OBS / GMT400 4-Hi Mod: Keep Low Beams On With Highs

Factory OBS / GMT400 lighting shuts the low beams off when you flip on the high beams. The 4-Hi mod keeps the low beams powered on so all four headlights stay lit together when you need maximum forward visibility. It’s one of the more useful lighting upgrades for a 1988–1998 Chevy or GMC full-size, but it only pays off if it’s wired correctly and isolated properly from the rest of the truck’s lighting circuits.

This guide covers what the mod actually does, which trucks it fits, what to verify on your own harness, how a proper relay-based install works, and where owners get into trouble — especially on trucks with daytime running lights.

Heads up — this is not transfer-case 4HI. “4 HI” in this article means four headlights on high beam — an electrical mod. It has nothing to do with the 4WD transfer case position.

What Is the OBS / GMT400 4-Hi Mod?

Stock GMT400 headlight wiring runs through the column-mounted dimmer switch. The dimmer routes 12V to either the low-beam supply wire or the high-beam supply wire — not both at the same time. When you click to high beam, the dimmer disconnects the low-beam supply, which is why the inner pair of bulbs drops out the instant the high beams come on.

The 4-Hi mod adds a single automotive relay that watches the high-beam circuit. When the high beams turn on, the relay coil energizes and the relay contacts send a fused 12V feed straight from the battery to the low-beam supply wire. Both pairs of bulbs end up lit together. Flip back to low beam, the relay de-energizes, and everything returns to factory behavior.

Operation comparison:

Why owners do it: Factory OBS lighting was thin even in 1988, and 30+ years of harness aging, ground corrosion, and cloudy housings haven’t helped. Lighting four bulbs together on high beam meaningfully widens the foreground spread without changing bulb wattage or buying new housings.

What it doesn’t do: This mod does not improve low-beam output, does not fix voltage drop in the factory headlight harness, does not clean up cloudy lenses, and does not fix bad grounds. Those are separate jobs. If your headlights are dim right now, fix the dim first — then this mod multiplies the result instead of doubling a bad pattern.

Which Trucks Does This Apply To?

The mod applies — with verification — to most GMT400-platform vehicles that use a separate low-beam supply wire and a separate high-beam supply wire at each headlight assembly:

• 1988–1998 Chevrolet C/K pickups — Cheyenne, Silverado, Scottsdale, WT trims, 1500 / 2500 / 3500.
• 1988–1998 GMC C/K and Sierra pickups — SL, SLE, SLT, and Z71 packages.
• 1992–1999 Suburban, Tahoe, and Yukon — GMT400-platform SUVs running the same headlight architecture.
• 1999 Cadillac Escalade (first-generation GMT400 Escalade) — same platform, same lighting layout.

Verify your truck before wiring. GMT400 lighting circuits vary by model year, headlight style, DRL equipment, market (US vs Canadian), and any previous owner modifications. Wire colors that match one writeup may not match your truck. Always confirm the low-beam supply and high-beam supply wires on your actual truck with a multimeter or factory diagram before tapping into anything.

A few platform-specific notes:

• Sealed-beam early trucks (1988 through roughly 1990 depending on trim) still have two bulbs per side — the principle is identical, but the splice point may sit deeper in the harness behind the headlight assembly.
• Composite-headlight trucks (1990+ on most trims) are usually the easiest because the low-beam and high-beam wires terminate at the connector behind each composite housing.
• DRL-equipped trucks (broadly 1990+ Canadian-market, 1995+ US-market) need extra care — DRL voltage on the high-beam circuit can falsely trigger the relay. See the troubleshooting and DRL sections below.
• Aftermarket projectors, retrofits, and dual-filament 9007-style swaps can change which wire carries which function. Verify with a meter first.

If you’re not sure which year or platform you’re working with, our guide to identifying GMT400 years fast walks through the front-clip, interior, and body cues that nail it down quickly.

What You Need for a Proper 4-Hi Mod

Built around a properly fused, single-relay setup. Skip the parts-store quality on a circuit that lives next to a hot exhaust and a vibrating frame — it’s not worth chasing a flickering headlight in three years.

