Relay Harness: Why and When You Need One

Relay Harness

Interface Solutions

Here at Race Sport Lighting we are commonly asked questions about interface solutions. What are they? Why do I need them? Well, we are going to be doing a 5 part blog series breaking this topic down. Interface solutions are all about keeping the vehicle’s Pulse-width modulation (PWM) satisfied and communicating properly with your HID or LED headlight upgrade. Although this modulation technique can be used to encode information for transmission, its main use is to allow the control of the power supplied to electrical devices, especially to inertial loads such as motors.

Relay Harness

A relay harness provides the HID ballasts or LED Driver with a steady twelve volts of power. Like amplifiers, these components require good solid power. Not providing this stable voltage could shorten the life expectancy and over all reliability of the component. A relay harness is commonly used in applications where these voltage issues are present:

  • Only one light comes on when starting the vehicle.
  • One light turns off after warm up. Then the light switch must be turned off and on repeatedly to get both lights to come back on.

This harness can be used in conjunction with an Anti-Flick cable to retain functionality of Day Time Running (DRL) lights. This is critical for installation of headlight conversions on 2014 and newer GM vehicles. Also, remember that a relay harness is required for 55 watt HID headlight upgrades.

Installing a Relay Harness

Be aware that one harness will do both left & right headlamps. Please see this helpful diagram below for proper wiring of the harness.


Race Sport Lighting Part Number: RELAY-HARNESS