240SX Swap Wiring & Grounding Fundamentals
How to plan power distribution, grounds, fusing, relays and signal wiring so a swap is diagnosable instead of mysterious.
Build a power map
Separate always-hot battery feeds, ignition-switched feeds, ECU/injector/coil power, fuel-pump power, fan power and accessory circuits. High-current loads should not be routed through small OEM signal wires. Document fuse size, wire size and relay trigger for each added circuit.
Grounds are circuits
Battery negative, engine block, chassis and ECU/sensor grounds must have low-resistance paths appropriate to their current. Paint, corrosion, small fasteners and braided straps with poor crimping can create voltage drop that looks like an ECU, starter or sensor failure. Test voltage drop under load rather than relying only on continuity.
Protect signal integrity
Keep crank/cam and other sensitive signal wiring away from high-current fan, alternator and ignition wiring where practical. Preserve shielding/twisted-pair construction when the donor harness uses it. Do not combine sensor grounds with high-current motor grounds without understanding the OEM strategy.
Make it serviceable
Use labels, documented connector pinouts, proper strain relief, sealed connectors where exposed, accessible relays/fuses and a printed wiring map. Future diagnosis is dramatically easier when every added wire has a purpose that can be traced.
Chassis condition, donor version, aftermarket geometry and local legal requirements vary. Measure your car and use the correct factory/component documentation before fabrication or safety-critical work.