Some harnesses are yellow, others orange, and others red or green. This varies by vehicle, but assume these brightly colored harnesses and plugs are verboten!
Take note of the bright yellow plugs on the right. Leave this to the pros! If your hot rod is a late model one, then you may want to consider purchasing a shop manual that has a diagram of the wiring harness. You can buy a consumer manual at your local auto parts store from Haynes or Chilton that has the basics. You should also check with your local library because many of them have quite a selection of these on the shelf.
A more advanced manual can sometimes be purchased directly from the manufacturer. When I was in the shop, we relied on manuals from Mitchell, and they were extremely accurate. OK, now that you know about the ignition switch and the wiring harness, you should understand their roles. As I said earlier, I consider the ignition switch to be the gatekeeper. A controller is considered anything that controls or governs the operation of an accessory—be it a simple switch or an on-board computer.
Switches are the simplest of all controllers. Most are manual, meaning that the operator of the vehicle has to operate them manually, such as a turn signal switch. Switches are typically used to control low- to medium-current accessories. Higher current switches are sometimes used for high-powered accessories, such as headlights or fog lights. When using a switch of any kind, it is important to match its current rating to that of the accessory it is controlling—a little bigger is never a bad idea.
As its name implies, it has a single pole common and a single electrical path that can be thrown open or closed. These switches are available in toggles, pushbutton, momentary, and many other styles. In the on position, the switch allows current to flow from the pole to the accessory. In the off position, the switch interrupts the flow of current to the accessory. These switches are easily identified, as they have only two electrical terminals.
This switch is simple in concept, but incredibly versatile as it can be used in a great many arrangements. All of the switches here are of the S. This third terminal is simply a ground input for the light. It is built into the toggle itself that goes on when the switch is closed. Of special interest is the push-button switch on the right.
Press the button once and the switch is thrown one way, press it again and the switch is thrown the other. In the center position, the C is not connected to anything. In the up position, the C is connected to one of the N. A switch like this Figure is what automakers use to control the turn signals. By adding a simple diode to the switch, the up position allows the parking lights only to be on, and the down position allows both the headlights and the parking lights to be on—clever.
The rheostat is built into the light switch assembly, which is used to dim the dash lights in my Mustang. Look closely for the white ceramic part with the spring nearby; this is the rheostat itself. Rheostats: A rheostat Figure is a switch that allows one to vary how much voltage is applied to an accessory. In addition, a rheostat is typically designed to allow the passage of a considerable amount of current. The dimmer switch controller for your dash lights is one such example.
A rheostat is really nothing more than a variable resistor. It has a wiper that rides along a carbon contact surface that has a varying level of resistance, which is linear by design. This allows the switch to have any voltage between 0 VDC and 12 VDC available at its output, based on where the dial is manually set.
Current Ratings: Most switches have a specified current rating. This is the amount of current that the switch can safely pass through its contacts. In fact, OEMs typically use a switch with a higher current rating than the load connected to it, so that the switch provides many years of service.
In addition, this can be a fire hazard. As I said earlier, this is my absolute favorite switch. A relay Figure is simply an electromagnetic switch, so you already know what it does.
The difference between a simple switch and a relay is how the switching is done. Relays are available in all shapes and sizes. Shown are S. Unlocking the Mystery: OK, so why are relays typically regarded as black magic? Since you already know all about switches, the relay is simply an extension of that knowledge. No different than switches, relays are also available in many different variations. This means that a relay really has two electrically isolated parts:.
They can have four or five terminals. Even though they may look similar from the bottom, not all five terminal S. I believe that this is where some of the confusion begins. After all, the five terminal S. T and the S. Diagram illustrates the differences between the four most commonly used automotive relays. How do you know for sure what kind of relay you have or need to purchase when looking at them in the store?
Easy—the body of the relay itself typically has an electrical diagram embossed or stamped on it; this is called the legend, and it is similar to the diagrams in Figure , allowing you to determine which relay you need.
Only S. Five-terminal S. The legends on the housings of this S. This means that you can-not use one of these in a circuit that calls for the other. To power the coil of a relay, voltage needs to be applied across terminals 85 and That being said, some relays have built in quenching diodes see the sidebar on diodes for an explanation so polarity must be observed with these.
The coil is powered to establish an electrical connection between the common and normally open terminals. Note the electrical difference in the two five-terminal S. Terminal 30 is labeled, but is just above the terminal on the left and out of the shot. This same relay has the electrical diagram screened on the side of the body itself.
Obviously, this is an S. Although the Bosch relay looks identical to its S. See, I told you relays were simple. No different than an S. The next chapter explains how to use them in your own projects. Boeing Global Services. Aircraft Spruce. Eastern Aero Supply. Falcon Crest Aviation. Omaha Airplane Supply NE. Omaha Airplane Supply CA. Quality Aircraft Accessories. Varga Enterprises. Aero Performance.
Allied Aircrafts. Textron Aviation Aftermarket Parts and Distribution. The ignition harness contains an insulated wire for each cylinder that the magneto serves in the engine. The ignition harness leads serve a dual purpose. It provides the conductor path for the high tension voltage to the spark plug. It also serves as a shield for stray magnetic fields that surround the wires as they momentarily carry high-voltage current.
By conducting these magnetic lines of force to the ground, the ignition harness cuts down electrical interference with the aircraft radio and other electrically sensitive equipment. A magneto is a high frequency radiation emanating radio wave device during its operation. The wave oscillations produced in the magneto are uncontrolled and cover a wide range of frequencies and must be shielded.
If the magneto and ignition leads were not shielded, they would form antennas and pick up the random frequencies from the ignition system. The lead shielding is a medal mesh braid that surrounds the entire length of the lead. The lead shielding prevents the radiation of the energy into the surrounding area. Capacitance is the ability to store an electrostatic charge between two conducting plates separated by a dielectric.
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