Agreed. A lot left to be desired of them.
The primary challenge between OEM halogen and HID headlight assemblies is the way GM sends the +12v signal to power the lights. Halogen uses a pulse width modulated (PWM) signal and HID is a constant +12v. For halogen, the modulated +12v signal travels from the body control module (BCM) through the under-hood electrical center (UEC) to the lights. On HID, the BCM sends a ground signal to the HID relays in the UEC to activate them, which then send a constant +12v signal to the lights.
Halogen schematic
HID schematic
What this means if you just swap in OEM HID headlight assemblies in an suv that originally had halogen lights, the HID bulbs will flicker rapidly due to the PWM signal activating & deactivating the HID ballasts. Besides the light not being useable, the ballasts and bulbs will most likely wear themselves out quickly being cycled rapidly like that.
There are two ways to address the PWM signal. One is to have your BCM reprogrammed for HIDs. It will require you to send it off for programming and circuits at the BCM and UEC to be moved. Another is a relay harness which is the route I took. The PWM signal cycles so fast the relay can use it to energize itself. Then it’s just a matter of bringing the now constant +12v signal to the HID headlight. I created a relay harness that uses OE connectors for a plug & play solution. I build them just as GM does their harnesses for a factory look. The only sign is the relays mounted to the driver side fender.
Here is my Yukon with OEM HID headlight assemblies and Osram CBI 66340 D3S bulbs.
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