You’ve just replaced the car alternator with a new one and then the dreaded symptoms kick in – the battery light warning on the dashboard and when you finally get to check the battery voltage again, you find that both the resting battery voltage and that when the alternator is running are below 12V.
Perplexed? Is it likely that a new alternator can fail so fast? If so, what causes this, and what is a good way to stop them from failing prematurely?
Unfortunately, a new alternator can fail after a few days or weeks too.
So, in this post, you’ll learn why a new car alternator can fail so fast, reliable ways to test an alternator, and also get tips to keep your car alternator working for as long as possible!
Why a Brand New Alternator Can Fail
Keep in mind that it is possible to receive a faulty alternator from the dealership. Beyond this, it could as well be that the underlying fault conditions that caused the previous alternator to fail in the first place are yet to be addressed.
The common reasons why even a new alternator can fail are listed below.
#1. A damaged car battery or battery is deeply discharged. If the car battery is faulty, for example, if it has a short or if the battery voltage is just low, then using a new alternator to charge it can result in its failure.
In either case, in an attempt to charge the battery, the alternator will supply a large current for longer which results in overheating and potential early failure.
Alternators are designed to keep the battery topped up and can fail if they have to supply high currents for a long time.
#2. The ground connections are loosely connected. A bad ground increases the charging circuit resistance which restricts the charging current of the battery.
The alternator takes longer to charge (because of the restricted current flow) which can lead to excessive temperatures which is generally bad for alternator longevity.
#3. Some alternator replacements are not of good quality and don’t last long. Whenever possible buy an OEM alternator or one that has genuine 4 or 5-star reviews to minimize the likelihood of early failure.
Signs that a Car Alternator is Faulty
Though not in themselves conclusive, the signs below can point to a bad alternator:
#1. The battery warning light comes on and while the battery voltage measured with the car engine running should be in the 13.5 – 14.5V range, it is instead under 13V.
Of course, it could as well be that there is a bad cable or connection that is causing a voltage drop too.
Another sign might be that the battery warning light comes on and the engine dies down possibly when the car battery charge runs out. This too could be caused by loose battery connections too.
#2. The alternator fuse blows upon installing the new alternator. Double-check the cable connections to rule out an accidental grounding of the alternator output.
How to Test an Alternator
Quick Indicative Test (Indicative and Not 100% Reliable)
Use a good-quality multimeter to measure the alternator output voltage. Connect the multimeter (set to read DC voltages) probes to the positive and negative battery terminals when the car engine running.
If the alternator works well and the cable connections are in good condition and firmly fixed, the voltage reading should be in the 13.5-14.5V range.
You can also have the alternator tested at an auto repair shop. This can be more reliable if it includes a load test on the alternator.
Tips to Stop it Failing
#1. Avoid charging deeply discharged or faulty batteries. If the battery is deeply discharged, charge it off a car battery charger first to avoid undue stress on the alternator.
#2. Do not attempt to charge faulty or damaged batteries as they can cause the alternator to fail. In fact, if you have to replace an alternator, have the car battery tested too as if it is faulty too, it can contribute to alternator failure.
#3. Periodically inspect and confirm that the alternator and battery positve, negative, and ground connections are firm and make clean contact to avoid voltage drops that too contribute to alternator failure.
#4. Confirm that the replacement alternator is able to provide the power output requirements of the car. An undersized alternator will be overloaded and will likely fail easily.
How Often to Replace an Alternator
A good quality alternator, with solid, firm, and clean connections and a well-maintained battery can last for 10+ years and over several hundred thousand miles before it fails.
Depending on how long you keep the car, you may not even need to do a replacement.
You may consider replacing the brushes or bearings at some point to extend the performance of the alternator. Contact a qualified auto mechanic to guide you on this.
You may also be interested in this post: Car alternator bearing noise – what you need to know
To stop your new car alternators from failing, always check and confirm that the ground connections are clean and firmly connected.
Avoid charging deeply discharged or damaged batteries using the car alternator, and buy well-regarded quality alternators whenever possible.