relay harnesses explained

HID Relay Harnesses Explained

xenon wiring harness
A typical relay power harness for HID Conversion Systems.

The primary function of an automotive headlight wire harness is to provide power directly from the vehicle’s battery to the lighting system. This allows the bulb, or HID ballast, to operate independently of the vehicle’s original wiring system and can provide a power source that is more consistent.

A lighting relay harness is most commonly used in aftermarket HID conversion kits to bypass the factory wiring in order to get around complicated electronic control systems or to allow a higher rate of current to be delivered to the bulbs. A Relay Wiring Harness utilizes a 30amp fuse protected power wire that directly connects to your positive terminal on the battery.  An Automotive Relay is used to pull signal from a source (Headlight connector) that triggers power to be drawn from the battery.  There are two sets of cables meant to provide power to an HID ballast.  There are also two separate ground wire terminals that need to be connected to the chassis to complete the electrical circuit.

relay harnesses explained
The different components of the Accessory Wiring Harness.

As previously stated, this product is most commonly found used in aftermarket xenon retrofits but can also be beneficial when trying to get the most light output from aftermarket or OEM halogen/incandescent light bulbs. Any lighting system can benefit from increased current capacity in the wiring. GTR Lighting offers a wide array of Plug and Play Relay Wire harnesses for H7, H8, H9, H10, H11, H13, 9004, 9005, 9006, 9007 and more. Visit to learn more about the different wiring products available.

What does a Wiring Harness fix?

An Relay Wire Harness is capable of fixing a variety of power related HID lighting issues i.e. intermittent on/off, flickering, delayed start, HID low light output, and uneven brightness.

Problem: Delayed Start
An HID Ballast requires more power to ignite on start-up than during normal usage.  This initial power surge can sometimes be problematic for vehicles with weaker electrical systems that cannot provide the power needed.  The vehicles stock headlight connectors are not capable of providing 7-8 amps per headlight and as a result, only one HID turns on when the switch is turned on.  After about 20 seconds when the ballasts power consumption drops to 3.2 amps, you can quickly turn the lights off then back on and voila, both HID lights turn on.  This scenario happens all the time and is the direct result of insufficient power.


An HID wiring harness with power supplied directly from the battery would eliminate this issue as it would be able to consume up to 30 amps through the batteries positive terminal to ignite both ballasts.

Problem: Flickering
An HID Ballast that is not getting sufficient power from a stock headlight connector will cycle on and off as it tries to ignite and power on.  This can also be a result of an inefficient ballast that is not able to convert the available power to properly ignite an HID system. I always recommend high quality HID components like the 35w CANBUS Pro ballast from GTR Lighting.

An HID wiring harness will provide the power necessary to ignite even an inefficient ballast for flicker free HID lighting.

Problem: Dim/Dark Light Output:
When a ballast is under-performing, light output can appear more blue and dimmer than it should be.  This is a result of the ballast not converting enough power to ignite the bulb at its optimal range.   A bulb that is 4300K connected to a ballast with a weak power source, can look like an 8000K blue color with substantially lower light output.

An HID Wiring harness will be able to provide more power to the ballast allowing the system to operate at 100%.  This however is not a guaranteed solution as a poor quality ballast which will not output sufficient power no matter how much is being inputted.

Problem: Uneven Brightness
My passenger side is brighter than my driver side!  Power in not always delivered in equal amounts to each headlight, and depending on your vehicles wiring configuration, can have a negative impact on lighting performance when installing HIDs.  If one headlight has less resistance to the power source i.e. length of wiring, fuse panels bridging connections etc, it will receive power first leaving less for the second headlight to consume.  In this scenario, one headlight could end up being brighter than the other, or powering up to full brightness faster than the other.

Solution: An HID wiring relay harness will provide equal amounts of power to each HID Ballast allowing for even and consistent brightness between both headlights.

To Buy or Not to Buy?
An HID wiring harness for some vehicles is a safety and assurance item to prevent future problems, and for other vehicles is a necessary requirement to a successful HID installation.  My general recommendation is to always use a harness where possible to help extend the life of your HID system, avoid unnecessary headaches, or to boost the output of your halogen light bulbs.  Not only are you providing your HID ballasts with adequate power, you are protecting both your vehicle and HID system with the inline fuse that separates the systems.

relay harness diagram
Single beam relay harness diagram from GTR Lighting.

27 thoughts on “HID Relay Harnesses Explained

  1. I have a problem with my HID. I installed them myself and they both work but only one of them actually uses the HID light the other other one uses they halogen light that comes with it. I can’t fix it and I’m not sure what’s wrong with it. Is there anyway you can help me. Thank you for taking your time to read this.

    1. Hi Jorge, Do you have the H4 style connector that has the 3 large, flat metal pins coming out of it to plug into your factory wiring? If so I think some of your wires in that connector need to be switched around so it activates the HID instead of the halogen on that one side. Also, try turning on your high beams. Does the problem go away?

  2. If one of my headlights turn on, and the other doesn’t even make a buzzing noise, would that mean it’s a ballast problem?

