A failing alternator can leave you stranded on the side of the road. Here’s how to make sure yours alternator is working properly, with tips from an expert. Is there a test for a bad alternator? Your multimeter will tell you if your alternator is working as it should. Your multimeter will tell you if the alternator’s electricity is not getting to the battery. Your multimeter will help you do a voltage drop test if you have a cable problem or a dirty connection.
Similar to testing your car battery, a multimeter can be used to quickly determine if your car’s alternator is adequately keeping the car battery at the correct charge and also running the electrical system at the correct voltage.
In most cases, car batteries have an open-circuit voltage of between 12.45 and 12.6 volts. However, your car alternator operates at a slightly higher voltage than the battery voltage for a number of reasons. For one, a phenomenon known as voltage drop causes the voltage at the alternator to decrease as it travels through the hundreds of feet of wiring that snakes through a vehicle.
This means that the voltage at the alternator can be 14.5 volts, for example. The voltage at the battery may be 14.2 volts, but way out at the rear tail light bulb, that voltage may be as low as 13.5 volts due to a voltage drop.
For this reason, the operating voltage measured on the battery by each vehicle may differ, but they should all be higher than the open-circuit voltage of the battery. In most cases you will see a voltage of 14.00 to 14.5 on most vehicles when the vehicle is running and all accessories are off. Now that you know that, it’s time to test the alternator.
Before reading any further, we recommend that you consult your car’s owner’s manual for specific instructions on how to check the condition of your alternator. The manufacturer’s suggestions may differ from the instructions below.
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How Do You Test an Alternator?
The following steps address how to test an alternator with a multimeter:
- Get a multimeter.
- Set your multimeter to voltage and ensure it’s adjusted to 20 DC volts, or if your voltmeter does not have incremental settings then simply set it to DC volts.
- Make sure your alternator’s positive and negative terminals are clean.
- Put the multimeter’s each probe to the correct terminal, touching negative to negative and positive to positive. Your multimeter and battery are probably both color-coded, so the negative terminal and probe will be black while the positive terminal and probe are red.
- Simply check the voltage of your battery when the car shut off. Take note of that number; it should be somewhere around 12.6.
- Then, start the car and check the voltage at the battery again while the vehicle is running. Ideally, it should be in between 14.2 and 14.7. “That’ll tell you that the alternator is producing voltage.”
- A reading of over 14.7 means the battery is being overcharged, while a reading under 14.2 means the alternator is undercharging the battery.
- Turn the lights, radio, fog lights, fan, and other electronics on, making sure the voltage reading doesn’t go below 13.
- When you shut off the car, make sure the reading is above 12.6.
Note: – With the engine off, the battery voltage should be between 12.5 and 12.8 volts. If it’s below that, charge the battery with a battery charger before you conduct the test again.
If any of these readings are off, you likely have an issue with your alternator and may want to visit an auto repair shop.
If you have a newer battery but your car won’t start, you may have a bad alternator. If you’re tempted to test an alternator by disconnecting the negative battery cable, don’t do it. While a good alternator can keep the engine running without a negative wire, that has never been a good test.
In the days before computers, you could pull it off without damaging anything. Nowadays you risk burning every electrical device in your vehicle. The second you disconnect the battery; the voltage regulator triggers the alternator to deliver maximum power.
Without a backup battery in the circuit, the alternator can deliver up to 150 volts, depending on the engine RPM. When the smoke clears, this “simple test” could end up costing you several thousand dollars in new electronics.