Alternator Test: How to Check Your Car Alternator
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Time to read 5 min
How to Test an Alternator
The alternator is a fundamental part of your vehicle's electrical system. It is responsible for keeping your car battery charged while the engine is running. The ability to perform an alternator test is a useful skill and can save you from hefty diagnostic fees - which can be over $165 per hour!
Symptoms of a Bad Alternator
- Slow, long cranking or no cranking of the engine at all.
- Battery light illuminated on dash.
- Reduced performance of electrical components - like flickering or dim headlights.
- Growling or grinding noise from alternator.
- Electrical or burning smell.
💡 Did you know
A faulty alternator can drain your battery, damage electrical systems or even prevent your vehicle from running.
Tools Needed to Check an Alternator
Digital Multimeter or Voltmeter.
Battery and electrical system tester - optional, but can accelerate the process.
Personal Protective Equipment (e.g. safety glasses).
Alternator Test Procedure
Step 1: Check Battery Charge with a Voltmeter
Before you begin an alternator test, it is important to check the battery first. A weak or dead battery can cause similar symptoms and is more common.
You can check you car battery's state of charge with a multimeter:
- Switch ignition off: Ensure there are no loads on the battery. Switch off ignition, headlights, air-conditioner, etc.
- Select DC voltage on your multimeter.
- Connect the multimeter: The red probe goes onto the positive (+) terminal and the black probe onto the negative (-) terminal.
- Read the voltage: An acceptable resting voltage is between 12.4V to 12.8V.
If your car battery voltage reading is below 12.4V, try charging it and retesting. It is important to know voltage reflects the state of charge and not the overall health. To test battery health, you'll need a battery tester.
A fully charged car battery should be 12.6V to 12.8V
Step 2: Alternator Test with the Car Running & No Load
The next step in an alternator test is to measure the voltage with the vehicle running and under no electrical load. This is tested at the battery with electrical loads - like headlights, interior lights, radio and aircon - switched off.
- Select DC Volts on your multimeter.
- Connect multimeter: Red probe to positive battery terminal and black to negative.
- Read the voltage: Engine must be running to check alternator output and with all electrical loads switched off.
A good alternator test result will be between 13.5V to 14.8V.
If it is below 13.5V then the alternator may be undercharging, and unable to keep up with the electrical load. If it is above 14.8V, it may be overcharging - which can kill a battery or damage the electrical system.
It is important to refer to the manufacturer's specifications as values may vary.
Step 3: Alternator Test Under Load
To test an alternator under load, leave the engine running and turn on all heavy loads. This includes the headlights (high beams), aircon and radio. There should be no significant drop in the alternator output. If there is a significant drop in voltage - below 13.5V - the alternator may be struggling to keep up with the electrical demand.
Step 4: Inspect Wiring & Fuses
Before you go ahead and replace your alternator, you should inspect your wiring and fuses. If you are seeing no change in alternator output when you start your engine, you may have a blown fuse or open circuit.
- Check Fuses: Look for a blown or damaged alternator fuse.
- Visual inspection: Look for damaged or broken wiring from your alternator.
- Test Output at the Alternator: This is the same procedure as before - except directly on the alternator.
How to conduct an alternator test:
Connect your voltmeter's red probe to the positive output terminal on the alternator and your black probe to a suitable ground (e.g. on the metal housing). Measure voltage at the alternator and compare to your readings taken at the battery. This will eliminate any wiring issues and ensure your alternator test is accurate. During this alternator test, Be sure that your limbs, clothing and testing tools are clear of any hot or rotating components.
Diagnosing Common Alternator Faults
Audible Alternator Test (Bearing Noises)
You can also perform an audible alternator test to diagnose physical issues like worn bearings or damaged internals. A rotational whirring or grinding noise from the alternator may be a failing bearing or a broken component hitting the alternator housing.
Damaged or Slipping Drive Belt
A damaged or loose drive belt could be the cause of low alternator output. The drive belt is what spins the alternator. If it is slipping it will not be spinning the alternator as fast, causing a failed alternator test. Inspect for cracks, damage or slippage and replace as necessary.
How to Diagnose Voltage Regulator Issues
The voltage regulator, usually built into the alternator, manages how much voltage is delivered to the battery. When the regulator fails, it can cause either overcharging or undercharging issues. During an alternator test, signs of a faulty regulator may show up as fluctuating dashboard lights, a burning smell, or voltage readings that are noticeably too high or too low.
Tools to Make Your Alternator Test Easier
While a multimeter works, using a dedicated battery and electrical system tester can save time and give more accurate results when you perform an alternator test. Instead of running multiple checks manually, these tools quickly diagnose battery health, charging performance, and potential alternator faults in one go.
At AutoTech Workshop Solutions, we recommend the Topdon BT200 and Topdon BT600. Both are trusted by DIY car owners and professional technicians alike for their reliability, ease of use, and ability to test an alternator in minutes. By spotting problems early, these testers help prevent costly breakdowns and keep your vehicle’s electrical system in top shape.
What is an alternator test?
An alternator test is a diagnostic check that measures how well your vehicle’s alternator is charging the battery and powering the electrical system. It helps identify issues like undercharging, overcharging, or regulator faults before they cause breakdowns. An alternator test can be done with a multimeter or dedicated tester.
How do I test an alternator at home?
To test an alternator at home, you can use a digital multimeter or a battery and electrical system tester. Start by checking the battery voltage with the engine off, then repeat the alternator test with the engine running. A healthy alternator should produce between 13.5 and 14.8 volts.
Can a battery tester perform an alternator test?
Yes. Many advanced battery testers, such as the Topdon BT200 and BT600, include a built-in alternator test function. This allows you to quickly test an alternator and the overall charging system in one step, saving both time and effort.
What are the symptoms that mean I should run an alternator test?
If you notice dim headlights, a dead battery, fluctuating dashboard lights, or strange noises under the hood, it’s a good idea to perform an alternator test. These signs often point to charging system issues.
How accurate is an alternator test with a multimeter compared to a professional tester?
A multimeter and a professional tester are both accurate for an alternator test, but a professional tester is far more time-efficient and user-friendly. Tools like the Topdon BT200 or BT600 simplify the process, making it quicker and easier to test an alternator.
