Not confident with automotive wiring? Have a professional harness builder or tuner do it, or check your work before first power-up. A wiring mistake can damage the ECU, the engine or the car, or start a fire.
Install modifications
Steps on the car itself, outside the harness. Every harness for this car inherits them.
- Cut the sensor-ground-to-chassis wirerequired
Cut the wire in the engine bay that ties the ECU sensor/signal ground (2C/2D, BLK/LT GRN) to chassis ground, so the standalone gets a clean sensor-ground reference. After cutting, verify with a meter: sensor ground ↔ chassis ground reads open (OL) unless the standalone manual says otherwise.
Why: The stock loom bonds sensor ground to chassis ground; a standalone ECU needs its sensor ground isolated from the noisy chassis/power ground, otherwise sensor readings are noisy or wrong.
Where: Engine-bay ground joint near the ECU (1991 EWD Fig.2 shows the sensor-ground run and its joint connector).
Correction reported by Justin Barrera
Unverified — reported by Justin Barrera (Motorsport Wiring Alliance), not yet sourced.
- Bypass the airflow-meter pump switchrecommended
The stock pump is run by the flapper airflow meter's pump-switch contact through the circuit-opening relay, not by the ECU. The standalone harness adds ECU control of the pump relay (ignition-switched, fused); remove/bypass the AFM pump switch so the two do not fight.
Why: There is no stock fuel-pump output at the ECU, so the standalone provides it. Leaving the AFM switch in place leaves a second, unswitched pump path.
Where: Airflow meter connector (pump-switch contact) and circuit-opening relay.
Correction reported by Justin Barrera
Source: 1991 Mazda Miata wiring diagrams (Mitchell reproduction, mellens.net community mirror)
Components
| Ref | Kind | Part | Exclude from BOM |
|---|---|---|---|
| MAXXECUMINI-C1 | connector | ||
| OEM-1 | connector | ||
| OEM-2 | connector | ||
| SP-0V | splice | ||
| SP-14V | splice | ||
| SP-GND | splice |
Create part
Cut list
Lengths need a layout (0 of 27 routed).
| Wire | From / To | Gauge | Signal |
|---|---|---|---|
| W1 | OEM-1-B / SP-14V-1 | 0.85 | ECU power +14V (main relay) |
| W2 | SP-14V-1 / MAXXECUMINI-C1-H4 | 0.85 | +12V (ECU power supply) |
| W3 | OEM-2-A / SP-GND-1 | 0.85 | Power ground |
| W4 | OEM-2-B / SP-GND-1 | 0.85 | Power ground |
| W5 | SP-GND-1 / MAXXECUMINI-C1-G4 | 0.85 | ENGINE GND |
| W6 | OEM-2-C / SP-0V-1 | 0.5 | Sensor ground (E2) |
| W7 | OEM-2-D / SP-0V-1 | 0.5 | Sensor ground (E2) |
| W8 | SP-0V-1 / MAXXECUMINI-C1-A2 | 0.5 | TRIGGER GND |
| W9 | SP-0V-1 / MAXXECUMINI-C1-B3 | 0.5 | SENSOR GND |
| W10 | SP-0V-1 / MAXXECUMINI-C1-B4 | 0.5 | SENSOR GND |
| W11 | SP-0V-1 / MAXXECUMINI-C1-D4 | 0.5 | SENSOR GND |
| W12 | OEM-2-K / MAXXECUMINI-C1-E2 | 0.5 | +5V sensor supply |
| W13 | OEM-2-E / MAXXECUMINI-C1-A1 | 0.5 | Crank position (CAS NE) |
| W14 | OEM-1-G / MAXXECUMINI-C1-F1 | 0.5 | Ignition 1 (coils 1+4, wasted spark) |
| W15 | OEM-1-H / MAXXECUMINI-C1-F2 | 0.5 | Ignition 2 (coils 2+3, wasted spark) |
| W16 | OEM-2-U / MAXXECUMINI-C1-G3 | 0.5 | Injector 1 & 3 (batch) |
| W17 | OEM-2-V / MAXXECUMINI-C1-H1 | 0.5 | Injector 2 & 4 (batch) |
| W18 | OEM-2-Y / MAXXECUMINI-C1-H2 | 0.5 | Injector 3 (California sequential) |
| W19 | OEM-2-Z / MAXXECUMINI-C1-H3 | 0.5 | Injector 4 (California sequential) |
| W20 | OEM-1-E / MAXXECUMINI-C1-G2 | 0.5 | Malfunction indicator lamp (MIL) |
| W21 | OEM-2-I / MAXXECUMINI-C1-G1 | 0.5 | Tachometer output |
| W22 | OEM-2-O / MAXXECUMINI-C1-C3 | 0.5 | Air flow meter (AFM) |
| W23 | OEM-2-N / MAXXECUMINI-C1-C4 | 0.5 | O2 sensor |
| W24 | OEM-2-Q / MAXXECUMINI-C1-C1 | 0.5 | Coolant temperature (ECT) |
| W25 | OEM-2-P / MAXXECUMINI-C1-C2 | 0.5 | Intake air temperature (IAT) |
| W26 | OEM-1-C / MAXXECUMINI-C1-B1 | 0.5 | Start signal |
| W27 | OEM-1-N / MAXXECUMINI-C1-B2 | 0.5 | Wide-open throttle switch |
Bill of materials
No wire parts assigned: 27 wires need a wire part.
| Part number | Manufacturer | Description | Qty | Unit | Unit price |
|---|---|---|---|---|---|
| connector2 lines | |||||
| 174515-6 | TE | AMP Multilock .040/.070 hybrid 22-pin female harness plug (Miata NA/NB ECU) | 1 | ea | |
| 174516-6 | TE | AMP Multilock .040/.070 hybrid 26-pin female harness plug (Miata NA/NB ECU) | 1 | ea | |