MX-5 Miata NA6 1990-93 (B6-ZE, 1.6) → Haltech Nexus S3 HT-213000 (auto)
MX-5 Miata NA6 1990-93 (B6-ZE, 1.6) — Haltech — Nexus S3
⚠ NOT CHECKED. Don't build from this.
Drawn by Michael Gnankou · Rev 0 · draft · 2026-10-09
Before you start
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)
| Status | draft · not yet verified: nobody has reported building and running this harness |
|---|---|
| Open errors | none |
| Size of the job | 33 wires · 7 connectors · 2 part lines |
| Tools | Crimp and extraction tools for each terminal family (see Connectors) · wire strippers · multimeter · heat gun + heat-shrink |
| Order of work | 1. Check every part against the parts list. 2. Cut and label every wire (labels: Export → Wire labels). 3. Crimp and pin connector by connector, ticking the build checklist. 4. Run the bench test with nothing plugged in and the battery disconnected. 5. Only then connect the ECU and power up. |
Build checklist
Tick each box as you go. Continuity: meter from end to end, expect under 1 Ω.
| Wire | From | To | Gauge | Colour | Cut | Label | Crimp A | Crimp B | Pin A | Pin B | Continuity |
|---|---|---|---|---|---|---|---|---|---|---|---|
| W1 | OEM-1-B | SP-14V-1 | 0.85 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W2 | SP-14V-1 | NEXUSS3-A-18 | 0.85 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W3 | SP-14V-1 | NEXUSS3-A-25 | 0.85 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W4 | OEM-2-A | SP-GND-1 | 0.85 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W5 | OEM-2-B | SP-GND-1 | 0.85 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W6 | SP-GND-1 | NEXUSS3-A-10 | 0.85 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W7 | SP-GND-1 | NEXUSS3-A-11 | 0.85 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W8 | OEM-2-C | SP-0V-1 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W9 | OEM-2-D | SP-0V-1 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W10 | SP-0V-1 | NEXUSS3-C-26 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W11 | OEM-2-K | NEXUSS3-C-25 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W12 | OEM-2-E | NEXUSS3-C-1 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W13 | SP-0V-1 | NEXUSS3-C-2 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W14 | OEM-2-G | NEXUSS3-C-3 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W15 | SP-0V-1 | NEXUSS3-C-4 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W16 | OEM-1-G | NEXUSS3-A-27 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W17 | OEM-1-H | NEXUSS3-A-28 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W18 | OEM-2-U | NEXUSS3-A-1 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W19 | OEM-2-V | NEXUSS3-A-2 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W20 | OEM-2-Y | NEXUSS3-A-3 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W21 | OEM-2-Z | NEXUSS3-A-4 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W22 | OEM-1-E | NEXUSS3-A-9 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W23 | OEM-2-I | NEXUSS3-A-12 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W24 | OEM-2-O | NEXUSS3-C-10 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W25 | OEM-2-N | NEXUSS3-C-11 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W26 | OEM-2-Q | NEXUSS3-C-12 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W27 | OEM-2-P | NEXUSS3-C-13 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W28 | OEM-1-C | NEXUSS3-C-5 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W29 | OEM-1-N | NEXUSS3-C-6 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W30 | OEM-1-P | NEXUSS3-C-7 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W31 | OEM-1-Q | NEXUSS3-C-8 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W32 | OEM-1-V | NEXUSS3-C-19 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | |
| W33 | OEM-2-L | NEXUSS3-C-20 | 0.5 | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ | ☐ |
Bench test (before first power-up)
Pre-power install steps (make these cuts/modifications on the car first, then verify the checks below): Cut the sensor-ground-to-chassis wire.
Battery disconnected, ECU and every sensor unplugged. Meter on resistance. Each pair below must read open (OL): a reading means a pinched wire, a stray strand or a mis-pinned cavity. Then check continuity of every wire (last column of the build checklist).
| # | Probe A | Probe B | Expect | Why | Reading |
|---|---|---|---|---|---|
| 1 | NEXUSS3-A-18 | NEXUSS3-A-10 | open (OL) | +12 V supply must not touch power ground | |
| 2 | NEXUSS3-A-18 | NEXUSS3-C-2 | open (OL) | +12 V supply must not touch sensor ground | |
| 3 | NEXUSS3-A-10 | NEXUSS3-C-25 | open (OL) | power ground must not touch 5 V sensor reference | |
| 4 | NEXUSS3-A-18 | NEXUSS3-C-25 | open (OL) | +12 V supply must not touch 5 V sensor reference | |
| 5 | NEXUSS3-C-2 | NEXUSS3-C-25 | open (OL) | sensor ground must not touch 5 V sensor reference |
If any reading doesn't match, stop and get help before powering up.
