Hi everyone, and thanks for checking this out. For the next few installments I will be going over electrical systems as used in trucks. As usual, I will strive for a basic approach and will not overload posts with meaningless theory (I hate that stuff, too). We don't need to know Ohm's law to identify and change a relay!
What follows is a basic description of each type of component. Each will be covered in more detail in later installments, along with picture charts for identification.
Fuses and circuit breakers protect circuits and devices in that circuit from shorts and overvolt situations by being the weak link in the circuit. The fuse or breaker has less current - carrying capacity than the rest of the circuit, so it will burn out or overheat before the circuit is damaged. Once a fuse does its job, it must be replaced, while circuit breakers can be reset and can function again as new. The circuit protector you may not know about is the fusible link. It is spliced into a wire and has a special insulation that causes it to overheat and burn in two if too much current is present. Fusible links are used in many starter engagement circuits and some alternator outputs. They are often hard to spot unless you know what to look for and can only be tested with a meter or test light.
A solenoid is a mechanical device that turns electrical power into mechanical power. An electromagnet is attached to a rod or a valve to do some sort of physical work. Mechanical fuel injection pumps on reefer units have solenoids to shut off fuel flow and shut off the engine. Power door locks use a solenoid to move the lock shut. A starter drive has a solenoid to move the drive gear into contact with the engine ring gear for cranking. An air solenoid uses an electrical signal to open a valve supplying air to an engine fan clutch.
Relays are the most common component in a modern electrical system, and their function is probably the least understood by the average person. They may also be the most important component, because without them, a computer would be unable to control anything, and cab wiring would be a huge, invasive bundle of large wires. A relay in general uses a small input to switch on a large output. For instance, an airbrake relay uses the small amount of air from the foot valve in the cab to switch on a large volume of air from a reservoir, to apply the brakes. A foot valve cannot supply the volume or pressure of air needed to effectively operate the brakes, so that small signal switches on another, high pressure, high volume source. An electrical relay operates the same way. A low power circuit switches on another, high power circuit, which does the work. For instance, the electric horn button on the steering wheel is not directly connected to the horn. Electric horns require a lot of power, so a relay is used. The switch side of the relay is a small electromagnet, so it needs both power and ground. Power in this case is supplied all the time, and the horn button supplies the ground. When the horn button is pushed, the signal closes a set of points in the high power part of the relay and the horn blows. The important thing here is that the relay's switch and power circuits are separate and have no electrical connection between them. Relays will be covered in more detail in a later installment, and the many types used will be illustrated.
Resisters are very important components, and are used to control all variable devices on a truck. A set of resistors is what gives cab heater blowers multiple speeds. The fan speed switch directs the path of power through the resistors and gives multiple speeds. Only two resistors of different sizes thus can give four fan speeds. On low speed, power goes through both resistors on its way to the fan motor, reducing voltage to a low level. On the next higher speed, power goes through the larger of the two resistors, and the smaller resister is used for yet the next higher speed. On the highest fan setting, power bypasses the resistor set and goes directly to the blower motor at full voltage. Actually, this control method almost always operates on the ground side of the motor's circuit, with power again supplied full time, and of course is supplied by a relay. Sets of resistors also control a wiper's intermittent settings, and can also tell a computer what setting is desired for computer - controlled devices.
Conductors are merely the wires, cables, and straps the electricity travels in. The important thing about them is that the capacity of a conductor of a given material is a function of its cross sectional area and its length. A longer wire needs to be larger in diameter to safely carry a given load, because resistance increases in proportion to length.
Well that's all for now, stay tuned next time when we will look at different kinds of fuses and circuit breakers, what they look like, and how they work.Thanks, Bruce
Below is a chart with the basic electrical components listed, what they are, and what they are used for. The text will expand on this chart and one blog will be dedicated to just relays, as they may be the single most important component of all. I will also provide pictures of the more common individuals of each component, for identification purposes.
For the first installment of this series, I will just overview the different components and lay the groundwork for what is to come. Some of the components we will talk about will be familiar, some less so, and some may be a surprise. Hopefully, I can clear up some common misconceptions about some of these devices and what they do, and make it easier for drivers to communicate with mechanics and parts people.
Maybe it would be beneficial to clear up some basic terminology before we proceed.
Maybe it would be beneficial to clear up some basic terminology before we proceed.
