Electrical systems part 2: what are fuses and circuit breakers, and why are they used?

 Hi, and welcome to the next installment of electrical systems for trucks. Although these are written to help truck and semi drivers, this information will also apply to cars and pickups. Heavy and light vehicles use many of the same circuit protection devices (fuses, circuit breakers, and fusible links), and other electrical components operate in the same manner. Above is a picture of the three most common fuses for automotive use. Many vehicles use two or even all three, often in the same fusebox.

     Let's begin by looking at why fuses are used and why we might need more than one type. As we saw in the first installment of this series, a circuit is a complete loop from a battery and back, with a device installed somewhere along that loop where electricity is used to do some kind of work or to perform a task.  An example would be a simple circuit to light a bulb. We would connect one contact of the bulb to battery +, and the other side to battery -.

 It doesn't matter to a light bulb which contact is hooked to which side of the battery. This makes a complete circuit. The bulb lights because it's filament is resistive to electrical flow, so it heats up and glows. Any time electricity is used to do something, heat is generated. A circuit is engineered to carry a certain amount of electricity, with larger wires used in circuits that carry more power. If a circuit is asked to carry more than it is supposed to, excessive heat will be generated and if enough heat is made, the circuit will burn apart at that part of the circuit that has the most resistance.

     In a well designed and soundly assembled circuit, the fuse provides that point of resistance, the weak link that will burn apart and open the circuit if the electrical flow exceeds the design limits. Two things can cause a circuit to overload and burn, or blow, the fuse. Sometimes someone adds something to a circuit that demands more power than the circuit can handle. This can result in burned connections, melted wiring, or blown fuses. Sometimes insulation on a wire can rub through, usually on a sharp bodywork edge, the edge of a frame or bracket, or where a wire goes through a hole. This allows the conductor to make contact with ground, and causes a short circuit and often melted insulation. If the short happens between the battery and the fuse, lots of wiring can be burned up if the short makes really good contact. For this reason, manufacturers go to great lengths to make sure the power supply to a fusebox is well protected and routed away from trouble. Sometimes a bigger problem is if the short is not making real good contact or makes contact once in awhile when the wire moves. These intermittent problems can drive mechanics and owners crazy, because they can be very hard to reproduce and find.

     Accidents can also cause wiring to short out and these are dangerous because fires can be started. Lightning occasionally strikes a vehicle and the results can be very interesting. Twice I have helped repair trucks that had been damaged by being welded on without the precaution of having the batteries unhooked. In one of those cases, an ECM and a complete engine wiring harness was needed. I also once repaired a wrecker that had been dragged onto a guardrail during a recovery and the main wiring harness under the truck was chopped in two, but not before shorting out the ECM, the injector driver, and the personality module. The cost of repairing these types of problems is usually very impressive.

      If you look closely at the first picture, you can see that the blue MINI fuse is blown. The metal conductor inside the fuse is burned in two after the fuse has done its job. Fuses have other features in addition to being able to be visually checked. The picture below shows that they can be checked with a meter or a test light, without having to remove them from the fusebox. If power exists at both contacts, the fuse is good. The fuse is bad if there is only power at one contact, and if there is no power at either, the fuse can still be checked by using a meter to check continuity between the contacts. Note also that in addition to having the amperage rating printed on the fuse, they are also color coded for amperage.

     Circuit breakers are the next circuit protection device we will look at. Circuit breakers perform the same function as fuses, but they are re-settable. They operate thermally, with a bi-metallic strip that bends and breaks contact when it gets hot. Auto reset breakers reset automatically when they cool off. Other types have a button that pops out, indicating an overload, and the button can be pushed back in to reset. Some heavy circuit breakers like those on liftgates have a flag that pops out and is reset by pushing it back in. Some can replace glass tube fuses, and some can replace blade fuses. Others have unique blade configurations that must be replaced with the same thing. There are studded types that bolt into trailer nose receptacles or power distribution panels. There are too many different types of breakers to list here, but you can go to  http://www.electerm.com/breaker.html  to see a good chart of the different types and their features.