Revisions
| Rev | Label | By | Date |
|---|---|---|---|
| No revisions yet. | |||
Schematic
Connectors
NEXUSS3-A
| Cavity | Function | Signal | Wire | Colour | Gauge (mm²) | Destination | Terminal | Seal | Plug |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Injector 1 | — | W18 | — | 0.5 | OEM-2-U | — | — | — |
| 10 | Battery ground | — | W6 | — | 0.85 | SP-GND-1 | — | — | — |
| 11 | Battery ground | — | W7 | — | 0.85 | SP-GND-1 | — | — | — |
| 12 | DPO 2 | — | W23 | — | 0.5 | OEM-2-I | — | — | — |
| 13 | Ignition switch input | — | — | — | — | — | — | — | — |
| 14 | DPO 3 | — | — | — | — | — | — | — | — |
| 15 | DPO 4 | — | — | — | — | — | — | — | — |
| 16 | DPO 5 | — | — | — | — | — | — | — | — |
| 17 | DPO 6 | — | — | — | — | — | — | — | — |
| 18 | +12V switched input | — | W2 | — | 0.85 | SP-14V-1 | — | — | — |
| 19 | HBO 1 | — | — | — | — | — | — | — | — |
| 2 | Injector 2 | — | W19 | — | 0.5 | OEM-2-V | — | — | — |
| 20 | HBO 2 | — | — | — | — | — | — | — | — |
| 21 | HBO 3 | — | — | — | — | — | — | — | — |
| 22 | HBO 4 | — | — | — | — | — | — | — | — |
| 23 | CAN 1 High | — | — | — | — | — | — | — | — |
| 24 | CAN 1 Low | — | — | — | — | — | — | — | — |
| 25 | +12V switched input | — | W3 | — | 0.85 | SP-14V-1 | — | — | — |
| 26 | ECR output | — | — | — | — | — | — | — | — |
| 27 | Ignition 1 | — | W16 | — | 0.5 | OEM-1-G | — | — | — |
| 28 | Ignition 2 | — | W17 | — | 0.5 | OEM-1-H | — | — | — |
| 29 | Ignition 3 | — | — | — | — | — | — | — | — |
| 3 | Injector 3 | — | W20 | — | 0.5 | OEM-2-Y | — | — | — |
| 30 | Ignition 4 | — | — | — | — | — | — | — | — |
| 31 | Ignition 5 | — | — | — | — | — | — | — | — |
| 32 | Ignition 6 | — | — | — | — | — | — | — | — |
| 33 | Ignition 7 | — | — | — | — | — | — | — | — |
| 34 | Ignition 8 | — | — | — | — | — | — | — | — |
| 4 | Injector 4 | — | W21 | — | 0.5 | OEM-2-Z | — | — | — |
| 5 | Injector 5 | — | — | — | — | — | — | — | — |
| 6 | Injector 6 | — | — | — | — | — | — | — | — |
| 7 | Injector 7 | — | — | — | — | — | — | — | — |
| 8 | Injector 8 | — | — | — | — | — | — | — | — |
| 9 | DPO 1 | — | W22 | — | 0.5 | OEM-1-E | — | — | — |
NEXUSS3-C
| Cavity | Function | Signal | Wire | Colour | Gauge (mm²) | Destination | Terminal | Seal | Plug |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Trigger + | — | W12 | — | 0.5 | OEM-2-E | — | — | — |
| 10 | AVI 1 | — | W24 | — | 0.5 | OEM-2-O | — | — | — |
| 11 | AVI 2 | — | W25 | — | 0.5 | OEM-2-N | — | — | — |
| 12 | AVI 3 | — | W26 | — | 0.5 | OEM-2-Q | — | — | — |
| 13 | AVI 4 | — | W27 | — | 0.5 | OEM-2-P | — | — | — |
| 14 | AVI 5 | — | — | — | — | — | — | — | — |
| 15 | AVI 6 | — | — | — | — | — | — | — | — |
| 16 | AVI 7 | — | — | — | — | — | — | — | — |
| 17 | AVI 8 | — | — | — | — | — | — | — | — |
| 18 | AVI 9 | — | — | — | — | — | — | — | — |
| 19 | SPI 5 | — | W32 | — | 0.5 | OEM-1-V | — | — | — |
