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When Is It Time To Replace My Alternator?

We discuss the important role alternators play, how they wear, signs of trouble, and how to test the output of one with a multimeter.

What Is An Alternator?

Bolted to an engine block and powered by a belt-driven pulley, an alternator is in motion whenever the engine is running. Essentially, a vehicle's battery is tasked with storing electricity to start the engine, while the alternator's job is to produce electrical power for the battery, ignition system, and other vehicle accessories.

Alternator In Action

Cars have almost always had an engine-driven means to recharge the battery. For years, the job was performed by a generator, which produced DC voltage. In the early 1960s, the AC-alternator replaced it, and that updated unit is found on every vehicle today. The primary advantage of an alternator over a generator is greater output, necessary for today's vehicles with their high electrical demand.

Alternator Components Scheme

If you were to open an alternator and look inside, you'd find a series of metal brushes attached to a rotating piece known as a "rotor". This rotor piece spins within a coil assembly known as a "stator", which contains wound copper coils. As the brushes pass by those stator coils, an electrical charge builds up.

That charge is then sent from the coils through a voltage regulator and rectifier that converts it from alternating current (AC) to evenly-flowing direct current (DC). Finally, the DC current exits the alternator via a main terminal on the housing where it travels through a heavy-duty cable to the battery.

Over time, these alternator components are subject to wear, engine heat, and the outside environment - ultimately needing replacement no matter how well the vehicle is maintained.

Alternators typically have a long lifespan, so they're not considered a preventative maintenance item that's automatically replaced at preset intervals. However, usage, climate, and other outside influences can affect their lifespan. In this article, we'll discuss things that cause alternator wear, how to recognize symptoms of a failing alternator, and how to test the output of an alternator with a multimeter to confirm if replacement is necessary.

Things That Cause Alternator Wear

The most common cause of alternator failure is when bearings that allow the rotor piece to spin freely wear out. As these long, flat "needle" bearings slowly wear down, they can no longer support the rotor in the flush and level position it must be seated in to spin freely. The pulley then sits at an odd angle, causing uneven wear and fraying of the accessory drive belt.

Alternator Worn Out Bearings
The most common cause of alternator failure is worn-out bearings.

Bearings can wear faster if excessive pressure is put on them by a drive belt that's too tight, or by engine fluids leaking onto the alternator.

A battery that won't take a charge properly because it's at the end of its own lifespan can also cause issues for an alternator as it works harder to try to keep the battery charged.

High-End BMW Equipped With Air Suspension System
With their heavy electrical draw, accessories such as air suspension systems can be taxing on an alternator - especially if the pump must work harder because of a leak in the system.

Additional accessories with heavier amounts of electrical draw can put extra strain on an alternator. Air suspension pumps and hardware are a good example - especially if they're required to work harder because of a leak in the system. Booming stereo equipment and related components can also be quite taxing. And if you've got powerful off-road lights, they use a bit of juice too.

Driving Through Snow Storm In Wintry Climate
Wintry climates that see heavy use of blower fan defrosting, electric window defogging, heated seats, fog lights, and more can really put an alternator through its paces.

Naturally, cold and damp climates that require heavy use of the defrost system means that the a/c compressor, electric window defrosters, heated seats/steering wheel, and blower fans are in use frequently. Similarly, hot climates see the heaviest use of air conditioning and fan use. All of these put an alternator through its paces.

Signs Of A Malfunctioning Alternator

When an alternator is going bad, its output can be irregular. Most noticeably, headlight output may fluctuate between bright and dim, or remain dimmer than usual. Interior lights are also subject to this.

Low Battery Warning Light
A "low battery" warning light is often one of the first signs of alternator trouble.

It's important to note that a reduced voltage condition can affect the entire vehicle. Electronics and on-board computers become confused, resulting in erratic and unpredictable performance of everything from power windows to diagnostics to the radio. A low battery warning light is usually the first one to come on - but other warning lights may appear for phantom problems that don't exist.

