Alternator Output Voltage Low when Idling? – Find out Why and What to Do

Is the car alternator output voltage low, dropping close to or into the red (below the 13.5V level) when the car is idling?

This is an issue that can happen to a new and recently installed alternator or with an existing alternator.

What should the output voltage of the alternator be at idle? Why is the voltage lower than expected and what can you do to fix it?

For answers to these questions and more, read on.

Normal Alternator Output Voltage at Idle

The expected alternator output voltage at idle, measured across the car battery terminals is in the 13.5-14.5V range.

This voltage range should be maintained even with the car accessories – headlights and radio switched on.

If it drops to lower than this then there’s a problem with the car’s electrical charging system that needs to be addressed.

What Causes the Alternator Voltage to Drop at Idle?

There are several reasons why the alternator voltage may be drop when the car is idling:

1. The car alternator fitted may not be suited to the car. Check the alternator specifications are suited to your vehicle.

It could be for instance that the factory alternator replaced with needs to be rotated at higher RPMs when the car is idling to supply a high enough voltage at idle.

If you’ve done a recent alternator change, make sure that the alternator pulley is the right diameter.

This is necessary for the alternator to rotate at the recommended minimum speed. You can contact the alternator manufacturer or a qualified car technician for support.

2. The voltage regulators in the alternator may be failing. A bad voltage regulator can cause a several problems including intermittent charging and under charging.

Note: These problems are not limited to failing alternator regulators and may be caused by other charging system faults.

To confirm if the alternator voltage regulator works or is defective, have the alternator tested at a reputable auto parts store.

3. The car battery ground connections may be loose or damaged. Inspect the car battery grounds from the battery negative to the chassis and from the chassis to the engine block.

Make sure that they are not corroded, or damaged as they can cause voltage drops too. Conduct a voltage drop test to confirm if the voltage drop is within the recommended limits and if not replace the ground wire connections.

4. The car’s throttle body may need cleaning. The throttle body is a mechanical valve that controls how much air flows into the car engine.

If it’s dirty, the airflow will be restricted, resulting in lower engine power which then leads to lower RPMs (at idle).

Contact a qualified auto mechanic to clean the throttle body.

How to conduct an alternator voltage ground test.

What you need: Good quality multimeter.

Testing the Negative ground and Positive circuit voltage drops

  • Start the car engine. Set the voltmeter to read voltage. Place the black probe of the multimeter connected to the negative terminal on a known ground connection for the car. Look for a stud on the car’s metal chassis.
  • Next place the red meter probe connected to the positive terminal on the alternator ground connection. The voltage reading should not exceed 0.2V. If higher, confirm that the connections are clean and make firm connections to the respective terminals.

If the voltage drop is still higher than 0.2V even after confirming the cable connections, then there is probably a broken or damaged cable connection that you need to identify and replace.

Can a Bad Alternator Cause a Car Engine to Fail when Idling?

Yes, a failing alternator will not be able to supply a large enough voltage to keep the car’s electrical system running especially at idle when the engine RPMs are low.

Should the battery voltage be low, the car engine will stall.

Related Questions

Does an Alternator Charge at Idle?

Yes, the car alternator can charge the battery even when idling. The car alternator supplies electrical power to the car’s electrical accessories even when idling including charging the battery.

Note: The car alternator is suited to maintaining a car battery as opposed to charging it when drained. To charge a drained battery, use an external car battery charger instead.

Why a New Alternator May Not Be Putting out a High Enough Voltage?

If the output voltage of the alternator is low:

  • check for voltage drops in the alternator wiring,
  • confirm that the alternator belt is in good condition, not slipping and
  • that you’ve installed the correct type of alternator for your car.

Closing Thoughts

The low car alternator output voltage may be attributed to wrong size of alternator, a bad voltage regulator, bad ground connections or a throttle body that needs cleaning.

Always contact a qualified car technician.

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