The 4-Hi mod keeps the low beams lit when you switch to high so all four headlights run together. Cheap, reversible, real improvement at night — when it’s wired correctly.
This thread is for GMT400 owners to share how they did it and what actually worked on their specific truck.
Looking for the basics, install steps, troubleshooting tables, and DRL handling? Read the full guide here:
OBS / GMT400 4-Hi Mod: Keep All Four Headlights On With High Beams — OBS / GMT400 4-Hi Mod: Keep Low Beams On With Highs - FullSizeChevy.com
What to share in your reply:
- Truck: year, model, US or Canadian, DRL equipped or not, sealed-beam or composite housings
- Wiring approach: single relay only, relay + isolation diode, plug-in harness kit, or your own twist
- Where you sourced the relay and fuse
- Wire colors you actually saw on your truck at the splice points (so other owners can cross-reference)
- Any quirks — flicker, DRL interaction, flash-to-pass behavior, anything you had to chase down
- Photos of your relay mounting location, splices, and the lit headlights before/after
Especially looking for input from owners with:
- 1990–1994 Canadian-spec trucks (early DRL hardware)
- 1995–1998 US-spec trucks with factory DRL
- HID or LED retrofits running through the 4-Hi mod
- Plug-in harness kit users — which one and would you buy it again
Post the truck, post the wiring, post the result. The more model-year specifics in this thread, the more useful it gets for the next owner standing in front of an OBS with a multimeter.