H4 wiring harness upgrade

[dahondr98]

i still dont quite understand where to plug them.

we rip out the old harness?

Yes, a Vinny said (I wasn't near a PC yesterday, as I wasn't at home), you plug the new harness into the old one. You don't rip out the old wires - they are still needed to power the new relays.

Specifically, you have a lead to the battery positive, a plug to plug into the old harness, two new sockets to plug into the headlight globes, and usually two new thick earth wires, one for each headlight socket. You unplug the old headlight sockets, attached to the Honda original equipment wires, and plug one of them into a male plug on the new harness. That gets the power from the old Honda wiring, to switch the relays on the new harness, for low beam and high beam. Then, the new harness has sockets which plug into your headlights. Put the earth wires to handy bolts in the engine bay near the headlights, to earth the non-battery sides of the headlight globes. That's it. Plug it in, attach several wires to battery and earth, mount the relays in a spot in the engine bay between the battery and closest headlight, and you're done.

Because the wires are nice and thick, and are all in the engine bay, the voltage drop is much less. The Honda wiring, while being much thinner, also travels from battery into the cabin of the car, via the headlight switch and dimmer stalk, back into the engine bay, and then to the headlights. It travels much further, and it's much thinner, so the old wiring drops a lot of voltage. The new wiring is much shorter, and much thicker, so is much more efficient. The old wiring is still needed to switch the relay coils, which only take a tiny amount of current, so the load is reduced on the old wires, and on the cabin headlight switch and dimmer switch.