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Some of my friends have asked about off-road lighting and how
things get hooked up. If you are putting it together from used parts or
have decided to pick and choose your own parts for the installation, you may
find this write-up helpful. Here are some basic off-road lighting diagrams
that should help you in your lighting project. Once you understand the
basic diagram, you can easily make a change or two to suit your needs.
Figure 1 shows a typical off-road lighting setup. The power relay is used to switch the current that lights up the headlights. The power relay can handle much more current than your typical dashboard switch. Using a power relay is highly suggested and will help prevent electrical fires. The power relay is shown in the off position. This means the two electrical contacts are NOT connected together, which can be seen in the diagram.
The power relay is really a small electro-magnet with a set of electrical contacts connected to a mechanical arm. One end of the power relay's electrical coil is connected to ground. The other lead is connected to +12V through another switch. When that switch is turned on, the current flows into the electrical coil and creates a magnetic field. This magnetic field causes the mechanical arm to move down and close the electrical contacts. This applies +12V to the off-road lights and turns them on. When the switch is turned off, the +12V is removed from the electrical coil. The magnetic field fails and the spring loaded mechanical arm moves up and opens the electrical contacts. The headlights lose the +12V and turn off. As you can see, the power relay is really nothing more than a remotely control switch, albeit one that can handle a lot of current for the hi-wattage off-road lights.
The switch shown in Figure 1 is typical of the switches used in
this type of circuit. It is a simple on/off switch which may also be
designated in a parts diagram as SPST (single pole, single throw) The +12V that comes from the battery is
applied to the other contact when the
switch is turned on (closed).
In Figure 2, a slight change has been made concerning the power source for the switch. In this setup, the +12V lead that powers the switch is connected to a voltage source that only has voltage applied while the ignition switch is turned on. Wiring your off-road lights in this fashion helps prevent you from accidentally leaving your headlights on after parking your vehicle. The drawback to this kind of setup is that if you want your lights, such as when helping a friend with a trail side repair at night, you will need to turn on your ignition in order to turn on your off-road lights.
Another variation to Figure 2 is to power the +12V switch lead
from your vehicle's headlight switch. In this case, when you turn off your
regular headlights, your off-road lights are turned off too. You can even
modify this configuration a bit such that your off-road lights only come on when
your hi-beam headlights are on. To do this, you would connect the switch's
+12V lead to the wire coming off of your factory headlight switch's hi-beam
Figure 3 show the pin out diagram for a typical 30 amp power relay. The two pins marked Coil, 85 & 86, are used to control the relay. You connect ground to one of these two pins and a switched +12V to the other. When you want to energize the relay, switch the +12V on. Pin 30 is a Common contact. When the relay is turned on (energized), pin 30 makes with the N/O contact, pin 87. When the relay is turned off (de-energized), pin 30 makes with the N/C contact, pin 87a. The N/C contact would not normally be used in a typical off-road lighting setup, so you can leave it disconnected.