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How to Wire a Relay in a Car for Lights
By K.K. Lowell, eHow Contributing Writer

When additional accessories are added to a vehicle's electrical system it is always better to modify the wiring slightly and add a relay to the circuit. This change allows more current to be supplied to the new lights or other accessory without increasing the load on the original wiring of the car of truck. In many vehicles the wiring is already nearly overloaded with original equipment needs. The real benefit from wiring a relay in a car for lights is quickly noted: The lights are brighter. And it won't cost you much: Most of the added wiring is less expensive small-gauge wire.
Difficulty: Moderately Easy
Things You'll Need:
· Aftermarket lights
· Automotive relay
· Automotive primary wire
· Circuit breaker
· Toggle switch
· Solderless terminals
· Crimping tool
· Self-tapping screws
· Electric drill
· Socket to fit self-tapping screws
1. Step 1
Mount the new lights in the recommended location, which is commonly as low as possible for fog lights and as high as practical for driving lights. Follow the instructions in your lighting kit for the proper mounting method.
2. Step 2
Locate a protected area near the battery to mount the circuit breaker and the relay using self-tapping screws to attach them to the vehicle. Drive the screws with a socket held in an electric drill. Connect the battery terminal on the circuit breaker with the positive battery terminal by stripping 1/4 inch of insulation from the ends of a piece of 12-gauge wire and installing a solderless connector on each end. Install the connector by placing the stripped end of the wire in the barrel of the connector and squeezing the outside of the colored insulator with the crimping tool until the wire is securely clamped in the connector. Pull on the wire and the connector to ensure a firm crimp. Also connect the ACC terminal on the circuit breaker to terminal 30 on the relay with this gauge wire and a solderless connector.
3. Step 3
Connect relay terminal 87 to the lights with 12-gauge wire and solderless connectors.
4. Step 4
Mount the toggle switch in an easily reached location near the driver. It may be necessary to use an electric drill to make a hole to mount the switch in or under the dash. An option is to purchase a switch with a mounting panel and secure that to the underside of the dash with self-tapping screws. Connect one switch terminal to a fused power supply using 16-gauge wire. Connect the other switch terminal to relay terminal 86.
5. Step 5
Complete the wiring by connecting relay terminal 86 to a metal part of the vehicle with a solderless connector and a self-tapping screw.
Tips & Warnings
  • Connecting your toggle switch to a fused wire that is only live when the ignition key is on will prevent a dead battery should you accidentally leave the lights switched on when you shut the engine off.
  • Always disconnect the negative battery terminal before performing any wiring work in your car.

How to Wire a Relay in a Car for Lights
One kind of handy thing you can do with a relay is to wire your low beam lights to trip with the high beam lights automatically. I haven't done it on my Challenger yet but I've done it on my motorcycle and Avalanche.

The principle this: Ordinarily when you turn on your high beam lights the low beam lights go off. On most cars, including the Challenger, you can slightly pull back on the high beam changer to have both high and low beams and you can easily see the difference.

On my Avalanche I found the hot wire for the high beam circuit and used a sealed splice connector to wire that to the "trigger" for a relay. The relay is then wired to its own power source (30 amp, fused) and then to the low beam circuit.

When the high beams are activated the relay is tripped and the low beams are powered by the relay power circuit. I added a second set of driving lights where the fog lights were and added a second relay for that circuit also. When I hit the high beams I get all three sets.

I haven't done the Challenger yet but I plan to. The relay method causes very little draw on the high beam circuit, only enough to trip the relay, so there should be no issues with the power circuits overloading.
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