The ground wire connects the car's battery to its metal chassis, completing a circuit. Without the ground wire, a car's electrical circuitry will not work.
Ground wires are important for completing a car's circuitry from its battery to metal and its electronic components like the audio system, fuel pumps and electronic engine controls. Good ground connections need to be free of paint, rust and dirt.
If electrical problems persist after checking fuses, cleaning connections and ensuring the battery doesn't need replacing, then a wiring check is in order. This requires a voltmeter, which measures resistance, current or voltage. To find where voltage stops or is low, wiring from the accessory to the engine block should be tested using a wiring diagram.