| 2 | Trigger - | — | W13 | — | 0.5 | SP-0V-1 | — | — | — |
| 20 | SPI 6 | — | W33 | — | 0.5 | OEM-2-L | — | — | — |
| 21 | CAN 2 High | — | — | — | — | — | — | — | — |
| 22 | CAN 2 Low | — | — | — | — | — | — | — | — |
| 23 | Knock 1 | — | — | — | — | — | — | — | — |
| 24 | Knock 2 | — | — | — | — | — | — | — | — |
| 25 | +5V | — | W11 | — | 0.5 | OEM-2-K | — | — | — |
| 26 | Signal ground | — | W10 | — | 0.5 | SP-0V-1 | — | — | — |
| 27 | AVI 10 | — | — | — | — | — | — | — | — |
| 28 | AVI 11 | — | — | — | — | — | — | — | — |
| 29 | WBI 1 Heater + | — | — | — | — | — | — | — | — |
| 3 | Home + | — | W14 | — | 0.5 | OEM-2-G | — | — | — |
| 30 | WBI 1 Input | — | — | — | — | — | — | — | — |
| 31 | WBI 1 Pump | — | — | — | — | — | — | — | — |
| 32 | WBI 1 Nernst | — | — | — | — | — | — | — | — |
| 33 | WBI 1 Heater - | — | — | — | — | — | — | — | — |
| 34 | WBI 1 Cal | — | — | — | — | — | — | — | — |
| 4 | Home - | — | W15 | — | 0.5 | SP-0V-1 | — | — | — |
| 5 | SPI 1 | — | W28 | — | 0.5 | OEM-1-C | — | — | — |
| 6 | SPI 2 | — | W29 | — | 0.5 | OEM-1-N | — | — | — |
| 7 | SPI 3 | — | W30 | — | 0.5 | OEM-1-P | — | — | — |
| 8 | SPI 4 | — | W31 | — | 0.5 | OEM-1-Q | — | — | — |
| 9 | +8V | — | — | — | — | — | — | — | — |
OEM-1
| Cavity | Function | Signal | Wire | Colour | Gauge (mm²) | Destination | Terminal | Seal | Plug |
|---|---|---|---|---|---|---|---|---|---|
| A | Constant +12V | — | — | — | — | — | — | — | — |
| B | ECU power +14V (main relay) | — | W1 | — | 0.85 | SP-14V-1 | — | — | — |
| C | Start signal | — | W28 | — | 0.5 | NEXUSS3-C-5 | — | — | — |
| E | Malfunction indicator lamp (MIL) | — | W22 | — | 0.5 | NEXUSS3-A-9 | — | — | — |
| G | Ignition 1 (coils 1+4, wasted spark) | — | W16 | — | 0.5 | NEXUSS3-A-27 | — | — | — |
| H | Ignition 2 (coils 2+3, wasted spark) | — | W17 | — | 0.5 | NEXUSS3-A-28 | — | — | — |
| N | Wide-open throttle switch | — | W29 | — | 0.5 | NEXUSS3-C-6 | — | — | — |
| P | Power steering pressure switch | — | W30 | — | 0.5 | NEXUSS3-C-7 | — | — | — |
| Q | A/C request | — | W31 | — | 0.5 | NEXUSS3-C-8 | — | — | — |
| V | Neutral/clutch switch | — | W32 | — | 0.5 | NEXUSS3-C-19 | — | — | — |
OEM-2
| Cavity | Function | Signal | Wire | Colour | Gauge (mm²) | Destination | Terminal | Seal | Plug |
|---|---|---|---|---|---|---|---|---|---|
| A | Power ground | — | W4 | — | 0.85 | SP-GND-1 | — | — | — |
| B | Power ground | — | W5 | — | 0.85 | SP-GND-1 | — | — | — |
| C | Sensor ground (E2) | — | W8 | — | 0.5 | SP-0V-1 | — | — | — |
| D | Sensor ground (E2) | — | W9 | — | 0.5 | SP-0V-1 | — | — | — |
| E | Crank position (CAS NE) | — | W12 | — | 0.5 | NEXUSS3-C-1 | — | — | — |
| F | NC | — | — | — | — | — | — | — | — |
| G | Cam position (CAS G) | — | W14 | — | 0.5 | NEXUSS3-C-3 | — | — | — |
| I | Tachometer output | — | W23 | — | 0.5 | NEXUSS3-A-12 | — | — | — |