• One 4-pin SPST automotive relay, 30A or 40A, with terminals 30 / 85 / 86 / 87 (standard Bosch-style ISO 7588 relay).
• Relay socket / pigtail with weatherpack or sealed connections preferred.
• Inline fuse holder and fuse for the battery feed. Size the fuse to your wiring, not to the relay — 15A–20A is typical for a low-beam pair on factory-style wiring.
• 16 AWG primary wire in two or three colors so you can tell circuits apart later.
• Crimp connectors — ring terminals for battery and ground, soldered and heat-shrunk splices preferred at the low-beam and high-beam wires.
• Heat-shrink tubing (adhesive-lined) and split loom or wire conduit for harness routing.
• A multimeter or test light — non-negotiable. You need to confirm which wire is high beam and which is low beam on your specific truck.
• Optional: a small isolation diode (1N5408 or equivalent, rated above the load current and above the system voltage with margin) if you choose to add diode protection between the relay output and the low-beam splice.
• Standard hand tools — wire strippers, crimpers, soldering iron, side cutters, zip ties, self-tapping screw or proper bolt for mounting the relay to clean fender steel.

A few things to not buy:

• Generic, no-name eBay 5-pin relays with no terminal markings. You won’t know which pin is which.
• Cheap “tap” connectors (the blue plastic squeeze-on splices). They corrode under the hood and they’re a constant source of flicker complaints.
• Bulbs labeled with wattages well above stock without first upgrading the wiring. The 4-Hi mod alone doesn’t carry that extra current — the factory low-beam wire still does, and that wire is decades old.

How the 4-Hi Mod Works

The whole mod is one relay solving one problem: when the dimmer switch routes power away from the low-beam supply wire, put power back on it from somewhere else.

The circuit logic:

• Trigger: The high-beam supply wire becomes the relay’s coil signal. When the dimmer is in HIGH, 12V appears on the high-beam wire, energizes the relay coil, and pulls the relay contacts closed.
• Output: With the contacts closed, +12V from the battery (through an inline fuse) is connected through the relay to the low-beam supply wire. Both pairs of bulbs are now powered.
• Off state: Click back to low beam. The dimmer cuts the high-beam wire, the coil de-energizes, the contacts open, and the low-beam circuit returns to being fed only by the dimmer switch in low position. Stock behavior, restored.

The relay is doing two jobs at once: it isolates the source of low-beam power (now coming from the battery, not from the dimmer) and it gates that power only when the high-beam circuit is hot. Done correctly, the factory dimmer never sees backfeed and the battery feed never reaches the low-beam wire unless the high beams are on.

Why a relay and not just a jumper. A jumper from the high-beam wire to the low-beam wire would technically light both. It would also send power back through the dimmer switch’s open contact, dump load through wiring not designed for it, and make flash-to-pass do strange things. A properly fused, isolated relay is the right tool.

Step-by-Step OBS / GMT400 4-Hi Mod Installation

Read this whole section before you start. The exact wire colors and splice points depend on your year and harness — the steps below describe the procedure; you verify the wires on your actual truck.

1. Disconnect the battery

Negative cable off first. You are about to splice into both a power-fed circuit and a battery-fed circuit — don’t skip this.

2. Plan the relay location

Pick a spot on the passenger-side inner fender or firewall near the battery, away from exhaust heat, sharp edges, and any moving steering or suspension components. The relay needs to be mountable to clean, paint-stripped metal for its ground.

3. Identify the high-beam supply wire and the low-beam supply wire on your truck

At the headlight connector behind one composite housing (or at the appropriate splice point on a sealed-beam truck), use a multimeter or test light against a known good ground:

• Headlight switch on, dimmer in LOW: the low-beam supply wire is hot, the high-beam wire is not (DRL trucks: the high-beam wire may show reduced DRL voltage even with headlights off — see DRL section).
• Headlight switch on, dimmer in HIGH: the high-beam supply wire is hot, the low-beam wire is not.
• Flash-to-pass momentary: the high-beam wire pulses hot.

Mark the two wires. Do not splice yet.

Wire colors vary. Community shorthand calls the high-beam wire “green” and the low-beam wire “brown” or “tan.” These names are common but not guaranteed for every year and configuration. Always confirm with a meter on the truck in front of you.

4. Run the relay’s battery feed

From the battery positive terminal, run 16 AWG (or heavier if your load demands it) through an inline fuse holder, then to the relay’s terminal 30. Place the fuse holder within 12 inches of the battery. Use a covered ring terminal at the battery.

5. Ground the relay coil

From the relay’s terminal 85, run a short ground wire to a ring terminal on a clean, paint-stripped fender bolt or a dedicated self-tapping screw to bare metal. A bad ground here is the number-one cause of mods that “don’t work right” later.

6. Wire the coil trigger

From the relay’s terminal 86, run a wire to the high-beam supply wire you identified in step 3. Solder and adhesive-lined heat-shrink this splice — don’t use a tap connector.

7. Wire the relay output

From the relay’s terminal 87, run a wire to the low-beam supply wire you identified in step 3. Solder and adhesive-lined heat-shrink this splice as well.

