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Understanding Electrical Accessories

A modern vehicle has many functions that are controlled by the electrical system. In the automotive world, even power steering is now often electrically controlled. Farm Equipment is no different. So far, the only component that manufacturers do not control electrically is the brakes. When things go wrong, it’s important to understand the basic system designs. So here’s a look at two common systems, lights and air conditioning, and some tips on finding faults.


Most light bulbs are simply resistors that get hot enough to give off light when electrical current passes through them. The glass globe of a bulb is filled with an inert gas creating an oxygen-free environment for the resistor — or filament — which is made of an exotic metal. If the glass globe breaks, the resistor is exposed to the atmosphere, causing it to immediately burn out.

The names given to halogen and xenon bulbs reflect the type of gas surrounding their resistor. Using an ohmmeter, you can tell if the bulb is burned out. Touch one lead to the bulb’s base and the other to the contact point (or “foot”) at the bottom, then check the meter’s reading. Current flows into a bulb from the contact point on the bottom (the small brass bulge). It passes through the filament and out through the base. Bulbs with two filaments will have two contact points on the bottom. For additional information on how to use an ohmmeter, check out

Don’t try an ohm’s law calculation (current=volts/resistance) on a light bulb to see what current it will draw. Because a light bulb gets very hot when it is on, the resistance of the filament changes substantially.

HID (high intensity discharge) bulbs do not work the same way normal bulbs do. Rather than using a resistor, high voltages cause a spark to jump across two electrodes, which means they require a ballast in order to operate. Although expensive, they do provide better illumination than most other bulb types.

LED lighting is now becoming very popular. LED stands for Light Emitting Diode. LEDs are replacing many of the incandescent (ordinary) bulbs on vehicles because they draw much less current. If you are thinking of replacing the regular bulbs in your vehicle with LEDs, be careful where you put them. Because LEDs usually do not draw enough current to operate an ordinary flasher for turn signals or hazard lights, flasher units in the same circuit will also need to be replaced by a type compatible with LED lights.

When installing extra lights of any kind, make sure the circuit can handle the higher current flow. Often simply adding extra lights to existing circuits can overload the system. By adding a relay and a new power wire you can use an existing light circuit to trigger additional lights and not create an overload.



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