How to Use a Voltmeter on a Car

A voltmeter is an essential tool when diagnosing problems with your vehicle's electrical system.

Voltmeters are handy instruments used to check battery voltage. They are especially useful when determining whether your car battery has enough voltage to power your vehicle. You can even use a voltmeter to check the charging system in your car or truck. If you don't have a voltmeter, they are fairly inexpensive, often less than $20, and can be purchased at most any automotive supply store.

Instructions

Testing Your Car Battery with a Voltmeter

1. Turn your car off in order to check just the voltage of the battery. You'll also want to ensure that all other electrical accessories are turned off as they draw energy from the battery. When you're ready to begin testing, pop the hood on your vehicle.

2. Locate the battery in the engine compartment. The battery may be covered by plastic shields. If so, remove them as well as any other guards or components that are in the way. You will need open access to the battery and its terminals.

3. Turn on your voltmeter if you have a digital model. Otherwise, begin connecting the meter to the battery. You must attach the red, positive clip to the positive battery terminal. The positive terminal is often marked with a plus sign. Next, connect the black, negative clip to the negative battery terminal, typically marked with a minus sign.

4. Examine the gauge or readout on your voltmeter. If you've attached the clips correctly and securely, you should see a reading of approximately 12.6 volts on a good and fully charged battery. A reading of less than 12.4 volts, however, is indicative of a battery that is in need of a charge.

Testing Your Car's Alternator with a Voltmeter

1. Turn your car or truck on and allow the engine to idle. Similar to testing the battery, though, you want to make sure all electrical accessories are turned off.

2. Open the hood and remove any shields or guards from the battery. Again, you need to have complete access to the terminals. Be careful not to touch any moving parts in the engine compartment.

3. Attach the positive and negative clips to the corresponding terminals on the battery. Make sure the clips are securely attached. If they are loose, you will not get an accurate reading.

4. Examine the gauge or readout on your voltmeter. With the engine running and your alternator working correctly, you should see a reading of approximately 13.6 to 14.3 volts. If you see a reading higher than 14.3, your alternator is overcharging, which can shorten the life of your battery. A reading less than 13.6 indicates that your alternator is not providing enough charge, which will cause your battery to drain more quickly. Either way, take your vehicle to a qualified mechanic to have your alternator replaced or fixed.

References

• AA1 Car: Diagnosing a Car Battery that Runs Down
• 2 Car Pros: How Do I Check My Car Battery?