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 |
|---|---|---|---|
| OEM-1 | connector | ||
| OEM-2 | connector | ||
| P8-C1 | connector | ||
| SP-0V | splice | ||
| SP-14V | splice | ||
| SP-GND | splice |
Create part
Cut list
Lengths need a layout (0 of 39 routed).
| Wire | From / To | Gauge | Signal |
|---|---|---|---|
| W1 | OEM-1-B / SP-14V-1 | 0.85 | ECU power +14V (main relay) |
| W2 | SP-14V-1 / P8-C1-19 | 0.85 | 12 Volts |
| 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 / P8-C1-15 | 0.85 | Power Ground |
| W6 | SP-GND-1 / P8-C1-34 | 0.85 | Power Ground |
| W7 | SP-GND-1 / P8-C1-55 | 0.85 | Power Ground |
| W8 | OEM-2-C / SP-0V-1 | 0.5 | Sensor ground (E2) |
| W9 | OEM-2-D / SP-0V-1 | 0.5 | Sensor ground (E2) |
| W10 | SP-0V-1 / P8-C1-46 | 0.5 | Signal Ground |
| W11 | SP-0V-1 / P8-C1-47 | 0.5 | Signal Ground |
| W12 | SP-0V-1 / P8-C1-49 | 0.5 | Lambda Earth |
| W13 | SP-0V-1 / P8-C1-51 | 0.5 | Signal Ground |
| W14 | OEM-2-K / P8-C1-10 | 0.5 | +5V sensor supply |
| W15 | OEM-2-E / P8-C1-8 | 0.5 | Crank position (CAS NE) |
| W16 | OEM-2-G / P8-C1-27 | 0.5 | Cam position (CAS G) |
| W17 | OEM-1-G / P8-C1-1 | 0.5 | Ignition 1 (coils 1+4, wasted spark) |
| W18 | OEM-1-H / P8-C1-38 | 0.5 | Ignition 2 (coils 2+3, wasted spark) |
| W19 | OEM-2-U / P8-C1-37 | 0.5 | Injector 1 & 3 (batch) |
| W20 | OEM-2-V / P8-C1-18 | 0.5 | Injector 2 & 4 (batch) |
| W21 | OEM-2-Y / P8-C1-54 | 0.5 | Injector 3 (California sequential) |
| W22 | OEM-2-Z / P8-C1-36 | 0.5 | Injector 4 (California sequential) |
| W23 | OEM-2-Q / P8-C1-26 | 0.5 | Coolant temperature (ECT) |
| W24 | OEM-1-J / P8-C1-41 | 0.5 | A/C relay |
| W25 | OEM-1-R / P8-C1-5 | 0.5 | A/C extra cooling fan relay |
| W26 | OEM-1-E / P8-C1-43 | 0.5 | Malfunction indicator lamp (MIL) |
| W27 | OEM-2-W / P8-C1-4 | 0.5 | Idle speed control (ISC) valve |
| W28 | OEM-2-X / P8-C1-42 | 0.5 | Purge solenoid valve |
| W29 | OEM-2-I / P8-C1-24 | 0.5 | Tachometer output |
| W30 | OEM-2-O / P8-C1-25 | 0.5 | Air flow meter (AFM) |
| W31 | OEM-2-M / P8-C1-48 | 0.5 | TPS signal (A/T) |
| W32 | OEM-2-N / P8-C1-12 | 0.5 | O2 sensor |
| W33 | OEM-2-P / P8-C1-45 | 0.5 | Intake air temperature (IAT) |
| W34 | OEM-1-C / P8-C1-13 | 0.5 | Start signal |
| W35 | OEM-1-N / P8-C1-14 | 0.5 | Wide-open throttle switch |
| W36 | OEM-1-P / P8-C1-31 | 0.5 | Power steering pressure switch |
| W37 | OEM-1-Q / P8-C1-32 | 0.5 | A/C request |
| W38 | OEM-1-V / P8-C1-33 | 0.5 | Neutral/clutch switch |
| W39 | OEM-2-L / P8-C1-11 | 0.5 | Idle switch (TPS) |
Bill of materials
No wire parts assigned: 39 wires need a wire part.
| Part number | Manufacturer | Description | Qty | Unit | Unit price | Buy |
|---|---|---|---|---|---|---|
| connector2 lines | ||||||
| 174515-6 | TE | AMP Multilock .040/.070 hybrid 22-pin female harness plug (Miata NA/NB ECU) | 1 | ea | Mouser ↗Digi-Key ↗Amazon ↗ | |
| 174516-6 | TE | AMP Multilock .040/.070 hybrid 26-pin female harness plug (Miata NA/NB ECU) | 1 | ea | Mouser ↗Digi-Key ↗Amazon ↗ | |