How to Use a Voltmeter on a 12 Volt Motorby C. Taylor
A 12 volt motor transforms electrical energy from a 12 volt battery into mechanical energy to perform work. Therefore, to test the voltage of a 12 volt motor, you would test the source, i.e., the 12 volt battery, or the leads coming into the motor to ensure proper current. An obvious example of a 12 volt battery is found in an automobile, where it is used to turn the starter, which in turn, allows the car to start up. Sometimes, however, it is difficult to know if a problem exists in the battery, starter or somewhere else, so you need to test the battery to be sure. The most accurate way to test your battery is with a voltmeter.
Testing a 12 Volt Battery
Turn off the car and anything that draws current from the battery, such as a dome or hood light, which turns on when the door or hood is open.
Remove or open any caps that cover the battery terminals, which are the studs sticking up from the top of the battery. The caps usually just clip in place.
Select DC current on your voltmeter, and if there's a scale, choose 12 volts.
Touch the red, negative (-) wire of the voltmeter to the red, negative terminal of the battery. At the same time, touch the black, positive (+) wire of the voltmeter to the black, positive terminal of the battery.
Read the voltmeter's display. A fully-charged car battery should read 12.6 volts at room temperature. Cold weather can cause the reading to be slightly lower, even on a full charge. For example, a fully-charged battery should read 12.5 volts at zero degrees Fahrenheit.
Testing at the Motor's Leads
Locate the wires coming into the motor. There are typically two, although there may also be a ground wire. If there are more than two, look for the black and red wires.
Touch the red probe of the voltmeter onto the post where the red wire is connected. At the same time, touch the black probe of the voltmeter onto the post where the black wire is connected.
Read the output from the voltmeter.