Voltage Drop: How to Tell When Your Alternator Is Failing

Old alternator

Modern vehicles rely on their electrical systems to move you down the road in safety and comfort more than ever. From multiple ECUs, to supplemental restraint systems, accessories, and high-powered lighting, your car needs to keep those electrons flowing to keep you going.

The hardest working component of your electrical system is no doubt the alternator. While a generally robust mechanism meant to last the life of the car, they can and do fail. When that happens, you won’t be going anywhere in a hurry. Why is this?


The alternator converts mechanical energy from your engine into electricity that charges your battery while you drive. As you go down the road, all the electrical components that your car needs draw their power from the battery, which the alternator keeps topped up as long as the engine is running. If your alternator is starting to go bad, its ability to maintain the battery’s charge will falter, and all those electrical components will start to drain the battery. Without the alternator to supply a new charge, your battery will die, and you’ll be stuck.

Luckily, there are a handful of tell-tale signs to watch for that may indicate your alternator is struggling, and maybe on the verge of giving up. 

Visual Signs Your Alternator is Failing

There are some visual cues you can use to diagnose your alternator’s general health. The first is, very simply, a gauge in your dashboard. Many vehicles have a voltage gauge – indicated usually by a graphic of a battery. Most voltage gauges are not calibrated, they simply indicate a correct range of voltage for your battery while the engine is running. Normally, the needle pointing straight up means the alternator is charging the battery correctly.

Car dashboard with gauges

If it drops below the correct range, it may mean the alternator is not charging the battery enough, and if the needle goes well above the range, it means your battery is being overcharged. This problem is less common than undercharging, but just as bad for your battery and your car in general.

Historically, alternators had an internal component called a voltage regulator that ensured the charge sent to the battery was within the correct range. Nearly all alternators in new cars today are called “smart alternators”, and their voltage is controlled by the ECU instead. A healthy battery should read between 12.4 and 12.9 volts when the car is off, and from 13.9 to 14.9 volts while the car is running. 

Even if your car isn’t equipped with a voltage gauge, all vehicles have a battery warning light – if this illuminates while the engine is running, it could indicate a problem with the alternator. You can measure the electrical system’s voltage yourself with a multimeter, which is another easy visual indicator of your alternator’s condition. 

Multimeter showing good battery voltage

A multimeter is a useful and inexpensive tool for many other automotive diagnostics, and for vehicles without a gauge, it’s the easiest way to tell how well your alternator is maintaining the battery’s charge. Turn your multimeter to the voltage measurement setting, place the positive probe on the positive terminal of your battery and the negative probe on the negative terminal. The reading will tell you where your battery’s voltage sits – do this both with the engine off, and with the car running.

Mechanical Signs Your Alternator Is Failing

Two of the primary signs of a faltering alternator are trouble starting your car and keeping it running. While hard starts could suggest that your battery is going bad, poor charging levels supplied by the alternator may be contributing to the lack of juice. If your car is hard to start, and also mysteriously sputters or stalls out while driving, this is a classic sign of a sick alternator.

In addition, many of your electrical accessories can give you a good measure of how well the alternator is supplying a consistent charge to the battery. Headlights that are dimmer than usual, or suddenly super bright, sluggish windshield wipers, a radio that cuts in and out, and wacky behavior from the gauges in your dash all point to alternator problems.

Check the Condition of the Alternator Belt and Wiring

Another mechanical inspection that you can easily do yourself is to check out the condition of the alternator belt. The alternator is driven from either the engine crankshaft or camshaft pulley by a belt. Over time it can become cracked, brittle, and worn smooth, causing slippage between the pulleys. 

Alternator belt

If the belt is slipping, the alternator won’t turn consistently, leading to voltage irregularities. If you’re lucky, just replacing it, or even simply adjusting the tension on the belt, may solve the problem. Belt tension is important – too loose and the belt may slip, too tight and it could put undue pressure on the bearings and shaft of the alternator, leading to premature failure. Gravely noises coming from your alternator may suggest that the bearings could be toast. 

Automotive wiring connectors

Lastly, you can also take a look at the wiring leading to and from the alternator – is it in good condition? No missing insulation or loose connections? No frayed ends or corroded attachments? If there are, damaged wiring can also contribute to poor charging performance from your alternator.

What Do I Do If My Alternator Is Failing?

There are really only two routes to remedying an alternator that’s on its last legs. 

It’s important to remember that you may be able to limp along with a dying alternator for a little while, but once it gives up the ghost, you won’t be driving anywhere. This could put you in a dangerous situation, for example, if it decides to cut out while you’re commuting in heavy highway traffic. Losing your vehicle’s entire electrical system all at once is a scary proposition, especially if you’re traveling at higher speeds, or find yourself on a particularly lonely stretch of road. It pays to be proactive in nipping an alternator problem in the bud, before it blossoms into something much worse. 

Disassembled alternator

First, most automotive electrical shops can rebuild alternators, especially those from older cars. In fact, rebuilding alternators was common practice for many years. Of course, this process takes a little bit more time than simply buying a new one, and you’ll be without wheels while they’re doing the repairs.

Second, these days it’s almost always just as cost effective to buy a brand new alternator, than it is to have it rebuilt. And, some of the new generation of smart alternators are even sealed, and thus not repairable at all. If you do replace or rebuild your alternator, it’s good insurance to change the belt out with a high quality replacement

Replacing Your Alternator

Finally, replacing an alternator on many vehicles is a fairly straightforward process for the confident driveway mechanic and rarely requires special tools. Often, alternators are mounted to the engine with only two bolts and a few wires, making it an easy afternoon fix. For safety, whenever you’re working on your car’s electrical systems, remember to disconnect the battery’s negative terminal.

Alternator installed on engine

Some cars can even accommodate upgraded aftermarket alternators that have higher amperage ratings. Increasing your amps means you can run more electrical accessories, or components that have a heavy draw, such as high wattage stereo amplifiers. When increasing current from a more powerful alternator, you may need to also upgrade your alternator wiring to a larger gauge. When in doubt, check in with an automotive electrician.

Has a failed alternator ever left you in a lurch? Questions about alternators? Let us know below in the comments.


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