OBS Headlight Conversion Guide (Sealed Beam to Composite)

GMT400 (1988–1998) – OBS Discussions Exterior
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OBS Headlight Conversion: Sealed Beam → Composite (Replaceable Bulb)

Straightforward upgrade. Better light output, easier maintenance, cleaner look.

What You’re Switching From

Sealed Beam

  • Bulb + lens + reflector = one unit

  • Replace the whole thing when it burns out

  • Basic 2–3 prong plug

Composite

  • Separate bulb + housing

  • Uses modern bulbs (9005/9006, 9007, H4, etc.)

  • Better beam pattern if done right

Parts You Need

  • Composite headlight housings (left/right)

  • Mounting brackets/buckets (composite style)

  • Adjuster screws + springs

  • Retainers / clips / bezels

  • Bulbs (matched to housings)

  • Bulb sockets or pigtails (if plugs don’t match)

  • Adapter harness (recommended)

Optional but smart:

  • Relay harness (more voltage = brighter lights)

  • Dielectric grease

  • Heat shrink + proper connectors

Planning

  • Verify core support compatibility (most GM OBS trucks are easy swaps)

  • Use matching parts (grille + brackets + housings if needed)

  • Avoid junk housings — OEM-style > cheap aftermarket

  • Don’t jam LED/HID into halogen housings unless designed for it

Removal (Sealed Beam)

  1. Disconnect battery

  2. Remove grille and headlight bezels

  3. Remove retaining ring

  4. Pull sealed beam forward and unplug

  5. Remove old buckets/brackets

Install (Composite)

  1. Bolt in composite brackets

  2. Install adjusters + springs

  3. Mount housings

  4. Ensure adjusters move freely

Wiring

Best Option: Adapter Harness

  • Plug factory connector → adapter → new bulbs

  • No cutting, no guessing

Custom Wiring (if needed)

  • Identify: ground / low beam / high beam

  • Match to new socket

  • Solder or crimp properly

Relay Harness (recommended)

  • Pulls power directly from battery

  • Reduces voltage drop

  • Saves your headlight switch

Final Assembly

  • Install bulbs (don’t touch glass)

  • Apply dielectric grease

  • Reinstall grille/bezels

  • Trim plastic if needed

Aim Your Lights (Don’t Skip)

  • 25 ft from wall

  • Mark headlight height

  • Low beam cutoff slightly below that line

  • Even left/right pattern

Common Issues

One side out

  • Bad bulb or ground

High/low not working

  • Wiring mismatch

Dim lights

  • Voltage drop → use relay harness

Bad beam pattern

  • Cheap housings or bad aiming

GM OBS Notes (88–98 C/K)

  • Usually bolt-on if you grab matching brackets + grille

  • Many trucks share core support

  • Easiest conversion compared to Ford/Dodge

Quick Checklist

  • Housings

  • Brackets

  • Adjusters

  • Bulbs + sockets

  • Adapter harness

  • Tools + wiring supplies

  • Time to aim properly

Clean upgrade. Worth doing once, doing right.