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Wiring Relays into your Headlight Circuit

by Kit Wetzler

Parts list:

  • 2 brighter bulbs. I used 80/100 clear bulbs ~$10-$15 each
  • 2 Normally open headlight relays. ~$4-$10 each
  • 8 female spade connectors to fit relays ~$2
  • ~12 ft of black 12 gauge wire ~$2
  • ~12 ft of red 12 gauge wire ~$2
  • 2 30a fuses ~$1
  • 2 fuse holders ~$1.50
  • 2 wire taps ~$1
  • 2 9004 sockets ~$10
  • A bunch of zip ties. ~$3


  • Wire stripper/crimper.
  • Test light.
  • Electrically tape or heat shrink tubing.
  • Soldering iron/solder/flux (optional)
  • Multimeter (optional)


1) First, figure out where you want to mount the relays. They should be close to one of the headlights, I chose the driver's side headlight, because it was close to the battery and there was more room than on the passenger side.

2) Take the headlight socket off of the bulb closest to where you want to mount the relays. I zip tied the old socket to the frame, so I could reverse this process if necessary. I used a simple test light to figure out which wire was hot when the low beams and the high beams were on. For me, with the connector's latch on the top, the passenger side terminal was high beams, the middle was low and the driver's side was ground. (The middle was red w/ a yellow stripe, the passenger (high beam) was black with a red stripe, and the ground was black). Attach a wire tap from each power wire to one side of the coil of the relay (terminals 85 or 86) with a female spade connector. If you look at the diagram on the relay, it should show this:

30 _______/ ________ 87
85 -------| |------- 86

85 and 86 are the trigger and ground and 30 and 87 are where the switched power wires go. [1] Then, use another spade connector and a known good ground and ground the other side of the coil. Repeat for other relay. At this point, turn your lights on and listen for the relays to click. You might want to label the relays at this point. If they don't click you've got some wiring wrong, go back and make sure the wires you tapped into are hot.

3) Measure out the wire you'll need to go from the battery to the relays. You need to run a power wire from the battery to the high beam relay and the low beam relay.[2] Splice a fuse holder into this wire close to the battery and attach a female spade on the other end. Put this wire onto one of the Normally Open (NO) terminals of the relay (30 or 87). Repeat for other relay.

4). Now, run power wires to your new headlight sockets. From the other NO terminal of the low beam relay, run power wires to both low beam terminals of the sockets. Repeat for high beams. Test the power wires, but turning on your lights and making sure the appropriate one is hot.

5) Ground each headlight socket to a good point on the body.

6) Put your brighter bulbs in and turn them on. Go light up the night! [3]


[1] The coil on mine worked both ways, and if yours is similar it doesn't matter which is which. You can test this with a multimeter. Some relays have a diode that prevents them from working both ways. Run DC current through the relays and see if it's the same both ways. (red on one, black on the other, then switch)

[2] Theoretically this could be combined into one, but I didn't. If you have the low beams on and you flash the lights both the lows and highs are on at the same time. I think you'd blow the fuses that way if you were drawing off one power line.And using a big enough fuse to handle the current of both lights on at once would probably not leave much protection for when you don't run both at once.

[3] You might want to aim your headlights a bit downward, to avoid being excessively flashed. With 80 watt low beams your lights will be BRIGHT.