The vehicle’s electrical system is growing in importance with each new vehicle model year. Many systems that were once driven mechanically are now being redesigned to operate electrically.
The major subsystems of the electrical system include the following:
The battery is the foundation for the entire electrical system. It provides electrical current for starting the engine, provides current to the electrical system when the load exceeds the output of the alternator, and acts as an electrical “shock absorber,” preventing voltage spikes when there is excessive current in the electrical system.
An automobile battery is made up of lead plates immersed in an electrolyte made up of sulfuric acid and water. This is why this type of battery is known as a lead-acid battery. As the battery discharges, the sulfuric acid in the electrolyte is reduced to water, and the lead plates become lead sulfate. Charging the battery restores the chemical composition of the lead plates and the electrolyte. Care must be taken when working with an automotive battery because of the great amount of electrical energy and the highly corrosive sulfuric acid stored in it.
In order to start the engine, an electrically operated starter motor is utilized. When the ignition switch is moved to the “start” position, an electrical current is sent to the starter solenoid, which engages the starter drive gear onto the engine’s ring gear (located on the flywheel). When the drive is engaged, the solenoid connects the battery to the starter motor, which then turns the engine at sufficient speed to start it.
Once the engine is running, the charging system provides electrical current to recharge the battery and power the vehicle’s electrical system. The main component of the charging system is the alternator. The alternator is belt-driven by the engine’s crankshaft, and converts mechanical energy into electrical energy. The alternator produces alternating current (AC), which is then rectified by an internal set of diodes known as the rectifier bridge. The rectifier bridge converts AC to direct current (DC), which can be used to power the vehicle’s electrical system.
A voltage regulator controls the output of the alternator. Normal system voltage during engine operation is around 14.5 volts. Turning on the headlights, heater motor, and other accessories on the vehicle increase the load on the electrical system and system voltage drops. The voltage regulator senses this decrease in system voltage and responds by increasing the alternator’s output to compensate for it. As long as the alternator’s output is able to match the load on the electrical system, the system voltage will remain close to 14.5 volts.
There are many lights built into a vehicle’s lighting system. Headlights illuminate the road ahead of the car, taillights mark the rear of the car for other drivers, and interior lights help the driver see the instrument panel and other areas inside the car when necessary. The various lights are controlled by the driver through switches that turn the electrical current to the lighting circuits on and off.
Fuses or circuit breakers protect the vehicle’s lighting circuits. If, for some reason, the circuit should draw more current than it has been designed for, the fuse will “blow” and cut off current flow. This protects the wires in the circuits from overheating and may even prevent an electrical fire.
Computer control systems work much like the human nervous system. Sensors provide data to the computer, which processes this information and sends signals to actuators to control vehicle functions. In the case of the human body, the eyes and ears send signals to the brain, which processes this information and sends signals to the various muscles to control body movement.
The major components of the automotive computer system include the following:
Automotive technicians communicate with the ECU using a scan tool, which allows them to get information from the computer concerning the operation of the various vehicle systems. The scan tool is connected to the vehicle’s computer system through a diagnostic data link that is typically located near the driver’s seat.
Now, let’s look at how you will use your understanding of electrical and control systems on Test Day.
| Question | Analysis |
| The alternator produces current. |
Step 1: This question asks you to recall the type of current produced by the alternator. Step 2: There is nothing to simplify here. Step 3: The rotating magnetic field of the alternator rotor produces an alternating current (AC) in the alternator stator windings. The AC produced by the alternator must be rectified into a direct current (DC) in order to be compatible with the battery and the rest of the vehicle’s electrical system. The rectifier bridge is the component that performs the rectification of AC into DC. |
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Step 4: Only answer choice (D) addresses the nature of the current produced by the alternator. So it is correct. |
Now try another question on your own.
The correct answer is (D). The PCM (or ECU) is the processor that performs the analysis of the sensor signals and sends control signals to the actuators which makes the PCM (or ECU) the brains of the computer system. Answer choices (A), (B), and (C) are supporting subcomponents of the computer system that do not perform any analysis. Therefore, they do not answer the question.