| K | +5V sensor supply | — | W11 | — | 0.5 | NEXUSS3-C-25 | — | — | — |
| L | Idle switch (TPS) | — | W33 | — | 0.5 | NEXUSS3-C-20 | — | — | — |
| N | O2 sensor | — | W25 | — | 0.5 | NEXUSS3-C-11 | — | — | — |
| O | Air flow meter (AFM) | — | W24 | — | 0.5 | NEXUSS3-C-10 | — | — | — |
| P | Intake air temperature (IAT) | — | W27 | — | 0.5 | NEXUSS3-C-13 | — | — | — |
| Q | Coolant temperature (ECT) | — | W26 | — | 0.5 | NEXUSS3-C-12 | — | — | — |
| U | Injector 1 & 3 (batch) | — | W18 | — | 0.5 | NEXUSS3-A-1 | — | — | — |
| V | Injector 2 & 4 (batch) | — | W19 | — | 0.5 | NEXUSS3-A-2 | — | — | — |
| Y | Injector 3 (California sequential) | — | W20 | — | 0.5 | NEXUSS3-A-3 | — | — | — |
| Z | Injector 4 (California sequential) | — | W21 | — | 0.5 | NEXUSS3-A-4 | — | — | — |
SP-0V
| Cavity | Function | Signal | Wire | Colour | Gauge (mm²) | Destination | Terminal | Seal | Plug |
|---|---|---|---|---|---|---|---|---|---|
| 1 | — | — | W8 | — | 0.5 | OEM-2-C | — | — | — |
| 1 | — | — | W9 | — | 0.5 | OEM-2-D | — | — | — |
| 1 | — | — | W10 | — | 0.5 | NEXUSS3-C-26 | — | — | — |
| 1 | — | — | W13 | — | 0.5 | NEXUSS3-C-2 | — | — | — |
| 1 | — | — | W15 | — | 0.5 | NEXUSS3-C-4 | — | — | — |
SP-14V
| Cavity | Function | Signal | Wire | Colour | Gauge (mm²) | Destination | Terminal | Seal | Plug |
|---|---|---|---|---|---|---|---|---|---|
| 1 | — | — | W1 | — | 0.85 | OEM-1-B | — | — | — |
| 1 | — | — | W2 | — | 0.85 | NEXUSS3-A-18 | — | — | — |
| 1 | — | — | W3 | — | 0.85 | NEXUSS3-A-25 | — | — | — |
SP-GND
| Cavity | Function | Signal | Wire | Colour | Gauge (mm²) | Destination | Terminal | Seal | Plug |
|---|---|---|---|---|---|---|---|---|---|
| 1 | — | — | W4 | — | 0.85 | OEM-2-A | — | — | — |
| 1 | — | — | W5 | — | 0.85 | OEM-2-B | — | — | — |
| 1 | — | — | W6 | — | 0.85 | NEXUSS3-A-10 | — | — | — |
| 1 | — | — | W7 | — | 0.85 | NEXUSS3-A-11 | — | — | — |
Pin map
| OEM point | OEM function | OEM family | Target point | Target function | Target family | Wire colour | Evidence |
|---|---|---|---|---|---|---|---|
| OEM-1-A | Constant +12V | power_in | — | — | — | single_source | |
| OEM-1-B | ECU power +14V (main relay) | power_in | NEXUSS3-A-18 + NEXUSS3-A-25 | +12V switched input + +12V switched input | power_in | single_source | |
| OEM-1-C | Start signal | digital_in | NEXUSS3-C-5 | SPI 1 | digital_in | single_source | |
| OEM-1-E | Malfunction indicator lamp (MIL) | low_side_out | NEXUSS3-A-9 | DPO 1 | low_side_out | single_source | |
| OEM-1-G | Ignition 1 (coils 1+4, wasted spark) | ignition_out | NEXUSS3-A-27 | Ignition 1 | ignition_out | single_source | |
| OEM-1-H | Ignition 2 (coils 2+3, wasted spark) | ignition_out | NEXUSS3-A-28 | Ignition 2 | ignition_out | single_source | |
| OEM-1-N | Wide-open throttle switch | digital_in | NEXUSS3-C-6 | SPI 2 | digital_in | single_source | |
| OEM-1-P | Power steering pressure switch | digital_in | NEXUSS3-C-7 | SPI 3 | digital_in | single_source | |
