to test your alternator for free with more sophisticated equipment. If you’re not comfortable using the voltmeter method, you can take advantage of that if your local parts store offers this.

What to Measure to Check your Car’s Alternator

Most of the time I perform a visual inspection every six months to check for any signs of damage. These could include dark marks, loose electrical wire connections, or burnt rectifier diodes.

Following this, I use my multimeter to measure the AC ripple voltage on the output of the rectifier box. (I ensure that the battery is fully charged before performing this multimeter test.)

Typically, a good battery should provide a 12V reading. I then measure the AC ripple voltage on the alternator. What I have typically seen is if you get an AC measurement of between 50 to 100 mV, and the battery is fully charged, there is a good chance that your rectifier or alternator itself needs replacement.

Mogale Modisane

Mogale Modisane

Mogale Modisane is the founder of toolsgalorehq.com, a premium online resource for all things tooling. He is also an electrical engineer specializing in renewable energy systems.
Tommy Pederson

Tommy Pederson

Tommy Pederson, CEO of Vekhayn. He is also a bodybuilder and cars guru.

A Free and Simple Way to Check a Car’s Alternator

Your local auto parts store will have a free alternator/battery testing tool that takes three minutes out of your day and doesn’t require any mechanic work to be done with your car.

It’s simple. You just drive up to the shop and ask them to test your battery and alternator. It’s completely free and you aren’t expected to purchase anything at the store either. The store attendant will come out and ask you to pop your hood. They attach a simple clip to your battery cables and will ask you to press the gas on your car a little bit.

After a minute or two, the device will tell you the health of your battery and alternator. It will tell you if your alternator is charging the battery correctly, what the voltage is, and/or if either the battery or alternator is showing early signs of failure.

This simple and free tool that takes three minutes out of your day can save you hundreds in a tow truck down the road.

If you do find an issue, you can just buy the alternator or new battery right there and swap them out in the parking lot. Anybody can do this; it is super simple. Usually, the employee at the store will even give you a manual on how to do it. This will save you hundreds at a mechanic shop, a tow truck, and keep your car in good condition.

Another thing to keep in mind is that if the alternator is bad, it will cause the battery to deteriorate much faster, too. Try and keep tabs on your alternator so you can catch it before it costs you a battery, too.

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