    1. That buzzing sound is the high voltage of the ballast arcing somewhere either inside the ballast housing, or where it connects to the bulb. This can be caused by a defective ballast, a bad wire connection, or a defective HID bulb. Really the only way to identify the real problem is to swap parts around. Try swapping your HID ballasts from side to side. If the problem follows the ballast, it’s a bad ballast. If the problem stays on the original side, then it’s a bulb or wiring issue. It’s impossible to tell without doing this test.

    1. What type of sound does the relay make? Normally it will make a slightly loud click sound, that’s the noise a relay makes. However, if it is buzzing or “chattering”, then you have a CANBUS problem and I can help you with that.

  3. I installed an HID kit myself. Works fine when not running, but got the flicker & bulb out warning when its running. Got a relay harness with resistors, now they wont work at all, nothing. Tried reversing the plugs, nothing. Any thoughts? Its a 2012 Ram, Spyder projector headlighs with HID on the low beam, H1 bulb.

    1. You’re probably running the wrong HID kit. There’s only one kit and only one CANBUS module out there that will work on that truck. Check this out:

      That video shows what’s REQUIRED in order to do the install on the original reflector headlights. It would be the exact same install on your truck. Just use your existing HID bulbs that are in your Spyder headlights, take out the relay harness and resistors, and install the Headlight Revolution ballast and Dodge-CANBUS-Module.

  4. Both my hid lights goes on but when I look at my dash the high beam indicater shows that it is on. I have a 2003 honda pilot

  5. I have a 2012 Jeep Compass that the light flicker on and off. I’ve put decoder cancels on them and wire harness and still flickers.
    Will a can bus hid kit solve this issue or will I have to buy other parts to stop it?

  6. I installed hid on my 2003 honda pilot and the high beam in my dash stays on. So when I switch to high bean, my high beam flickers.

  7. Hi, with the signal power only connecting to one side,the side that is left unconnected, will that not flag a fault up? As in, make the car think you have a bulb out? As my car gives a warning light when there is a bulb out? Thank you in advance for your help

    1. The answer to your question is, yes for some vehicles. If your vehicle uses a computer to monitor the headlight bulbs then you’ll have to trick the side NOT plugged into the relay harness to think that there is a bulb there. You can do this by either running 2 relay harnesses. Or adding a CANBUS fix like resistors or a CANBUS module:
      CANBUS Module:

  8. HI I’ve had my HID kit installed on my 09 mercedes GL450 for several years now and yesterday both the low beam went out. My high beam is still working. I have a similar kit to the picture above, relay, ballast, canbus module? What could be the problem. I’ve check the fuse on the kit and in my car and it is fine. Please help.

    1. Seth, generally speaking if both all of a sudden go out it’s not an issue with any of the HID components but rather something wrong with the vehicle (switch, computer, wiring, fuse). Put in your OEM bulbs to test and I’m guessing they won’t turn on either. Figure out what’s wrong with the vehicle and go from there.

  9. I have a 2014 Ford Fusion had nothing but problems installing an HID conversion kit. The lights work with the car off but when it’s running they go out. Bought and installed a wiring harness and now they won’t work at all. Double and triple checked all connections, reversed polarity, but still nothing. The relay doesn’t click at all but when I checked it on a separate power source it clicks on and off as it should. It just doesn’t click as part of the wiring harness. Man I’m sick of taking the front bumper on and off just to put the originals back in. Please help

  10. Hi, my 2014 gmc sierra denali had a halogen headlights i put identical looking a new gmc HID headlights they are working fine when car off but when i start the car the hid bulbs blinks two times and turns off and all the DRL are on like they suppose to be

  11. Maaaaan…i bought HID headlights(whole dang headlight, BOTH of em, from PRO-TUNING LAB) they were ABSOLUTELY BEAUTIFUL n FLAWLESS…for bout THREE MONTHS!!!! I’d take it all back now, if ur car don’t already have them? Go LED, they’re predominantly gettin better n brighter, and sooooo much LESS headaches, and ALWAYS lookin, to make sure ur “eye”(inside projector, i.e., LOW BEAM BULB) is on, turnin ur lights, or even car, ON n OFF..its just such a waste..put it this way, if ur woman is pregnant, n goes into labor after dark?? If ur havin problems with ur HID’s, sorry, y’all ain’t makin it ta the hospital! Mine? Like i said, 3 months, beautiful, now? Ill turn car over, ONE FIRES UP(projector bulb) and this time, its driver side, shut my ride down, turn over again, POOF, PASSENGER SIDE IS ON, drivers is OFF?? i changed ballisters, COMPLETE DIFF kinds that were sent IN my headlights(pre-wired), and for THAT DAY, n NIGHT, they worked like the first day i bought them, next night tho??? SAME CRAP!!! Could it be, my battery isn’t big enuff? I have LED’s under my dash, back seats, and in my speaker panels on door, and just 200 watt sony DVD player, no amps or woofers…just infinity 6.5’s thru out, and 4 loud tweets…idk..i wana smash them..

  12. I have a 98 4runner installed a set of HID lights with relay harness both works the first time, the next time only the driver side works and it flicks when I turn it on. what do you think is the problem? thanks.

  13. I have a Navara 2006 i got the Xenon lights already installed and has been working fine for the past year an suddenly both just died. Wat can it be

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