| OEM-1-Q | A/C request | digital_in | NEXUSS3-C-8 | SPI 4 | digital_in | single_source | |
| OEM-1-V | Neutral/clutch switch | digital_in | NEXUSS3-C-19 | SPI 5 | digital_in | single_source | |
| OEM-2-A | Power ground | ground | NEXUSS3-A-10 + NEXUSS3-A-11 | Battery ground + Battery ground | ground | single_source | |
| OEM-2-B | Power ground | ground | NEXUSS3-A-10 + NEXUSS3-A-11 | Battery ground + Battery ground | ground | single_source | |
| OEM-2-C | Sensor ground (E2) | sensor_ground | NEXUSS3-C-2 + NEXUSS3-C-26 + NEXUSS3-C-4 | Trigger - + Signal ground + Home - | sensor_ground + trigger_in | corroborated | |
| OEM-2-D | Sensor ground (E2) | sensor_ground | NEXUSS3-C-2 + NEXUSS3-C-26 + NEXUSS3-C-4 | Trigger - + Signal ground + Home - | sensor_ground + trigger_in | corroborated | |
| OEM-2-E | Crank position (CAS NE) | trigger_in | NEXUSS3-C-1 | Trigger + | trigger_in | single_source | |
| OEM-2-F | NC | — | — | — | — | corroborated | |
| OEM-2-G | Cam position (CAS G) | trigger_in | NEXUSS3-C-3 | Home + | trigger_in | single_source | |
| OEM-2-I | Tachometer output | tacho_out | NEXUSS3-A-12 | DPO 2 | low_side_out | single_source | |
| OEM-2-K | +5V sensor supply | vref | NEXUSS3-C-25 | +5V | vref | single_source | |
| OEM-2-L | Idle switch (TPS) | digital_in | NEXUSS3-C-20 | SPI 6 | digital_in | single_source | |
| OEM-2-N | O2 sensor | lambda_nb_in | NEXUSS3-C-11 | AVI 2 | analog_in | single_source | |
| OEM-2-O | Air flow meter (AFM) | analog_in | NEXUSS3-C-10 | AVI 1 | analog_in | single_source | |
| OEM-2-P | Intake air temperature (IAT) | temp_in | NEXUSS3-C-13 | AVI 4 | analog_in | single_source | |
| OEM-2-Q | Coolant temperature (ECT) | temp_in | NEXUSS3-C-12 | AVI 3 | analog_in | single_source | |
| OEM-2-U | Injector 1 & 3 (batch) | injector_out | NEXUSS3-A-1 | Injector 1 | injector_out | corroborated | |
| OEM-2-V | Injector 2 & 4 (batch) | injector_out | NEXUSS3-A-2 | Injector 2 | injector_out | corroborated | |
| OEM-2-Y | Injector 3 (California sequential) | injector_out | NEXUSS3-A-3 | Injector 3 | injector_out | corroborated | |
| OEM-2-Z | Injector 4 (California sequential) | injector_out | NEXUSS3-A-4 | Injector 4 | injector_out | corroborated |
Cut list
| Wire | From | To | Gauge (mm²) | Colour | Length | Signal | Ident |
|---|---|---|---|---|---|---|---|
| W1 | OEM-1-B | SP-14V-1 | 0.85 | no route | ECU power +14V (main relay) | — | |
| W2 | SP-14V-1 | NEXUSS3-A-18 | 0.85 | no route | +12V switched input | — | |
| W3 | SP-14V-1 | NEXUSS3-A-25 | 0.85 | no route | +12V switched input | — | |
| W4 | OEM-2-A | SP-GND-1 | 0.85 | no route | Power ground | — | |
| W5 | OEM-2-B | SP-GND-1 | 0.85 | no route | Power ground | — | |
| W6 | SP-GND-1 | NEXUSS3-A-10 | 0.85 | no route | Battery ground | — | |
| W7 | SP-GND-1 | NEXUSS3-A-11 | 0.85 | no route | Battery ground | — | |
