How Does the Ground in a Car's Electrical System Work?

by Don Bowman

Whenever an electrical system is being worked on, it is important that everything that requires power have an appropriate ground. All electrical components need power, and the power must make it back to ground. It must be a complete circuit. That is the reason it is called a circuit.


The power on a vehicle starts at the battery. There are ordinarily two wires coming off the positive terminal. One large-diameter wire goes from the battery positive terminal to the starter. The second wire goes to the fuse and relay block. It is attached to a common buss bar. A buss bar is a flat rectangular piece of metal that more than one wire can be attached to, to supply power to various areas of the vehicle. This buss bar is always protected by fusible links to prevent fire if a short occurs. These main wires coming off the buss bar go to circuits such as the radiator fan relays, the horn relay, the ECM computer, the interior of the vehicle and the air conditioning. Each of the circuits has an independent fuse just for that circuit.

Fusing and Grounds

The main fuses can be recognized by amperage. They are all from 50 to 60 amps, where the independent fuses will range from 7.5 to 30 amps. Every piece of equipment that operates off battery voltage must have a way to get back to ground. All components will utilize one of these methods. The first method is a common ground that connects several grounds to one wire that then goes to a good location on the frame or to the engine.

The second method is to have a wire from the negative side of the part go to a switched grounding device like the on-board computer or ECM. When this is the case, the computer controls the function of the part by switching the ground on and off. The last method is to have a power wire go to one side of the part and the other side goes to the nearest ground (via the ground wire). Remember, no part will operate unless it has power and a good clean ground to allow the power to continue back to the battery.

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