Alternator Humming with Key On and Engine Off? (Try this)

Note: The information provided here is only intended as a guide. You should always consult a qualified auto mechanic

With the car engine still off and key ignition turned on, your alternator should be as silent as a stone.

But what if, you instead hear a distinct buzz from under the hood when you turn on the ignition?

Should you be worried? What are the likely dangers of this?

This post explains the possible dangers that there can be if there is a hum from the alternator when the key is turned on, what checks you can do to identify the problem, and shares some tips to help you fix it.

Is this Normal? What are the Dangers?

No, this is not normal. In fact, it should be silent until the engine starts running and driving the alternator.

The Dangers

Some car owners who have noticed the alternator hum have also noted that their batteries lose charge.

So, not only are you at risk of not being able to start the car when you need to, there is always a risk that the repeated battery drain can cause permanent damage to the car battery.

You may also be interested in the post: Can an alternator charge a dead car battery?

What Checks to Do when you hear the Hum?

When you hear the hum (which can be hardly noticeable unless you are in a quiet environment), confirm that it is indeed coming from under the hood.

#1. Pop the hood open and listen intently. Is it from the alternator? You can confirm by removing the alternator plug first?

Does the humming stop? If so then it is from the alternator.

Note, that the hum can also be from the blower fan.

You can use an infrared thermometer to monitor the alternator temperature safety in case it is extremely hot.

Is the alternator warm or hot? Care must be taken when checking this as you may suffer severe burns and injury.

#2. Start the car and measure the alternator’s voltage output when the engine running. Is it in the recommended 13.5 -14.5 V range?

If so, then probably your alternator is still good. Note, that additional tests may need to be done at an Autozone as this test is not confirmatory.

Why the Car Alternator may make a Humming noise with Key on (+suggested fixes)

#1. Might there have been changes to the alternator wiring? Is the wiring in the harness connected the right way?

The car alternator can hum if it is not wired correctly. There are instances when disconnecting and firmly reconnecting the alternator plug has stopped the hum.

#2. The alternator is faulty. In some cases, an alternator with an internal faulty – possibly a faulty voltage regulator alternator may hum.

You can also have faulty yet brand-new alternators. So, have the alternator tested if it produces a humming sound.

Final Word

If you hear a buzz from the alternator when the engine is less, it is likely that there is a fault with the alternator. You should check the wiring to the alternator.

Confirm that the connections are firm and double-check the wiring especially if you have recently made changes to the alternator wiring. Don’t rule out a faulty alternator as alternators with internal faults can cause the hum.

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