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The reason why the chassis is used as part of the electrical circuit is to reduce the amount of wires needed and also to make the installation of the various electrical components easier. This is achieved by only needing a single wire from the positive side of the battery to provide power to the component whilst using a short wire to connect it to the chassis or body to complete the circuit (as the body is bolted to the chassis it also provides an electrical path.) In some cases the component itself is bolted directly to the body or chassis and provides its own earthing point.
If we take it a step further and include some more components then you can see how it all starts to come together. You may notice that the
bulbs share one fuse. This is to show how this is done because a series 3 uses the same system to fuse its components.
The fuse box on a series is located beneath the steering column under a small cover. It only uses 3 fuses to protect all its components although there are 4 in the fuse box. Fuse number 7-8 protects the Fuel gauge, Water temp gauge and brake light circuits. Fuse number 5-6 protects the indicators and wiper motor circuits. Fuse number 1-2 protects the lighting circuit and fuse number 3-4 is a spare. However, yours may be different as Land Rover had a habit of changing things around, or a previous owner may have intervened and either changed things around or added extra wires to cover additional electrical items. To be 100% sure remove yours one by one and see what stops working. Write down these in the back of your manual for future reference as it may help you identify problems in the future. The fuses fitted should all be rated at 35 amp, so if one keeps blowing check its rating is correct. If it is correct then there is a problem else where. Incidentally the diagram above does not represent the actual wiring in a series 3 but its just a means to show you how things are wired together.
Unfortunately the actual wiring on a Land Rover isn’t laid out as neatly and as straight forward as a diagram. Instead wires are routed behind the dash, inside taped together looms, along the chassis, under the wings etc. So to help you trace a wire Land Rover (or to be more correct Lucas) made the wires different colours to help identify which is which without having to actually trace the wire along its actual length. For example if you had an issue with the nearside indicators not working, then provided the bulbs and fuse are ok (check these first) you could check its connection on the indicator stalk by simply removing the cowling and identifying in this case the Green and red wire. The information about what colour a wire is is gained from a wiring diagram. These at first glance look like a nightmare but with a little time and a logical approach these can help you enormously and could save you hours in trying to identify a problem