Top 5 Signs of Alternator Problems

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It all starts with a few telltale problems: A dash light coming on for a brief moment, maybe dimmed headlights and a few flickering gauges. Perhaps there's even an odd smell, or a growling sound coming from under the hood.

Is this a case of automotive possession? No. Most likely it's one of many possible alternator problems, and without a little attention, this problem can cause car trouble ranging from slow starts all the way up to a dead car.

While an alternator is a relatively simple component containing only a few parts, it plays a critical role in any vehicle's operation. Essentially it turns the mechanical energy of the engine's rotating crank shaft into electricity through induction. Wires within the alternator cut through a magnetic field; this in turn induces electrical current. That current is used to power your car's accessories, which can be anything from headlights to the electro-hydraulic lifts on a snow plow. The alternator also keeps the battery fully charged, providing the power it needs to start the car.

Because the alternator is connected to (and critical for) other vehicle systems, any mechanical auto problems can have an affect on its function and have an affect on diagnosing car problems. However, paying attention to the five trouble signs on this list can make diagnosing car trouble related to the alternator a little easier.

Keep reading to find out what you should be on the lookout for.

Warning Light
Keep an eye on your dashboard warning lights for an early indication of possible trouble.
Keep an eye on your dashboard warning lights for an early indication of possible trouble.
© Jackson

Within the instrument cluster of most cars built in the last decade is a warning light dedicated to signaling an alternator issue. In most cases the light is shaped like a battery, though some show "ALT" or "GEN," meaning alternator or generator, respectively. Many people see this light and instinctively think they have a battery problem, which is symptom that will be covered later, but that's not really why the light goes on.

This light is linked to computer systems within the car monitoring the voltage output of the alternator. If the alternator's output goes below or above a pre-set limit then the dash light comes on. Once the output is within range the light remains unlit. In the early stages of alternator problems the light can seem to flicker -- on for just a second and then off again. Or maybe it lights up only when accessories are activated. For instance, let's say its nighttime and the headlights are on and everything is working just fine. Then it begins to rain. As you flick on the windshield wipers the warning light comes on. Turn off the wipers and it goes away. While that may initially seem like an aggravating problem, the warning light is doing its job exactly.

Most alternators have an output between 13 and 14.5 volts that they try to maintain at a constant level. As more power is demanded by the headlights, the windshield wipers, your radio, the heated seats, the rear window defroster and so on, the alternator needs to work harder to maintain the necessary voltage. If your car's alternator is not working to its full potential, or demands are placed on it that it can no longer meet, the voltage will either go above or below the set level and switch on the warning light.

Dim Lights
Dim headlights are often a sign that your alternator is on its way out.
Dim headlights are often a sign that your alternator is on its way out.
© Radian-Alexandru

Since the alternator supplies the vehicle's electrical needs, when it begins to lose its potential so do the accessories that draw on that electricity. Drivers may begin to experience erratic symptoms ranging from dimming or extremely bright headlights and dash lights, to speedometers and tachometers that simply stop working for no reason. Other accessories, like heated seats or power windows may experience a slowdown as well.

A driver's exact experience is usually dependent on a number of factors. The first is how well the alternator is still producing power and also where it is in its death cycle. The second is how the car is programmed. In most new vehicles auto manufacturers have a sort of preprogrammed priority list for where electricity will be sent just in case an alternator problem arises. This is usually based on safety considerations. For example, the heated seat will switch off, or the radio will quit before the headlights dim and fade away. The reasoning behind this is that a driver would need to be able to see in order to safely pull over and stop if the car suddenly quit working -- but they definitely wouldn't care about a heated seat or what's on the radio at that point.

Sight, Sound and Smell

These symptoms are grouped together as signals from the senses. They're less definitive than a warning light; however, these signs of alternator trouble can tip off a driver that there's something going on that shouldn't be happening:

  • Sight: Mechanics are accustomed to customers complaining of classic alternator issues, and then opening the hood to find the belt missing, hanging off the engine block or so loose it's flapping around the engine compartment. A quick visual check of the belt for cracking, excessive wear and other age issues can give an indication of a future problem. Also, the belt must have the proper tension to run the alternator correctly; too much tension is just as bad as not enough. A quick check of the belt tension is usually enough to determine if a problem exists.
  • Sound: Drivers will sometimes complain about hearing a "growling" or "whining" noise before an alternator gives out. The alternator is driven by either an accessory belt or a serpentine belt in conjunction with the crankshaft pulley. The alternator pulley typically spins about two or three times faster than the crankshaft pulley to produce the power necessary at lower engine speeds, such as at idle. The alternator pulley spins on a shaft, which in turn is supported by either bearings or bushings. If the pulley is not in correct alignment with the belt, if it's canted on the shaft or if the bearings and bushing are worn out, the growling or whining noise will let you know there's a problem.
  • Smell: The scent of burning rubber or hot wires will often accompany alternator failure. A pulley that isn't in alignment or not turning freely will cause more friction on the belt, which creates heat and then the smell of burning rubber. The hot wire scent can be caused by an overheated alternator, one pushing too much power through the rotor and stator.
A Dead Battery
A bad battery or a bad alternator?
A bad battery or a bad alternator?
© Henry

While not technically an alternator problem, a car that won't start is a definitive notice from your car that there's a problem in the starting system. When the key is turned, you'll hear a tell-tale clicking of relays ticking over but nothing else happens. After a time, even that noise stops. The immediate problem is usually a dead battery; but the question a driver should ask is, "Why is it dead?"

When an alternator begins to fail, or fails outright, the car's battery begins to take up the slack rather than acting as a capacitor for the system by receiving a constant infusion of electrical power from the alternator. However, even the best car battery will run down eventually leaving a driver stranded in the driveway or worse, on the side of the road. Car batteries are not designed for long-term power use. They're designed to provide the vehicle with enough electrical oomph to start by juicing the starter motor with a surge of power and getting the whole works spinning.

Diagnosing a dead battery versus a dead alternator is relatively easy. Simply jump-start the car and then remove the cables as quickly as possible. Then wait. If the alternator is failing to charge the system, the car will soon die again and the problem has been pinpointed. If the car runs and continues to run, then it's likely the problem is with the battery. Caution should be used, however, as a dying battery can only receive a charge for so long and may go completely dead at a later point despite the best efforts of the alternator. Testing a battery can be accomplished with a voltmeter, and most auto stores have the facilities to check a suspect battery's state of charge and even its charge capacity.

Broken or Loose Connections

In this case, everything with your car's alternator seems to check out OK -- no belt issues or other visible signs of trouble -- but the car's battery is dead, as is most of the car's electrical equipment. What's happening is the alternator is producing electricity, but it's either not going anywhere or it's not the right type.

Alternator electricity is piped through large cables and smaller wires. Any problems within the wires, cables or the connections at either end can reduce or stop electricity from getting through. Occasionally, a symptom of this issue might be brighter lights as the alternator produces more energy to overcome the resistance in a bad wire or a broken or loose connection. This symptom is usually accompanied by the smell of hot wires, too. Higher resistance within a wire creates heat, similar to the way an electric burner on a stove heats up by resisting the flow of electricity.

Another problem could be the alternator's diode rectifier. Alternators produce alternating current (AC) electricity in three phases, but a car's accessories require direct current (DC) to operate. The rectifier changes the current from AC to DC. Without that critical component operating properly, the electricity the alternator produces can't be used.

For more information about alternators and other related topics, follow the links on the next page.


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