8. Mount the relay and protect the wiring

Mount the relay securely. Route all new wiring inside split loom or conduit, zip-tied away from heat and moving parts. Anchor at multiple points so vibration doesn’t work a splice loose.

9. Reconnect the battery and run the test sequence

See the testing checklist below — do it before you button anything up permanently.

10. Final cleanup

Once everything tests good and operates correctly across all modes, finish the harness routing, double-check every splice, and confirm nothing is touching exhaust, sharp metal, or a moving component.

Testing the Mod Before You Drive

Step through this checklist with the truck parked, in front of a wall or garage door so you can clearly see what’s lighting up. Don’t drive on the mod until every line passes.

• Headlight switch off → all headlights off, no DRL behavior changed.
• Headlight switch on, dimmer LOW → low beams on, high beams off, no flicker.
• Headlight switch on, dimmer HIGH → low beams and high beams on, all four bulbs lit, no flicker.
• Click back to LOW → high beams off cleanly, low beams stay lit.
• Click headlight switch off → all headlights off completely. Nothing stays lit. If anything stays on, stop and re-check wiring before continuing.
• Flash-to-pass → high beams flash, low beams remain on through the flash, then settle back to LOW behavior cleanly.
• DRL-equipped trucks: with engine running, parking brake released, headlight switch off → confirm low beams are not energized by DRL. If they are, see the DRL section below.
• No relay chatter, no buzzing, no smell, no warm wires after a few minutes.

Anything failing the checklist? Don’t drive — troubleshoot first.

Common Problems After the 4-Hi Mod

These are starting points, not guaranteed diagnoses. If the symptom doesn’t match the most likely cause, walk the entire new harness end to end with a meter before throwing parts at it.

Should You Also Upgrade the Headlight Harness?

Probably yes, if your truck has more than a couple hundred thousand miles on the factory wiring.

The 4-Hi mod changes which circuits are lit at the same time. It does not change how much voltage reaches the bulbs through the factory wiring. On a 30+ year-old truck with original headlight harness, original connectors at the housings, and the original headlight switch carrying every amp of bulb load, voltage drop at the bulb is typically meaningful — often well below 12V even with the engine running and the battery charged.

A proper fused relay harness — separate from the 4-Hi mod — runs new heavy-gauge wire from the battery to the bulbs through relays triggered by the factory wiring. The factory switch becomes a low-current trigger, and the actual bulb current rides on new copper. Combined with the 4-Hi mod, you end up with brighter low beams, brighter high beams, and four bulbs at high beam.

What not to do:

• Don’t bump bulb wattage through factory wiring. That’s how the OEM headlight switch gets cooked, and how the harness near the dimmer ends up melted.
• Don’t combine a fused relay harness with the 4-Hi mod without re-checking trigger and output points. Once the headlight bulbs are fed by their own relays, the wires you used to call “low beam supply” and “high beam supply” at the housing are now the relay outputs — your 4-Hi mod needs to be wired to the right side of that.

For a wider look at what’s worth doing to an OBS once it’s yours, see our 9 best OBS Chevy mods that actually matter.

Is the OBS 4-Hi Mod Worth Doing?

For most OBS owners who drive rural roads, do any nighttime work, or just want better forward visibility on a truck that came from the factory underlit, yes.

• It’s inexpensive — a relay, some wire, a fuse, and a couple of solder joints.
• It’s reversible — un-splice the two taps, pull the relay, and the truck is back to factory.
• It plays well with other lighting upgrades — fused relay harness, better housings, properly aimed bulbs.
• It’s a real visibility improvement, not a cosmetic one.

The catch: it’s only worth doing if it’s wired correctly. A poorly executed 4-Hi mod that backfeeds, drops voltage, or fights the DRL system is worse than no mod at all. Take the extra hour and do it right.

Final Thoughts

The 4-Hi mod is exactly the kind of small upgrade that makes an old GMT400 more usable without changing what the truck is. Wire it correctly, protect the circuit, test everything before you drive, and the truck gets a meaningful nighttime improvement without hacking up the factory harness or making the dimmer switch do work it wasn’t designed to do.

Verify the wires on your specific truck. Use a proper fused feed. Use a real relay, not a tap. Solder the splices. And if the truck has DRLs, plan for that before you wire the coil trigger.

For platform background, history, and what to look for when buying a GMT400, our OBS (GMT400) buyer’s guide covers the ground.

Done the 4-Hi Mod on Your OBS?
Post your setup, wiring method, headlight combination, and before/after results in the FullSizeChevy.com forum. Include whether your truck has DRLs, aftermarket housings, or a relay harness so other GMT400 owners can compare what actually works.
→ Share your build and wiring in the FSC 4-Hi Mod forum thread