| W8 | OEM-2-C | SP-0V-1 | 0.5 | no route | Sensor ground (E2) | — | |
| W9 | OEM-2-D | SP-0V-1 | 0.5 | no route | Sensor ground (E2) | — | |
| W10 | SP-0V-1 | NEXUSS3-C-26 | 0.5 | no route | Signal ground | — | |
| W11 | OEM-2-K | NEXUSS3-C-25 | 0.5 | no route | +5V sensor supply | — | |
| W12 | OEM-2-E | NEXUSS3-C-1 | 0.5 | no route | Crank position (CAS NE) | — | |
| W13 | SP-0V-1 | NEXUSS3-C-2 | 0.5 | no route | Trigger - | — | |
| W14 | OEM-2-G | NEXUSS3-C-3 | 0.5 | no route | Cam position (CAS G) | — | |
| W15 | SP-0V-1 | NEXUSS3-C-4 | 0.5 | no route | Home - | — | |
| W16 | OEM-1-G | NEXUSS3-A-27 | 0.5 | no route | Ignition 1 (coils 1+4, wasted spark) | — | |
| W17 | OEM-1-H | NEXUSS3-A-28 | 0.5 | no route | Ignition 2 (coils 2+3, wasted spark) | — | |
| W18 | OEM-2-U | NEXUSS3-A-1 | 0.5 | no route | Injector 1 & 3 (batch) | — | |
| W19 | OEM-2-V | NEXUSS3-A-2 | 0.5 | no route | Injector 2 & 4 (batch) | — | |
| W20 | OEM-2-Y | NEXUSS3-A-3 | 0.5 | no route | Injector 3 (California sequential) | — | |
| W21 | OEM-2-Z | NEXUSS3-A-4 | 0.5 | no route | Injector 4 (California sequential) | — | |
| W22 | OEM-1-E | NEXUSS3-A-9 | 0.5 | no route | Malfunction indicator lamp (MIL) | — | |
| W23 | OEM-2-I | NEXUSS3-A-12 | 0.5 | no route | Tachometer output | — | |
| W24 | OEM-2-O | NEXUSS3-C-10 | 0.5 | no route | Air flow meter (AFM) | — | |
| W25 | OEM-2-N | NEXUSS3-C-11 | 0.5 | no route | O2 sensor | — | |
| W26 | OEM-2-Q | NEXUSS3-C-12 | 0.5 | no route | Coolant temperature (ECT) | — | |
| W27 | OEM-2-P | NEXUSS3-C-13 | 0.5 | no route | Intake air temperature (IAT) | — | |
| W28 | OEM-1-C | NEXUSS3-C-5 | 0.5 | no route | Start signal | — | |
| W29 | OEM-1-N | NEXUSS3-C-6 | 0.5 | no route | Wide-open throttle switch | — | |
| W30 | OEM-1-P | NEXUSS3-C-7 | 0.5 | no route | Power steering pressure switch | — | |
| W31 | OEM-1-Q | NEXUSS3-C-8 | 0.5 | no route | A/C request | — | |
| W32 | OEM-1-V | NEXUSS3-C-19 | 0.5 | no route | Neutral/clutch switch | — | |
| W33 | OEM-2-L | NEXUSS3-C-20 | 0.5 | no route | Idle switch (TPS) | — |
Bill of materials
| Category | Part number | Manufacturer | Description | Qty | Unit |
|---|---|---|---|---|---|
| connector | 174515-6 | TE | AMP Multilock .040/.070 hybrid 22-pin female harness plug (Miata NA/NB ECU) | 1.000 | ea |
| connector | 174516-6 | TE | AMP Multilock .040/.070 hybrid 26-pin female harness plug (Miata NA/NB ECU) | 1.000 | ea |
ECU setup sheet
| Role | Engine ref | Engine signal | Sensor | ECU point | ECU function | ECU class | Setup flag |
|---|---|---|---|---|---|---|---|
| No configured channels. | |||||||
Findings
Open (5)
| Severity | Code | Subject | Message |
|---|---|---|---|
| warn | NO_FUEL_PUMP | fuel pump | This engine harness has no fuel-pump control path: no fuel-pump relay output is wired to the new ECU. If the stock car switches the pump outside the ECU (an airflow-meter contact or a separate module), the standalone must drive the pump relay itself — see the install notes for this car. |
