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How to check an alternator with a multimeter

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If your car’s engine is having trouble starting, or if the battery keeps dying, it may be time to check the alternator. The alternator is what charges the battery, so if it’s not working properly, the battery won’t stay charged. This can be easily checked with a multimeter, I’ll explain to you how.

First connect the multimeter to the battery terminals. If you’re testing a car battery, make sure the engine is off and the lights are off. Set the multimeter to DC voltage (usually 20V or less).Touch the red lead to the positive terminal and touch the black lead to the negative terminal.The multimeter should read around 12.6 volts. This is called an “open circuit voltage” and it means that the battery is fully charged.

Next, start the engine and rev it up a bit. The alternator is now providing power to the electrical system and charging the battery, so you should see an increase in voltage. With the engine running, the multimeter should read between 13.8 and 14.4v

What if the voltage remains the same or drops when you rev the engine?

If you’re running the car and the volts reading from the battery is still around the 12.6v mark, this means the alternator is not working properly. If your alternator is old it might be slowly dying and only doing half its job. Checking the exact amount of voltage that it’s providing is a good way to find out to condition of the alternator and how much life it has left.

If you see a voltage reading that’s higher than 14.4, this means the alternator is overcharging the battery and will damage it if left unchecked. It might be worth having a mechanic look at it if this is the case.

What Signs Indicate a Weak Alternator?

If the headlights on your car are dimming or flickering, this is a sign that the alternator isn’t providing enough power.

Another symptom of a weak alternator is if the battery dies quickly even after being fully charged.

If you notice any of these symptoms, it’s time to check the alternator with a multimeter.

How Often Should You Replace the Alternator?

Ideally, you should replace the alternator every 4-5 years or so. Of course, this depends on how often you use your car and how well you take care of it. If you drive frequently or for long periods of time, you may need to replace it more often.

When Should You Check the Alternator?

It’s a good idea to check the alternator every few months or so, just to make sure it’s in good working condition. If you notice any of the symptoms I mentioned above, it’s definitely time for a check.

How do you know if the problem is with the battery or the alternator ?

If the battery is more than three years old, it may be time to replace it regardless of whether or not the alternator is working properly. A common cause of battery problems is sulfation, which happens when the lead sulfate crystals on the battery’s plates grow too large. This can happen if the battery isn’t used often, or if it isn’t charged properly.

To find out if the problem is with the battery or the multimeter, first start with the alternator. If the car starts easily and the alternator is providing around the 14v mark when idling then it’s time to have a closer look at the battery. A battery that is sitting in a car before the car is running should show around 12.6v.

If you test your battery before running the car and its not holding this amount of charge, there’s a good chance that the battery cannot hold charge it doesn’t matter how much charge to try put into it. This can be caused my a number of issues such as:

  • Leaving the battery flat for an extended period of time
  • Old battery past its life cycle
  • The battery might have dropped cell

Checking the alternator with a multimeter is a quick and easy way to diagnose electrical problems in your car. If you have any questions send us a message or ask a professional mechanic for help. Thanks for reading!

Toby Ashby

I am an automotive specialist with over 20 years of experience in and around electronics