Signs of Car Alternator Failure

Updated March 23, 2017

Alternators are essential components to a car's engine, since it houses the power train which is the source of power. Without a properly operating alternator, the vehicle's headlights, air conditioning and system will all fail. Therefore, it is important to know and understand the symptoms of a bad or failing alternator so that it can be repaired or replaced quickly.

Indicator Service Light

On the dashboard of most every vehicle manufactured since the early 1980s is a light, which indicates the condition of the alternator. If the alternator indicator light flickers or if it stays on one minute and then fades out for minutes at a time, it could indicate a warning that the alternator is failing.

Battery Voltage Output

A sure sign that a car's alternator is not functioning properly is a low functioning battery. The voltage output of the battery can be checked through the usage of a hand held digital multimeter or DC voltmeter, which can be purchased at most any auto supply store. If the vehicle's battery is consistently functioning on a low level (less than 13V) and diminishing, this indicates the car's alternator is at least partially, if not fully failing. While another cause could be that the battery itself is bad, it is important to have the alternator checked so that the problem can be correctly diagnosed.

Reduced Accessory Performance

Another symptom of a faulty alternator is the diminished performance of the vehicle's electrical accessories. Reduced performance of the car's electrical accessories, such as the head lights, radio, dashboard lights or starter may indicate a low charging battery, which can be the result of a faulty alternator. The voltage on alternators can be checked through the use of a handheld digital multimeter, which can be purchased at most any auto supply store. A properly functioning alternator should read at levels between 13.8 - 15.3 of voltage. If a bad alternator is the cause, the vehicle will experience a gradual shutdown of all electrically operated accessories.

Unusual Engine Noise

Another warning sign that the vehicle's alternator may be failing is the presence of unusual noise coming from under the hood. When a failing alternator leads to failure of the bearings, it may cause a rattling or spinning noise coming from the location of the alternator. This sound is normally a strong indication that the alternator is failing. Therefore, the vehicle should be inspected by a trained mechanic to properly diagnose the problem.


If the vehicle's engine begins stalling, that's one of the strongest indications that the alternator is shot. A stalling engine can be a major indicator that your alternator is too weak to keep the car running. Stop driving the car and be sure to have a mechanic or technician check it right away in order to prevent additional damage to the car. Continuing to drive the car in this condition can cause the motor to overheat and can lead to major problems such as a blown engine.

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About the Author

Rebekah Worsham began writing professionally in 2007 and has been published on eHow. She has expertise in the fields of law, parapsychology and the treatment of drug and alcohol addiction. She holds a degrees in law from Beckfield College.