| info | NET_FAMILY | NEXUSS3-C-26 ↔ NEXUSS3-C-4 | NEXUSS3-C-26 "Signal ground" [sensor_ground] is wired to NEXUSS3-C-4 "Home -" [trigger_in]. Trigger return tied to sensor/trigger ground: correct for single-ended trigger inputs. |
| info | NET_FAMILY | OEM-2-I ↔ NEXUSS3-A-12 | OEM-2-I "Tachometer output" [tacho_out] is wired to NEXUSS3-A-12 "DPO 2" [low_side_out]. Tacho as a low-side output: check the tach needs a pull-up. |
| info | NET_FAMILY | OEM-2-P ↔ NEXUSS3-C-13 | OEM-2-P "Intake air temperature (IAT)" [temp_in] is wired to NEXUSS3-C-13 "AVI 4" [analog_in]. Thermistor on a plain analog input needs a pull-up; check the channel can enable one. |
| info | NET_FAMILY | OEM-2-Q ↔ NEXUSS3-C-12 | OEM-2-Q "Coolant temperature (ECT)" [temp_in] is wired to NEXUSS3-C-12 "AVI 3" [analog_in]. Thermistor on a plain analog input needs a pull-up; check the channel can enable one. |
Accepted (1)
| Code | Subject | Reason | By |
|---|---|---|---|
| REQUIRED_UNMAPPED | OEM-1-A | Battery-constant (memory) feed of the stock ECU: the new ECU has no such input, so it is not needed. | mapping-proposal |
Notes
Auto-mapped by auto_map() from the library pin data on 2026-10-09. Review every wire against both manuals before building. Open items: - Fuel-pump control: Haltech Nexus S3 needs to drive the fuel pump, but the stock ECU has no fuel-pump output (an airflow-meter contact or a separate module runs it). Add a fuel-pump relay on a spare low-side output — see the install notes for this car. Not wired: - OEM-1 A: Constant +12V: Haltech Nexus S3 has no battery-constant input; not needed - NEXUSS3-A 13: Ignition switch input: not fed (the stock ECU has no ignition-sense pin); feed it from a switched-ignition source
Sources
| Reliability | Kind | Title | URL | Archive | Captured | SHA-256 |
|---|---|---|---|---|---|---|
| 4 | vendor_doc | Haltech: Nexus S3 Plug 'n' Play adapter QSG (Subaru WRX/STI ADM, HT-186300) — ECU Connector A & C pinout | https://www.haltech.com/product/ht-186300-nexus-s3-plug-n-play-adapter-harness/ | archive | 2026-10-05 | 3cab7cd337c2 |
| 4 | vendor_doc | Haltech: Nexus S3/S2 ECU Overview (QSG) — specifications | https://store.haltech.com/product/ht-213000-nexus-s3-ecu-only/ | archive | 2026-10-05 | 388299668207 |
| 4 | vendor_doc | Link G4X MX5X install manual (NA 1.6/1.8) | https://linkecu.com/documentation/MX5X.pdf | archive | 2026-10-05 | b7f730de9f6e |
| 2 | community_copy | 1991 Mazda Miata wiring diagrams (Mitchell reproduction, mellens.net community mirror) | https://trull.org/alex/pubmirror/www.mellens.net/mazda/Mazda-Miata-1991-1993/1991_wiring.pdf | — | 2026-10-08 | 6f957a38ed49 |
| 2 | expert_report | Justin Barrera — Miata NA/NB wiring corrections | — | — | — | — |