So your battery’s playing up, the lights are dimming, or your car won’t start properly and now you’re wondering, ‘how can I test an alternator to figure out if that’s the issue?’. You’re not alone. The alternator is one of the most overlooked components under the bonnet, yet it plays a massive role in keeping everything ticking along.
Luckily, you don’t need to be a mechanic to run a few basic tests. Here’s a simple step-by-step guide to help you check whether your alternator is doing its job or whether it’s time to call in the pros.
Step 1: Know what the alternator actually does
Before you dive into testing, let’s get clear on what the alternator does. It’s not the same as the battery. The alternator is what keeps your battery charged while you drive. It also powers your lights, radio, aircon, and everything else electrical once the engine is running.
So, if your battery keeps dying, you’ve got flickering lights, or warning symbols on your dash, the alternator might not be doing its job.
Step 2: Listen for the early warning signs
You can often catch alternator issues before they fully knock your car out of action. Here are a few signs to watch for:
- Battery warning light
- flickering or staying on dim or overly bright headlights
- Strange smells (burning rubber or electrical)
- A whining or grinding noise from the engine bay
- Car starts but quickly stalls or struggles under load
- If you’re ticking any of those boxes, it’s time to run a quick test.
Step 3: Get a multimeter (they’re easier to use than you think)
To test your alternator properly, you’ll need a digital multimeter. They’re cheap (you can get one from a hardware store for under $30) and once you’ve used one, you’ll wonder why you didn’t grab one sooner.
Set the multimeter to DC voltage (20V range) and you’re ready to go.
Step 4: Test the battery first (engine off)
Before you start the engine, check your battery voltage. Connect the red probe to the positive terminal and the black probe to the negative.
A healthy, fully charged battery should read between 12.4 to 12.7 volts with the engine off. If it’s reading lower, your battery might be the problem or it could be undercharged due to a dodgy alternator.
Step 5: Start the engine and test again
With the car running, test the voltage again at the battery terminals. This is where the alternator comes into play.
You should now be seeing a reading between 13.8 to 14.5 volts. That’s your alternator charging the battery. If it’s not at least 13.8 volts or it’s bouncing all over the place, there’s a good chance your alternator isn’t pulling its weight.
If you’re getting more than 15 volts, that’s too much, and your voltage regulator might be shot (it’s part of the alternator in many vehicles). That can damage the battery and other electronics.
Step 6: Turn on the accessories
To really test your alternator under pressure, keep the engine running and turn on your headlights, radio, AC, and any other big electrical loads.
Watch the multimeter, your voltage should stay stable between 13.5 to 14.2 volts. If it drops significantly or can’t hold steady, your alternator might not be keeping up.
Step 7: Check the alternator belt (optional but useful)
If everything’s pointing to a failing alternator, it’s worth taking a peek at the alternator belt. A loose or worn-out belt can cause poor alternator performance even if the alternator itself is fine.
Look for cracks, glazing, or slack in the belt. Give it a gentle tug; if it moves more than 1–1.5cm, it may need tightening or replacing.
When to call in the pros
If you’ve run these tests and things aren’t adding up or you’re just not sure, bring your car into Autospark. We’ll do a full diagnostic check and confirm what’s going on with your charging system.
Sometimes the issue isn’t the alternator itself but a wiring problem, fuse, or a failing battery. Our team will pinpoint the issue, explain your options, and get things sorted with no guesswork.
Final thoughts
So if you’re wondering ‘how can I test an alternator’, now you’ve got a solid answer. A simple multimeter and a bit of DIY testing can tell you a lot about your car’s charging health. But, unless you’re handy with a wrench and know your way around a car, alternator replacement is best left to the experts.
But if you’re seeing signs of trouble and want a professional opinion (or a proper fix), book in with Autospark. We’ll get your vehicle charging properly again without the guesswork.