An alternator with bad bearings will make an audible growling noise as it spins. Open the hood and listen closely near the alternator with the engine running. If bearings are indeed the problem, you'll be able to hear them over the sound of the engine. You may even be able to hear bad alternator bearings from inside the vehicle.

Although not as common, an alternator on its last legs can produce an acrid burning smell. The reason may be burned out brushes or coils, or it could be from the accessory drive belt slipping over a pulley that's dragging because of bearings that have given up the ghost (this might also cause a rather noticeable squealing noise).

Determining Battery Vs. Alternator Issues

Jump Starting

As we mentioned, a failing alternator can affect the whole vehicle. But most importantly, it can cause the battery to drain and die as the ignition system and electronics continue to draw from it. What may seem like nothing more than a bad battery can have deeper roots. But how do you determine if a dead battery is just a dead battery, or if it's the fault of the alternator?

One way to find out is to jump start the dead battery. If the engine starts then stalls shortly after the jumper cables are removed, then it may be that the alternator is not providing electrical power to keep the vehicle running.

Testing An Alternator

Any time professional technicians come across a dead battery, they test both the battery and the alternator. First, a battery tester will be used to confirm if the battery is still capable of holding a charge, or if it has dead cells and must be replaced. Once the battery has been either recharged or replaced, the output of the alternator is checked with a multimeter. To use a multimeter for testing, set the knob to a number greater than 15 on the DCV (DC volts) section.

Alternator Tested By Professional Technician

A fully charged battery at rest should measure between 12.6 and 13.2 volts. With the engine running, the alternator must exceed that voltage at the battery. These initial tests should include basics like cleaning and tightening all wiring connections, especially the main battery terminals. Check the accessory drive belt to ensure it's got enough tension to spin the alternator pulley without slippage occurring.

After starting the engine, touch the positive lead of the multimeter on the positive terminal of the battery. Then, touch the negative multi-meter lead to the negative battery terminal. A numerical voltage output reading will be displayed on the multi-meter. Ideal output for an alternator at idle is between 14.2 and 14.7 volts. A reading of 15 volts or higher signifies a problem with the alternator and/or the voltage regulator, which is supposed to set a maximum upper range to avoid overcharging the battery.

If the voltage is in the 14 range, switch on some accessories inside the vehicle such as headlights, blower motor, and the radio. Under this slightly heavier load, the output will drop a little bit (maybe into the 13 range), but it should remain steady and not dip below 13.

If the reading is below 13 volts, the alternator is not charging the battery enough to keep up with the use of accessories. Also check all wiring between the alternator and battery to confirm wires are in good shape, without cuts, kinks, or visible signs of corrosion. Have an assistant rev the engine up to around 1500 rpms to see if voltage output increases. If it stays the same, the alternator is probably to blame.

Opportune Times To Replace An Alternator

Since the typical alternator is bolted directly to the outside of the engine block, removing and replacing one is relatively simple in terms of labor. If your engine is equipped with a serpentine accessory belt, you'll just need to release the pressure on the belt tensioner and then remove the belt. On older motors with individual accessory belts, the alternator mounting bracket is designed to allow the alternator to slide away from the belt to release tension after anchoring bolts are loosened.

Older Engined Outfitted With Alternator
Shown here is an older engine with an alternator that can be loosened and slid within its bracket in order to adjust belt tension.

A best practice is to conduct an annual charging system check. With a multi-meter, check the battery at rest, then check the alternator output. Any early warning of a failing alternator can save you headaches by avoiding a potential roadside breakdown.

Normally, any time an accessory drive belt is removed would be an opportune time to swap in a new alternator. We have heard of some extreme cases where owners replace the alternator every 6 years, no matter its condition. That may be overkill, as the part is not inexpensive. We've helped to lower the cost of purchasing an alternator by offering a choice between new and quality remanufactured units. For more details on value-priced options, we invite you to read our related article on this topic.

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