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)