Rick's Free Auto Repair Advice

Alternator failure — most common causes

What causes alternator failure?

Top 3 causes of alternator failure

You can prevent premature altnerator failure by knowing the 3 most common causes of alternator failure.

#1 Cause of alternator failure — High heat from excessive use

Alternators are designed to operate under high underhood heat conditions. But they’re not designed to generate extremely high loads for prolonged periods of time. Charging a severely discharged battery can overheat the rotor and stator windings, causing the insulation to degrade and short. The high generating load also overheats the diodes, causing them to fail early. Finally, high generating loads overheat the grease in the front and rear bearings, causing them to fail early.

What is considered excessive alternator use?

Using your alternator to charge a severely discharged battery

The alternator’s job is to recharge the battery to replace the electrical power lost during startup and to provide the power needed to run electrical accessories while driving, like your headlights, wipers, blower motor, defogger, heated seats, etc.

The power needed to recharge your battery from a normal startup is a fraction of the power needed to fully recharge a severely discharged battery. If you use your alternator to recharge a severely discharged battery instead of using a battery charger, you will shorten the life of the alternator. 

battery charger

One example of a logic-based battery charger that’s safe for modern vehicles

Your alternator can handle short periods of high output to recharge the battery for power lost from a normal start. But recharging a dead battery is a completely different story. In fact, it can take up to a full hour of driving to fully recharge a dead battery. That kind of high output can kill your alternator.

Recharge a dead battery with a battery charger, not your alternator

The numbers are simple. A modern battery charger costs less than $100. A typical alternator costs over $400 including labor. It makes no sense to kill your alternator when you can recharge it with a battery charger.

Stop and go driving wilth high electrical loads is considered excessive use

A typical alternator in a late model vehicle is capable of outputting up to 160-amps when running at engine speeds of 1,500- 2,000 RPM. That’s enough power to replace battery power lost from starting and to run all electrical accessories. However, your alternator can only output about 1/3 of its maximum capacity when the engine is running at idle speeds and low RPMs. Here’s what that means:

If you run all your electrical accessories in stop and go traffic, your alternator can’t provide all the power and it will have to come from the battery, discharging it at a rapid rate. And which component recharges your car battery? Yeah, the alternator.

Continued cycles of stating, running high loads in stop and go traffic or short run times will not only decrease the life of your battery, but cause your alternator to run hot and shorten its life.

#2 Cause of alternator failure— high electrical resistance

corroded battery terminals

You can have high resistance at your battery posts and terminals even without this kind of buildup.

Battery post and terminal corrosion is the #1 cause of high electrical resistance. The high resistance causes the alternator to run at a higher voltage to overcome the resistance, which causes alternator overheating. Battery post and terminal corrosion is not always noticable with a visual check. In other words, the posts and terminals can have high resistance even when they’re not covered in a powdery coating.

#3 cause of alternator failure—a worn belt tensioner

Engines don’t produce a smooth steady stream of power. It’s just the opposite, engines produce a power pulse every time a cylinder fires. It’s the job of the harmonic balancer and the damper inside the automatic belt tensioner to smooth out the power pulses.

The dampener inside the automatic belt tensioner has a useful life of slightly less than 100,000 miles. Once it fails, the belt tensioner will “dance” slightly with each power pulse and that will cause the belt to tighten/loosen rapidle. The belt dance causes vibrations that can prematurely wear out the bearings in your alternator, AC compressor and power steering pump.

What causes repeat alternator failure?

Excessive resistance. In addition to battery terminal corrosion, internal battery resistance can dramatically reduce the life of your alternator. Even if your battery voltage tests at or near a full state of charge, internal resistance may still be too high.

How to check for high internal battery resistance

A voltage and load test tells you what’s coming out of the battery, but it doesn’t tell you how power is flowing into the battery. That’s where a charging amp test comes into play.

Connect an amp probe to your fully charged battery and start the engine. You should see an inrush of approximately 40-50-amps for about 5-mins. The amps should drop down to below 10-amps by the 5-min mark. If the amps remain high and all the cables and posts are clean, the battery has too much internal resistance. A current draw of just 28-amps for longer than 5-mins can kill a replacement alternator in as little as 3-days.

©, 2020 Rick Muscoplat


Posted on by Rick Muscoplat

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