04 Wiring
Power & relays
Battery, ignition switch, main relay, fuel-pump relay and fusing.
8 min readUpdated 2026-08
The recommended Race Core power scheme is ECU-controlled main relay. The ECU wakes on ignition, then it turns on the EFI relay that feeds injectors, coil 12 V, pump relay and the ECU’s own +12 V MR pin.
Recommended (ECU-controlled main relay)
- 01Battery + → 5 A fuse → ignition switch → B18 (+12 V Raw).
- 02Battery + → 20 A fuse → main-relay pin 30 (or 87, depending on relay convention — the high-current contact).
- 03Main-relay coil: ignition-switched 12 V on one side, Low-side 13 (A10) on the other. The ECU grounds the coil.
- 04Main-relay output → B23 (+12 V MR), injector 12 V rail, smart-coil 12 V rail, fuel-pump relay pin 30, fan relay pin 30.
- 05Power grounds A17, A18, A24, B19 → engine block stud → battery negative with a 20 mm² (4 AWG) or better run.
Fuel pump and fan
| Load | ECU pin | How |
|---|---|---|
| Fuel pump | Low-side 16 (A23) | Grounds the pump-relay coil. Pump current does not go through the ECU. |
| Radiator fan | Low-side 15 (A12) | Grounds the fan-relay coil. Same rule. |
Fusing cheat-sheet
| Circuit | Typical fuse |
|---|---|
| B18 ECU logic | 5 A |
| Main relay feed | 15–25 A (ETB + high-sides + coil 12 V) |
| Injector 12 V rail | 10 A (8 saturated injectors) |
| Fuel pump | 15–25 A at the pump relay, sized to the pump |
| Fan | Sized to the fan, at the fan relay |
| Wideband heater | 5–10 A, on the main-relay output |
See the Power wiring diagram for a drawn version of this scheme.