What is needed to incorporate the LS harness into the stock wiring of an older truck?
Keep the LS harness fully standalone and only tie it into the truck at a few points. The LS side needs switched ignition that stays hot while cranking, constant battery power, solid grounds, and a fuel pump feed; the truck keeps its lights, wipers, heater, and original starter wiring untouched. Ground everything — motor to frame, body to frame, motor to body, and battery to motor — because most swap no-starts are grounds or ignition power that drops out during crank.
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Who this applies to
- Computers: P59
- Engines: LM7, LQ4, LQ9, LS1, LM4, L59, L33
- Transmissions: 4L60E, 4L80E, TH350, manual
- Not for: None — same approach works for cars
Standalone harness + keep the truck's chassis harness intact (recommended)
- Land the LS harness's constant battery, switched ignition, and ground leads at the truck fuse block — use the old HEI/coil pink wire for switched ignition HEI pink = ignition 1
- Reuse the truck's original starter wire from the key to the starter; keep the truck's constant-power feed at the firewall junction
- Feed the fuel pump from the LS harness's fuel-pump relay output, fused; minimum 4 fuses for injectors and PCM circuits min 4 fuses
- Run the four grounds: motor to frame, body to frame, motor to body, battery to motor 4 ground paths
DIY rework of the factory truck harness
- Depin unused circuits and plug the empty cavities; add a fuse/relay box for injectors, pump, fans, ignition
- Use a labeled fuse/relay panel (basic 12-circuit: 6 fuses, fuel-pump and ignition relays; standard adds fan and A/C relays) 12-circuit panel
Careful
- Switched ignition must stay hot while cranking — many accessory circuits drop out during crank and the PCM will never fire.
Go deeper
Check this first
- Key on, engine off: confirm the LS harness has constant battery power and switched 12V on the ignition feed that does NOT drop out while cranking
- Ohm the ground straps from block to frame and body — should read near zero
Try this, then this
- Land ignition (HEI pink wire) and all four grounds first, then power up — Most swap no-starts are ignition that drops during crank or bad grounds. Worked if: PCM powers up, pump primes
How you know it's fixed
- Fuel pump primes at key-on and the engine starts and idles
Key numbers
| What | Value |
|---|---|
| BP DIY fuse/relay panel contents | basic: 12-circuit block, 6 fuses, fuel-pump relay, ignition relay, OBD-II port |
Every option we know of
| Maker / product | Part # | Fits | Price | Where | Status / notes |
|---|---|---|---|---|---|
| BP Automotive DIY standalone fuse/relay panel (basic / standard) | LT1, 5.7 Vortec, LS1/2/3, LSA, LS9, 99-07 Vortec | bp-automotive.com | In production Requires wiring experience; labeled with install guide | ||
| Painless / American Autowire Universal chassis harness for the truck side | 1960s-1980s GM trucks | Summit Racing, Jegs | In production Full rewire — biggest job of the three routes |
Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.