There are two types of ground wires: a household AC, or alternating current, and an automotive-type DC, or direct current. The household ground wire, when working properly, should not carry any current at all. A DC ground wire in an automotive-type circuit, although always referred to as a 12-volt circuit, will actually carry 12.6 volts of electricity to complete the circuit on an automotive-type wiring system. This system could be in a car, motorcycle or truck.
Set your multimeter to read 120 volts AC to test a household ground wire. You are going to use it as an ammeter to measure the voltage between two points.
Flip the circuit breaker to the "off" position on the ground wire circuit that you wish to test.
Unscrew the ground wire with your screwdriver from the electrical ground post. Pull the wire from the post and allow the end to remain accessible.
Flip the circuit breaker for that particular circuit to the "on" position.
Touch one probe of the multimeter to the ground wire and touch one probe to the ground wire electrical post. Because your multimeter is now functioning an an ammeter, it will register any current that is flowing between the post and the wire. A correctly grounded wire will show zero voltage.
To test a DC circuit ground wire, remove it from the appliance to which it is connected, such as the radio, heater fan or cigarette lighter. Set your multimeter to 20 volts DC. Touch one probe to the end of the ground wire and the other probe to the appliance electrical post. Check the voltage reading. It should be zero. Anything more than that means you have a broken wire, which will read 12.6 volts, or a shorted ground wire, which will read anywhere between 12.6 and 0.