     Circuit breakers are nice because they let you keep going if a problem is intermittent, and don't need replacement every time they trip. Rarely they do need to be replaced, but repeated overloads can damage the mechanism and arcing can burn the contacts. Typically they will still work but may trip prematurely or may not allow enough voltage through the circuit. Fuses are still used for the circuits a customer would most likely check, because they are easy to check and can be bought almost anywhere. Fuses also protect computer power sources, because a breaker may allow a momentary voltage spike to get through, where a fuse will immediately blow in an overload.

    Confusion often exists as to whether something is a breaker, a relay, or a module of some kind. Note that some breakers are fairly large and made of plastic like a relay or module. They may have blade type connections where they plug into a fusebox or electrical panel like a relay, but if it has only two blades, it is a circuit breaker. A relay or module will ALWAYS have more than two blades, and a breaker will NEVER have more than two. The only exception I can think of is a two contact flasher, and those have a unique blade arrangement that is different from any breaker. A breaker will always have an amperage rating printed on it, but they often aren't coded for color like fuses, nor are they all universally interchangeable like standard blade fuses.
    I will say just a few words about the third type of circuit protection, the fusible link. A fusible link is a special piece of wire that burns through at a specific load, like a fuse. The wire is covered with an high temperature insulation that won't burn when the link melts, and this insulation makes it impossible to tell visually if the link is bad. A meter or a light must be used to check for continuity through the circuit. Links are rated by the diameter of the conductor, or more precisely the area of the wire's cross section, in millimeters. Unless you know what to look for, they are hard to spot, and are not used for more than one or two circuits. They are used for things like alternator outputs and starter switch power.
    Well, that's all for this installment, but stay tuned next for a look at relays, which are one of the most important electrical components on a modern vehicle.
Thanks, Bruce
  

Electrical systems Part 1: Electrical system components

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!

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.

• 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.

These definitions may seem a little picky, but understanding the difference between them means that you understand the basic rules of circuits and can understand how the components work.

Electrical components, functions, and properties

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

Communication or: how to get what you want when your vehicle is repaired

Hello again. I thought I would address a problem common in the repair industry, and one that is unfortunately probably getting more common

Often you will not talk to a mechanic when you bring a vehicle in for service. This leads to one of the biggest problems we as mechanics have, that is that we often don't have complete information on what the problem is. The driver usually gives good information as to what happened or what is wrong, but this information sometimes doesn't get relayed to the mechanic. A really good service writer or shop manager will write down what you tell him (or her), then will go through the list with you to make sure everything is covered. Unfortunately, really good service writers are very rare. I am convinced that poor communication between driver and mechanic via this third person is a major reason people are dissatisfied with how repairs are done. If we don't know the when, where and why of the problem, it at best takes longer to diagnose, and at worst leads to misdiagnosis or not getting something fixed the first time. As a driver you must insist that what you say about the problem is written down. Maybe in some cases it is better to just write it down yourself, and take the time to include everything connected to the problem, even if it doesn't seem important. You would be surprised at how often we as mechanics are given some little "unimportant" fact that turns out to be just what we need to figure something out.

Make sure you include when the problem first appeared and any noise, vibration, or unusual feeling that went along with the incident. Did it happen all of a sudden while driving, or did you start the truck up after lunch and find it inoperative? Or did it just kind of gradually get worse over the last three days? Did you smell something hot or burning when it failed? Has anybody else worked on the system lately, and what was done? Are there any fluid leaks in the cab or under the hood? Did anything else quit working at the same time?

Intermittent problems give both drivers and mechanics fits, because it is often hard to reproduce the problem. Here especially the more information, the better. Does the problem happen just when it rains? On rough roads? When the fuel gets low? At a certain road speed? If you can get the problem to appear at will, it often is worthwhile to have the mechanic or service writer ride along so you can show him and let him feel or hear the problem firsthand.

Thanks for your time

Bruce

Hi. New to The Toolbox, soon blogs on truck mechanical systems will be appearing. These may interest new or  beginning mechanics, or anyone who wants to know how truck mechanicals work. Also look for a format change soon.