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)

• Land the fuel-pump lead on the circuit-opening relay control side (Light Green wire)required

  The standalone's fuel-pump output is a low-side driver: land it on the circuit-opening relay's control-side wire — the Light Green wire (Link MX5X manual §1.4.4). Grounding that wire energises the relay and runs the pump, so the ECU output drives the relay directly. The 1991 EWD shows this circuit (Fig.2 "AIRFLOW METER → PUMP SW → CIRCUIT OPENING RELAY") but its scan does not name the relay control-side wire colour; the Light Green wire is from the Link manual, which is the wiring source of record.

  Why: The NA6 has no stock fuel-pump output at the ECU, so the standalone harness adds one (the NEW-FUELPUMP lead on the diagram). It must reach the pump relay's control side to actually run the pump.

  Where: Circuit-opening relay (under the dash, near the steering column) — control-side Light Green wire.

  Source: Link G4X MX5X install manual (NA 1.6/1.8)