How do I reuse the stock GM underhood fuse block to make a truck harness standalone for a swap?
- Fuse/relay center holds engine fuses plus spare positions. (applies to E38 ECM; Chevrolet Performance NA LS control kit 19171935; LS2/LS3/LS376/LS7/LSX454; 58x/4x; 2006+)
- Current Speartech fuse order: battery, injection/ignition, O2 heaters, fuel pump. (applies to E38/E67 ECM; Speartech 58x manual-transmission harness)
- Published figures differ: An older Speartech LS-58X harness revision uses: #1 fuel pump (orange), #2 battery (orange), #3 O2 heaters (brown), #4 ignition/injection (pink).
- Older Speartech revision lists a different fuse order. (applies to E38/E67 ECM; Speartech 58X with 4L60/4L70/4L80 transmission)
- Published figures differ: Current Speartech 58xman/58xA4 fuse order: #1 battery (orange), #2 injection and ignition (pink), #4 fuel pump (pink).
- BP Automotive includes fuse/relay box, pump relay, fan controls, CEL, OBD2. (applies to Gen III DBW: blue/green P59 PCM; Gen III DBC: red/blue 0411 PCM; 1999-2007 Vortec; 4L60E/4L80E)
- DIY fuse-box rework: removed circuits are unfused or re-fused, then re-loomed. (applies to 0411/P59 factory harness strip-down)
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Confirm against the instructions for your exact parts before wiring. More: Harness and power in our tuning guide.
We recommend a stock truck underhood fuse block from a wrecking yard. It costs about $50 and is small enough to mount on the driver-side inner fender. Key circuits are C1-B11 red (12V battery to the ignition switch), C1-A9 pink (hot in RUN/START, powers the fuse block), and C3-F1 gray (fuel pump relay power). C4 has only one stock A/C wire and can be skipped. Unplug the engine harness's C100 connector under the fuse block; don't cut it off. Then cut and label the wires one at a time. C100 pin C pink needs a fused 10-amp CRANK/RUN 12V+ source because it feeds the transmission solenoids and the PCM. The OBD-II wires and the check-engine lamp also run through C100. Run 12V battery power to the large terminal on the forward side for the fuse BATT supply, and you'll have spare fuses and relays for things like headlights.
Check this first:
- Wire colors and pin positions against your specific fuse block before cutting
- C100 pin C is on a fused 10A CRANK/RUN source
- OBD-II and check-engine lamp wires are kept and labeled
Careful: Unplug C100 instead of cutting it off, and label each wire before you cut it, because a miswired battery feed can damage the PCM or start a fire.
Based on what we've seen work across our own builds, our friends' builds and the wider LS community, reviewed by our lead shop tuner (over 20 years of LS and LT tuning experience). Every combination is different, so use at your own risk and check our safe tuning limits before any wide-open pass.
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Every option we know of
| Maker / product | Part # | Fits | Price | Where | Status / notes |
|---|---|---|---|---|---|
| Walbro / AEM In-tank fuel pump | varies (e.g. Walbro 255) | LS swap fuel systems | about $100-150 | Summit, Jegs | Current production Size to power level |
| Generic Fuel pressure gauge | n/a | Schrader valve on LS rails | about $30-50 | parts stores | Current production Verify 58 PSI at prime |
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.