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Automotive Electrical Wiring



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This section has diagrams that will explain how a relay works, what the terminals on the relay control and how to wire a relay to control nitrous systems, line locks, transbrakes, etc.  Toward the bottom is a diagram showing how to wire a transbrake and nitrous system together so the nitrous system deactivates while the transbrake is activated.

The standard 12 volt relay is single pole, double throw. There are five terminals; two to activate the relay and the other three are the switching terminals. A typical relay has three terminals that are facing up and down, and two that face side to side. The top terminal (#30) is connected to a 12 volt power supply.  The two side terminals (#85 and #86) are the "activation" terminals and have no polarity. They simply require +12 volts on one and ground on the other to activate the relay. The terminal on the top will be the common "switching" terminal. This means that it will be the one which is always connected to one of the other two switching terminals. When the relay is not "active," the top terminal is internally connected to the middle terminal (#87a). As soon as the relay becomes activated, the top terminal will break it's internal connection to the middle terminal and become connected to the bottom terminal (#87). It's just like flipping a switch, but the flipping action is controlled from a remote source.

One of the main reasons to use a relay is to control a high power source from a lower power source. Let the relay carry the load, not the switch.  As an example, lets look at an electric cooling fan.  For argument purposes lets say this fan draws 30 amps.  A typical rocker or toggle switch isn't going to last long for this application without a relay.  Sure you can get a big bulky switch that will carry the load safely but why not run a relay and use a nice looking switch that will look nice in your dash, console or where ever you choose to put it.  Most standard relays can safely handle loads of 30 amps or more.  If you tap into another switch for activation a relay becomes useful as well.  An example would be using a headlight switch to also control a set of fog lights.  You can also keep your relays close to the device which they control so you can keep the main power wires all in one area and close to the device you are controlling if you so desire.  You can also get creative with relays and use the ground wire as the activation wire which adds another degree of safety because you have ground wires running thru the vehicle which will not short out if they rub thru.

In high current applications, the life span of a switch can be drastically shortened. Even though it may not seem like it, when you flip a rocker switch, it actually takes a long time for it to make contact inside. A long time in electrical terms is still very short, but a switch takes much longer to make its contact than a relay. This long switching time causes internal damage to the switch as an arc is passed across the terminals as they come in contact. The longer it takes to make contact, the longer that arc will burn in the switch. As a switch deteriorates, the internal connections start to build up resistance. This resistance can offer intermittent connections and voltage drops, both of which can be disastrous.  Because a relay is activated much faster, the arc causes minimal damage.

Another use for relays is the ability to control multiple switches at the same time like head lights and fog lights, two cooling fans, a cooling fan and electric water pump, etc.  Yet another use is being able to supply one device from one of two sources. Example, you have a temperature gauge and you want to be able to monitor the engine coolant temperature and the temperature of the coolant in the radiator. You could use two gauges but using a relay you could use one gauge which would be cheaper and possibly be less cluttered. With a relay, you could have a small switch control to control relay with  the gauge going to the top terminal (#30) and each temperature sender going to each of the other switching terminals (#87a and #87).

Two of the top quality relays are Bosch and Potter & Brumfield. Stick with those two brands to minimize any problems you might run into.  Most products that are supplied with relays contain cheap relays.  If you buy a product that includes relays and they are not one of the above mentioned brands, throw the relay(s) away and buy a Bosch and Potter & Brumfield.  A good relay will run you around $8, well worth the money.