- A circuit is a loop that electricity travels in. If you connect two wires to a light bulb and it works, you have a circuit.
- A short circuit is just that - a circuit too short to do the job it is supposed to do. If the power wire going to a light bulb has a bare spot before the light that is touching the frame and the light doesn't work, you have a short circuit, or a short. You know a short because there will be a spark every time the circuit completes, and they cause fires and component damage.
- An open circuit is an incomplete circuit. If a wire breaks or a bulb burns out, that creates an open circuit.
- Amperage and voltage are the two things that characterize the amount of power a circuit carries. An analogy I like is to think of a water hose. The pressure of the water is like electrical amperage and the amount of water coming out in a given period of time is like voltage. If you restrict the circuit by putting your thumb over the end of the hose, you increase the pressure at the cost of less water flow. If you put a bigger resister in an electrical circuit, you increase the amperage in the circuit at the cost of less voltage available after the restriction.
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| Electrical components, functions, and properties |
Fuses and circuit breakers protect circuits and devices in that circuit from shorts and overvolt situations by being the weak link in the circuit. The fuse or breaker has less current - carrying capacity than the rest of the circuit, so it will burn out or overheat before the circuit is damaged. Once a fuse does its job, it must be replaced, while circuit breakers can be reset and can function again as new. The circuit protector you may not know about is the fusible link. It is spliced into a wire and has a special insulation that causes it to overheat and burn in two if too much current is present. Fusible links are used in many starter engagement circuits and some alternator outputs. They are often hard to spot unless you know what to look for and can only be tested with a meter or test light.
A solenoid is a mechanical device that turns electrical power into mechanical power. An electromagnet is attached to a rod or a valve to do some sort of physical work. Mechanical fuel injection pumps on reefer units have solenoids to shut off fuel flow and shut off the engine. Power door locks use a solenoid to move the lock shut. A starter drive has a solenoid to move the drive gear into contact with the engine ring gear for cranking. An air solenoid uses an electrical signal to open a valve supplying air to an engine fan clutch.
Relays are the most common component in a modern electrical system, and their function is probably the least understood by the average person. They may also be the most important component, because without them, a computer would be unable to control anything, and cab wiring would be a huge, invasive bundle of large wires. A relay in general uses a small input to switch on a large output. For instance, an airbrake relay uses the small amount of air from the foot valve in the cab to switch on a large volume of air from a reservoir, to apply the brakes. A foot valve cannot supply the volume or pressure of air needed to effectively operate the brakes, so that small signal switches on another, high pressure, high volume source. An electrical relay operates the same way. A low power circuit switches on another, high power circuit, which does the work. For instance, the electric horn button on the steering wheel is not directly connected to the horn. Electric horns require a lot of power, so a relay is used. The switch side of the relay is a small electromagnet, so it needs both power and ground. Power in this case is supplied all the time, and the horn button supplies the ground. When the horn button is pushed, the signal closes a set of points in the high power part of the relay and the horn blows. The important thing here is that the relay's switch and power circuits are separate and have no electrical connection between them. Relays will be covered in more detail in a later installment, and the many types used will be illustrated.
Resisters are very important components, and are used to control all variable devices on a truck. A set of resistors is what gives cab heater blowers multiple speeds. The fan speed switch directs the path of power through the resistors and gives multiple speeds. Only two resistors of different sizes thus can give four fan speeds. On low speed, power goes through both resistors on its way to the fan motor, reducing voltage to a low level. On the next higher speed, power goes through the larger of the two resistors, and the smaller resister is used for yet the next higher speed. On the highest fan setting, power bypasses the resistor set and goes directly to the blower motor at full voltage. Actually, this control method almost always operates on the ground side of the motor's circuit, with power again supplied full time, and of course is supplied by a relay. Sets of resistors also control a wiper's intermittent settings, and can also tell a computer what setting is desired for computer - controlled devices.
Conductors are merely the wires, cables, and straps the electricity travels in. The important thing about them is that the capacity of a conductor of a given material is a function of its cross sectional area and its length. A longer wire needs to be larger in diameter to safely carry a given load, because resistance increases in proportion to length.
Well that's all for now, stay tuned next time when we will look at different kinds of fuses and circuit breakers, what they look like, and how they work.Thanks, Bruce
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