TRS Tech
Technology: Wire Harnesses Explained
A wire harness is necessary to adapt your factory harness to work with your newly retrofitted HID headlights. Generally speaking you can't just plug the ballasts into where the halogen bulbs went before and expect them to work reliably or even at all. A harness works this way: Your factory socket that originally connected to the halogen bulb now switches on a relay. The relay is a switch that draws power from the car battery and sends it out to the ballasts. It's pretty simple, but this tends to be the part that confuses and deters people from doing a retrofit. In the end, the harness will allow the HID retrofit (Bi-xenon or low beam only) to operate just like stock.
A bi-xenon harness is a bit more complicated compared to a standard low beam wire harness. A typical dual-filament halogen bulb has 3 pins: one for the low beam, one for the high beam, and a ground. In low beam mode, the low beam pin gets power from the factory output harness, and in high beam mode the high beam pin does. This is to say that when you turn on the high beams, your low beams are no longer running.
In a Bi-xenon retrofit, you need the low and high beams powered on at the same time to create the high beam. Otherwise, when you hit the high beams, the projectors will turn to high beam mode but the ballasts will turn off - leaving you with nothing but darkness. A properly configured harness will correct for this problem. There are two ways a bi-xenon harness can be setup:
- With a diode
- the diode will be installed between the low and high beam input wires for the harness. A diode is a like a one-way road that lets current flow from the high beam lead into the low beam lead - allowing full functionality within the harness even though the low beam pin is no longer being activated by the factory output harness.
- With a control box
- this will electronically control the function of the low and high beams via it's circuitry. When the control box reads "low beam input pin active" it will only power on the ballasts. When the control box reads "high beam input pin active" it knows to send current to the ballasts and the bi-xenon solenoids.
Ground switched vs. Positive switched harnesses
Positively switched headlights are much more common, but a lot of applications nowadays run what is known as ground switched headlights. (Toyota trucks and some newer Chryslers for example) The difference here is in pin that becomes active to complete the circuit, and thus trigger the harness to activate the headlights. In the more common positively switched setups, the ground is constant and the positive pin switches on and off. In ground switched setups, the positive pin stays hot, and the system is grounded when the headlight switch is turned on inside the cabin.
Last and often confusing wiring topic
When your car has a separate bulb for the low and high beams but you want to use a bi-xenon projector. A typical bi-xenon relay harness won't work because you don't have the proper 3 pin output for it, but a simple low beam harness does not have all the outputs needed to run the bi-xenon projectors. Here, you can easily wire up the projectors so the bi-xenon solenoid activates the high beam normally. Basically all you need to do is take the 2 wires coming out the back of the projector, run them over to the halogen high beam side of your headlight and use some "crimp on" wire taps to splice the +/- wires together. It is relatively easy and effective.
Our recommendation for retrofitters
There are only a couple important steps in choosing a wire harness.
- What kind of headlight bulb did your vehicle originally take? (ie 9006, H4, etc)
- Are you doing a low beam, or a bi-xenon retrofit?
With that, you'll be able to determine what kind of harness you need. (ie 9006 low beam, or H4 bi-xenon etc)
