Your alternator is arguably one of the most important parts of your engine. Usually found attached to the front of the block, it’s responsible for keeping your battery charged as you drive down the road. Without it, your battery would quickly run out of juice, leaving you stranded. Think your alternator might be failing? Here’s how you can check on its health yourself, with tips from an expert.
Road & Track spoke with Kevin Hines, senior technician at McLaren Philadelphia, to learn the correct way to check whether your alternator is working properly. Hines is North America’s only factory-certified McLaren F1 technician, which means his day job is working on $20 million exotics. If anyone understands the ins and outs of alternators, it’s him.
Before reading any further, we suggest consulting your car’s owner’s manual for exact instructions on how to check the health of your alternator. The manufacturer’s suggestions may differ from the instructions below.
The Tools You’ll Need
At minimum, we recommend picking up some sort of multimeter to help test out your alternator. A multimeter is a handheld device used to measure voltage and test electrical connections. Affordable examples can be found both online or at your local auto parts store. Here’s one we love:
WeePro Digital Multimeter
In case you need to disconnect or remove your battery from your car, we also suggest keeping a standard set of wrenches and sockets on hand. Depending on what type of car you drive, you may have an onboard tool kit that contains the stuff you need.
How to Test Your Alternator
In order to get a conclusive result as to whether an alternator has gone bad, Hines says he uses a lot of fancy tools and expensive machinery. Proper load testers, normally found at professional shops and dealerships, cost around $2000 on the low end, not exactly cheap enough for the average DIYer. But there are still a few low-budget methods you can try to give you a good idea of the health of your alternator.
One of those methods involves checking your battery’s voltage. Simply check the voltage of your battery by touching the multimeter prongs to the terminals with the car shut off. Take note of that number; it should be somewhere in the 12-to-13-volt range, according to Hines. Then, start the car and check the voltage at the battery again while the vehicle is running. “Ideally, it should be in the high 13s or somewhere in the 14s,” says Hines. “That’ll tell you that the alternator is producing voltage.”
Here’s what the voltage reads with the car turned off.
Aaron Brown
And here’s what it reads when the car is running.
Aaron Brown
If the number remains constant or drops when the vehicle is running, you may want to investigate further, either by trying more methods below or taking the car to be inspected by a professional.
Another method Hines suggests is putting a bunch of load on your car’s electrical system. “With the car running, you can turn on all the heavy loads; turn on the heater blower motor, turn the lights on, etc.,” he tells us. “Then check the voltage at the battery terminals. That’ll give you a general idea of the health of the alternator.”
If the number has dropped significantly compared to when everything was turned off, it could be a sign your alternator can’t keep up with the electrical system’s demand.
Aaron Brown
Another, riskier method to check the alternator involves disconnecting the battery from the car altogether; With the car running, disconnect the battery and see if the car continues to run. If it does, that means the alternator is still producing sufficient voltage to at least keep the engine firing, according to Hines. If it doesn’t, that means the alternator isn’t producing enough voltage.
Please note: We wouldn’t recommend using this method, as it could cause some serious damage to the alternator and leave your car in worse shape than it was before. But it can work if you’re in a pinch.
Brian Silvestro Road & Track staff writer with a taste for high-mileage, rusted-out projects and amateur endurance racing.