Tech Tip: Aux. Lighting Wiring Harness

By Frank Twarog

The proper installation of any auxiliary lighting to a Land Rover - whether it be a pair of driving lamps or a work lamp - requires a wiring harness, a relay and a switch.

The first step for installation is to disconnect the battery. (if your vehicle has a radio code - make sure you have the correct code before disconnecting the battery)

Now locate the relay in a position that is both convenient for wiring purposes and is easily accessible (Hella offers a fused relay - if the fuse blows you'll want to be able to replace it easily). It is important to include a relay in your installation, as it helps to prevent voltage drop - and thus produces optimal performance in your lights. Keep in mind that a mere 10% reduction of optional operating voltage will produce a 33% drop in optimal lighting intensity.

Find a main power source - your battery or starter - and run a fused lead to the relay itself (#30 on the Hella and PIAA relays). Solder correct fittings onto your wire (rather than using crimp-ons), as this creates a connection that will not corrode and will prevent the wire from loosening from the fitting over time.

Next, you'll want to install the switch - either the one provided with a pre-assembled wiring harness, a genuine Land Rover switch appropriate to your model or an aftermarket switch of your choice. If you are passing any wires through your bulkhead (firewall) try to use existing holes - if you must drill new ones always touch-up the metal and insert a rubber grommet to prevent the sharp steel from stripping the insulation of your wires. Run a lead from the switch directly to the relay (#86 terminal)

Now it is time to determine how you want to activate the lamps. The correct specification for fog lamps is to only operate on low beam - driving lamps should only operate on high beam. By including the switch you can choose to turn the lamps off in situations where fog or driving lamps are not required. Consult your workshop manual's wiring diagram or use a test lamp to determine which lead powers the main beam (for fog) or high beam for driving. Splice a lead to that wire and connect it to the other terminal on your new switch (for newer model owners, an additional option is to run another lead from the same terminal or the back of the new switch up to one of the unused indicator lamps within the dashboard cluster. Then run a ground lead from the dash bulb itself. This will allow you to illuminate one of these dash bulbs when the lamps are in use).

It is now time to ground the relay. Attach a lead to the #85 terminal on the relay and secure the other end to an appropriate ground - if you are grounding to a painted area, use a star washer underneath the end fitting of the head in order to score the surface and create a good contact. Be sure to use some type of silicon grease to stop corrosion at the wire ends.

The final step is to connect the lamp assemblies to the #87 terminal of the relay. Certain model lamps will ground themselves to the base that they attach to - if not, ground the negative terminal of the bulb in the same manner as above. Reconnect your battery, turn on the lamps and adjust accordingly.

For a complete listing of off-road driving, fog and european lamps, see our Hella and PIAA on-line selection.

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