If you’re dealing with dim interior lights, a slow-cranking engine, or a lit battery warning light, consider it a sign that your alternator needs testing.
Not sure how to test an alternator?
Don’t worry — we’ve got you covered.
There are two common ways to check if your alternator is properly charging the battery and supplying electrical energy to your car’s systems: the multimeter and the headlight test.
Keep reading as we’ll walk you through each method to test a bad alternator.
This Article Contains:
- How to Test Alternator Using a Multimeter
- How to Test the Alternator Without a Multimeter: Headlight Test
- 3 FAQs about Alternator Testing
How to Test Alternator Using a Multimeter
NOTE: It’s always a good idea to have a professional mechanic test your alternator to avoid electrical issues that could damage your car. Any mistake during testing can cause overvoltage or short circuits, which might harm critical electrical components or start a fire.
For an accurate diagnosis, the mechanic will first perform some basic checks, followed by using a multimeter to test if your alternator is delivering the correct voltage to your car’s electrical system.
Here’s a look at the steps involved:
Step 1: Check the Dashboard Warning Lights
A battery warning light can come on when the alternator isn’t charging the battery correctly.
If your battery light is on, the mechanic will check the battery to rule out the chances of a weak or degrading unit triggering the warning light. Once they’re sure the battery’s not the issue, they’ll check the alternator.
Step 2: Locate the Alternator
The mechanic will open your car’s hood to locate the alternator, typically near the top front of the engine.
It’s usually easy to spot because the alternator housing is a round, vented metal casing with visible copper wiring inside.
Step 3: Listen to Engine Sounds
With the engine on, they’ll listen closely for any unusual engine noises, such as grinding or squeaking, which could indicate a faulty alternator.
To pinpoint the sound, they may use a rubber hose — placing one end on the alternator and the other near their ear. If the noise is loud, it could suggest a failed bearing, meaning you may need a new alternator.
If there are no noises, the mechanic will proceed to inspect the alternator and other related components for physical damage.
Step 4: Turn Off the Engine
Before inspecting the alternator, the mechanic will park your car on a level surface and turn off the engine to ensure safety.
Step 5: Check the Engine’s Drive Belt
The mechanic will first check the drive belt (also called the serpentine or alternator belt), as a loose or damaged belt can hamper your alternator’s ability to charge the car battery.
They’ll press the belt between two pulleys to test its tightness — it should feel firm with little movement. If it’s loose, they’ll tighten it, but if it’s worn or damaged, they’ll replace it.
Step 6: Inspect the Alternator for Physical Damage
If the belt is fine, the mechanic will check your alternator housing and the voltage regulator (if it’s external) for corrosion, frayed wires, or loose electrical connections. They’ll also examine the alternator pulley to ensure proper tension and alignment.
If the electrical connections are loose, they’ll tighten them using a screwdriver and replace any damaged parts.
However, if the alternator shows no signs of physical damage, they’ll perform a voltage test.
Step 7: Prepare the Multimeter
The mechanic will turn on the multimeter and set it to 20 DC volts. If the meter doesn’t have incremental settings (2, 20, 200, etc.), they’ll set it to DC voltage.
Then, they’ll connect the positive wire to the car battery’s positive terminal (+) and the negative wire to the negative battery cable (-).
Step 8: Test Battery Voltage with the Engine Off
With the car engine off, the mechanic will check the battery voltage. If it reads around 12.6 volts, the battery is in good condition.
Step 9: Test the Battery Voltage with the Engine On
They’ll start the engine and recheck the battery voltage. At idle speed, a reading between 14.2 and 14.7 volts indicates a healthy alternator that’s charging the battery correctly.
If the voltage reading is below 13 or above 15 DC volts, the alternator may be undercharging or overcharging the battery.
Step 10: Perform a Load Test
Finally, the mechanic will perform a load test by turning on electrical components like the headlights, interior lights, radio, and air conditioner.
While these systems run at idle speed, they’ll monitor the multimeter to ensure the battery voltage stays above 13 volts. Afterward, they’ll turn off the car and check if the reading drops to 12.6 volts.
If the battery voltage remains steady under load, it confirms a healthy alternator. However, a voltage drop could signal a failing alternator.
Now, if you don’t have access to a multimeter or a mechanic but want to check for a faulty alternator, there’s a simple test you can perform yourself.
How to Test the Alternator Without a Multimeter: Headlight Test
Here’s how you can do a DIY alternator test using the headlight method:
Step 1: Start the Engine and Headlights
Start your car and switch the headlights to a dipped beam (low beam). This will make it easier for you to notice any changes in the lights.
Step 2: Check the Headlights
While the car engine is running, check if the low-beam headlights are bright and steady. If the alternator output is good, the lights should remain bright and stable without flickering.
Step 3: Rev the Engine
Gently rev your car engine. If the headlights get brighter, the alternator is charging your battery properly. But if the lights dim or flicker, it could point to a weak alternator struggling to provide electrical energy to your car’s accessories.
Once you’ve confirmed that your alternator is failing, call a mobile mechanic (since driving your car in this case would be risky) to test and replace the bad alternator.
Have more questions?
Check out these FAQs.
3 FAQs about Alternator Testing
Here are answers to some commonly asked questions related to alternator testing:
1. What Causes an Alternator to Fail a Test?
Alternator failure can be a result of many problems, including:
- Jammed alternator bearing: If the bearings get jammed from wear or bad repairs, the alternator won’t spin smoothly, resulting in failure.
- Stator winding burnout: Water can damage the alternator’s stator winding, causing a short circuit.
- Worn alternator brushes: The brushes that help transfer current between the rotating and stationary alternator parts can wear out with use.
- Voltage regulator relay failure: A faulty regulator relay can disrupt voltage regulation and cause alternator output to fluctuate, resulting in too-low or too-high voltage readings.
- Diode bridge failure: The diode bridge converts AC voltage (alternating current) to DC voltage (direct current) and can fail due to a bad battery or short circuit.
2. Can You Test an Alternator by Disconnecting the Car Battery?
No, disconnecting the battery while the car is running can cause the alternator to produce too much power (up to 150 volts), which could severely damage your car’s electronics.
3. How Much Will an Alternator Test and Replacement Service Cost?
The cost of testing a failing alternator can vary by repair shop but typically falls around $100.
If the tests reveal an alternator problem, replacing it can cost anywhere from $200 to $1,200, depending on your vehicle make and location. This range includes:
- New alternator: $100 to $1,000
- Labor: $100 to $200 per hour
Additionally, if a new alternator belt is needed, you’ll have to spend another $90 to $250 for parts and labor.
Catch Alternator Issues Early with AutoNation Mobile Service
An alternator is vital to keeping your car’s electrical system running. Regularly testing and replacing a weak alternator can help avoid unexpected breakdowns and save on costly repairs.
The best part?
With AutoNation Mobile Service, you can have car maintenance experts perform a simple test and fix the alternator problem right from your driveway. You can also easily book our services online and enjoy a 12-month, 12,000-mile repair warranty.
Contact us to schedule an alternator test, replacement, or other car maintenance services.