How Does a Car Ground Strap Work?

by Richard Rowe
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Automotive electrical systems are ostensibly simple and hardy, but they do have a few weak links. The ground strap and grounding system that helps keep your system simple may also prove its undoing, causing vehicle-wide malfunctions resulting from a few little frayed wires.

Electrical System Basics

Electricity itself is the movement of electrons along a substrate, known as a "conductor." Electricity comes in two forms, either alternating current -- where the electrons "vibrate" back and forth along the conductor -- or direct current, where electrons run around the circuit in a single direction. In a direct current system like the one used in your car, electrons flow out of the battery's positive terminal, through the car's electrical system and appliances and back to the battery through its negative terminal.

Power Flow

The car's chassis and engine block actually function as part of its electrical system. The battery's negative terminal connects to the car's frame via a short length of cable, as do the electrical accessories and engine block. Electrons flowing from the battery's positive terminal flow through the ignition system, chassis lights, dashboard, fuel pump and anything else that uses electricity, and back to the battery through the metal chassis. Using the frame itself as part of the electrical system cuts by half the amount of wire required to build the car.

Ground Strap Function

The ground strap or ground wire is the cable that connects the engine block to the chassis, or sometimes directly to the negative battery terminal. This strap completes the circuit for electrical accessories grounded to the engine block instead of directly to the chassis. The accessories may include the ignition system, alternator or any number of sensors. Of these, the alternator requires the most power flow because it produces upward of 200 amps to recharge the battery through its positive output wire.

Ground Strap Failure

Most ground straps aren't wires or cables as you might normally think of them; they're more like ribbons of braided steel lacking any sort of insulation. Over time, constant engine movement and vibration can work-harden the individual strands of steel in the braid, causing them to snap. Once the ground wire becomes compromised, anything that relies on it will weaken, including the starter motor, alternator and ignition system. The results may include a chronically dead battery, hard starting and cylinder misfire resulting in poor fuel economy, loss of power and a probable check engine light.

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