Today’s cars are filled with energy-thirsty electronic devices. The list of electronics includes everything from electric steering and dashboard infotainment to headlights and heated seats. Your car’s battery stores energy, but it’s the alternator that provides the necessary power to those systems. So it’s important to know how to test and repair an alternator.
Automobile Electronics 101
An alternator keeps a car battery fully charged.
First, let’s go over a few basics. Your car’s 12-volt battery stores the energy used to start the vehicle. The battery also powers those electronics when the vehicle ignition is off.
When the engine is running, it powers a drive belt that’s connected to the alternator. The belt drives a pulley that turns the alternator’s rotor—effectively converting the engine’s mechanical energy into electric power.
Here’s another way to put it: The alternator turns rotational power into electricity. Some of that power is sent to the battery to keep it fully charged. The rest of the juice keeps the lights on and the car’s electric stuff powered up.
Did the Alternator Fail?
The alternator has a device called a rectifier that converts AC to DC power, which is the type of current used by your car’s electrical systems.
The reasoning why alternators produce AC is a long story. But you probably don’t care about those background details when your alternator fails. You’ll have the more immediate problem of a dead battery, dim lights, or accessories that don’t work.
Car owners should fix any issue with the alternator promptly. The alternator is a small round metallic object about the size of a softball. It’s usually located toward the front of the engine—with a pulley on the front end and a belt going around the pulley.
There are a few things to check if you suspect your alternator failed.
- Did the dreaded Check Engine light come on? One cause could be the alternator not providing sufficient power to the battery. (See our article: What Does the Check Engine Light Mean?)
- If you smell an acidic odor under the hood, it could be the alternator. (Also see this: Why Does My Car Smell Like Eggs, Crayons or Other Odd Things?)
- Is your car taking more time to start? The battery might be losing power due to a bad alternator.
- If you notice the headlights flickering at night, especially when the car is idling, that’s another danger sign.
How to Test an Alternator
Without any input from the alternator, the battery outputs 12.6 volts.
It’s easy to test an alternator with a multimeter or voltmeter. A multimeter is a handy tool for measuring various electrical signals. We prefer a digital readout tester that makes it easy to understand the units of measure.
Shop now for a multimeter
If you work with electronics, a high-quality multimeter is a good investment.
How do you test an alternator? Follow these steps:
- Set the voltmeter/multimeter to about 20 volts of DC power.
- With the engine off, connect the red and black leads from the voltmeter to the negative (black) and positive (red) terminals of the car battery.
- Move the leads away from any parts that could move when the car is later started.
- A typical car battery should show 12+ volts. A reading between 12.0 and 12.9 would be typical.
Now start the car and check the reading on the voltmeter. A good alternator should now be sending around 14 volts to the battery. Again, expect a reading between 13.3 and 14.9 volts.
If you see no change in the reading, that would indicate the alternator is not working. Likewise, if the reading is above 16 volts, the battery is being overcharged. That means the alternator’s regulators (or diodes) are probably bad. You can now turn off the car’s engine and remove the voltmeter.
Many modern cars offer digital readouts of the engine’s diagnostics, like battery voltage.
If you don’t have access to a voltmeter, many modern cars offer a set of digital readouts, including for the battery voltage. If your car offers that option, you can check the battery voltage before starting the car and again after it starts. The readings should be similar to the numbers above.
Replacing an Alternator
If you determine that your alternator is bad, consider these three options.
- Take the car to a mechanic to confirm your readings and take steps to fix the alternator.
- You could disconnect the alternator and take the alternator to a repair shop where they can replace the components that failed. Or skilled DIY folks can buy an alternator repair kit to fix it yourself. Owners of classic cars might want to take this hands-on approach to ensure that parts remain stock.
- Finally, you could replace the entire alternator with a new one.
When choosing a new alternator, the amperage rating while idling is the most critical factor. Don’t choose an alternator with a rating lower than the factory standard. If you recently added a powerful stereo or aftermarket electric accessory, you might opt for an even higher rating. While you’re upgrading, consider adding some engine bling with a chrome alternator.
To remove the alternator, watch relevant YouTube videos while consulting with a shop manual.
You’ll need to remove the drive belt, as well as the bolts that typically hold the alternator in place. There is also a wire connector that likely needs to be removed. To swap in the new alternator, reverse these steps outline above.
Replacing an alternator is an easy job. Most handy folks equipped with the right tools can complete the job in about an hour.
