What Causes a Car Starter Relay to Vibrate (Buzz)? (Tips to Fix)

Is your car struggling to start and instead there’s a loud buzz from the car starter relay?

You may hear the buzz even when you have to make start attempts before the car starts or even fails to start altogether.

If this is what you’re experiencing, read on. In this post, you’ll find possible reasons why the car starter relay vibrates and what you can do to fix it and get the car starting again without a buzz.

Why a Car Starter Relay Buzzes (+Fixes)

If the car starter relay buzzes, it may be because of the following:

#1. Loose or damaged wire connection(s) from the car battery to the starter. Inspect the wire connections for any damaged or frayed cables. Look for signs of corrosion.

Make sure the wire connections from the battery and starter are clean and firmly connected. You can also carry out a voltage drop test.

How to Conduct a Voltage Drop Test

You’ll need two voltmeters (or Multimeters set to read 12V DC).

  • Connect the first voltmeter to the starter positive terminal and starter ground.
  • Connect the second voltmeter to the battery positive terminal and the battery negative post.
  • Next, crank the engine and note the voltmeter readings from both voltmeters.
  • Subtract the reading from the first voltmeter from the second voltmeter. The difference should not be greater than 0.5V

A higher voltage drop indicates a significant voltage drop that can lead to car starter problems.

Check for loose or damaged wire connections including bad ground connections on both the starter and battery side.

#2. A weak or faulty battery. The starter relay may be buzzing because the battery voltage is not high enough for the contacts to close.

You can measure the car battery voltage using a digital multimeter set to read DC volts as an indicative test though not always accurate.

A fully charged battery has a resting voltage of about 12.6V. If the car battery voltage is less than 12V then it’s likely because the battery voltage is low (or at least one of the reasons).

Additionally, you can also have the battery tested at a reputable auto repair shop.

If the battery voltage is low, use an external charger, or if faulty replace it with a new battery.

#3. Faulty solenoid or starter. It is possible that the starter may be faulty. While there are tests one can do such as directly powering the starter by connecting it to the car battery directly, I’d recommend that you contact a qualified auto mechanic for assistance.

If the starter or solenoid is faulty. Replace the faulty solenoid or starter and check if this clears the problem.

Closing Thoughts

A buzzing sound from the car starter may be a result of loose or damaged wire connections, a weak or damaged battery, or even a faulty starter itself.

A voltage drop test can be used to check for any loose wire or bad ground connections at the starter or battery side.

Hope this helps!

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