How Does a Car’s Electrical System Work?

An Overview of Your Car’s Electrical System

A belt-driven alternator generates alternating current, which is converted to direct current by means of a rectifier incorporated in the body of the device. The final current output is regulated by a regulator, also incorporated in the device, which acts as kind of pressure regulator, maintaining the output current at between 14.2 and 14.6 volts in order to prevent damage to the battery. Voltages below or above this will invariably damage the battery, starter, the alternator itself, and most of the critical electronic equipment and control systems.

The alternator is designed to keep the battery charged, while at the same time replacing the electrical energy that is continually being drawn by the various components that require electrical power to keep the vehicle in motion. Alternator output current is delivered to the battery and then distributed to where it is needed through fuses, relays, and the wiring harness. If untangled and placed end to end, the wiring harness can be a mile or more in length.

How the ECU Functions

Almost all operational functions of the vehicle are controlled by the ECU (engine control unit), which every second obtains several thousand impulses and readings from up to a dozen or more sensors placed around the engine, transmission, and security system, as well as from the sometimes interconnected suspension and braking systems.

The failure of any sensor or system will at best illuminate a warning light and at worst affect one or more other systems. For example, a defect in the transmission control system software may cause the engine not to accelerate or even to shut off unexpectedly.

Similarly, a failure of the wheel speed sensors may cause the ABS braking system to malfunction, which in turn will affect the active handling and other systems meant to assist the driver in maintaining control in difficult driving conditions.

The main components of the electrical system are:

• Battery
• Alternator, incorporating a rectifier and voltage regulator.
• Fuse box(es).
• Fuses of various capacities. Fuses sometimes number in the dozens.
• Relays. Relays act as switches to minimize the electrical load on the wiring harness.
• Wiring harness.
• Electrical consumers, i.e. everything that needs an electrical current to operate.
• Many sensors in the brake, suspension, transmission, fuel control, and engine management systems.