Hello, and welcome to truck electrical systems and components part 3. Relays may be the most important component in a truck electrical system, which is ironic because they are also the most misunderstood. Even a few mechanics do not really understand how they work and why they are used, and some cannot even tell them from other components.
There are several reasons relays are used in modern vehicles:
If you study the picture, several interesting things about relays become clear. Note that the wire going to the steering wheel for the horn switch is a ground wire. This keeps you from getting a shock if the wire should short out inside the wheel or the column. Ground is supplied to the horn all the time, and power only when the button is pushed. The really important thing is that this way of powering devices allows the relays to be grouped together a long distance from the switches that turn them on, without needing to run a lot of large, high amperage wires throughout the cab, this makes the wire bundles smaller, less prone to shorts, and creates less heat. Fuse boxes can be mounted on or near the firewall, so the large power wires needed to supply them are short and mostly outside the cab.
This basic arrangement is used for other devices demanding lots of power such as lights, starter solenoids, fuel pumps, and wiper motors. The relay switch coil is shown between terminals 86 and 85. The high power circuit feeding the horn is terminals 30 and 87. Terminal 87a gets power when the relay is not actuated, is not used for this application, and is eliminated in many relays. This terminal numbering system is standardized so that the same terminal has the same function, no matter who made the relay or what it is used for. The relay depicted is a generic and ubiquitous 5-blade relay. It is a cube about 1" square, usually black (although they can be almost any color), and are found in virtually all vehicles. In the picture below, this relay is on the right. Some other common relays are also shown.
There are several reasons relays are used in modern vehicles:
- Relays provide the means by which the low voltage and amperage of a computer output can control another electrical component or system.
- Relays can provide feedback to tell a computer if a device is functioning properly.
- Relays allow much lighter gauge wire to be used behind the dashboard and control panels.
- Relays can and often are, controlled from the ground side of a circuit, so that fewer and shorter live wires go through the firewall and inside the cab. This lessens the chances of short circuits and possible fires.
If you study the picture, several interesting things about relays become clear. Note that the wire going to the steering wheel for the horn switch is a ground wire. This keeps you from getting a shock if the wire should short out inside the wheel or the column. Ground is supplied to the horn all the time, and power only when the button is pushed. The really important thing is that this way of powering devices allows the relays to be grouped together a long distance from the switches that turn them on, without needing to run a lot of large, high amperage wires throughout the cab, this makes the wire bundles smaller, less prone to shorts, and creates less heat. Fuse boxes can be mounted on or near the firewall, so the large power wires needed to supply them are short and mostly outside the cab.
Relays were used for many years before computers were put in vehicles, but as more and more vehicle systems are computer controlled or monitored more relays, and more kinds of them, are used. Computer control would simply not be possible without the use of relays. Computer output circuits operate at very low power and cannot turn other components on or off, by themselves. I was a dealer mechanic when Navistar came out with the Diamond Logic system, where everything on the truck, except the entertainment radio, is controlled by a computer controlling modules on a LAN system. The computer outputs for things like lights, in addition to having hard fuses for the circuits, also had virtual fusing in the computer to protect the outputs against overloads. Many, if not all major truck and car makers now have systems such as this. They have relays in fuseboxes under the hood as well as in the cab, and more remotely mounted in both places. Those not mounted in fuseboxes are never marked for function. I guess the assumption is that they are not customer-serviceable. Luckily relays as a group tend to be very reliable.
As an aside, the transistors used in computer circuits and the earlier vacuum tubes are a kind of relay. For a very interesting tutorial on how transistors work, go tohttp://amasci.com/amateur/transis.html .
If you can get your head around what the author is saying, you will have a good idea of transistor function, as well as what electricity is.
Of course, not all relays are connected to such complex circuits. The relay for a starter, for cab power, or for a liftgate may have 4 or only 3 terminals. 3 terminal relays have 2 large power studs and only one for the switch power, and are grounded by the mounting surface. If you are as old as I am, you remember the older Ford cars and pickups that had a starter relay mounted on the inner fender. Everyone called this a starter "solenoid", but it is really a relay. A solenoid is a different device that has a large electromagnet to move a rod or some other mechanical component. Solenoids are used for starter engagement, reefer engine fuel shutoffs, and power door locks.
Well, that covers the basics of relays, stay tuned next time for resisters, another essential component in modern trucks.
