ECU - Electronic Control Unit Connector.
The 1991–1994 Acura NSX uses Honda’s PGM-FI (Programmed Fuel Injection) Engine Control Unit to manage the mid-mounted 2,977 cc DOHC 90-degree C30A V6 VTEC engine. The ECU operates in the OBD0/OBD1 era: 1991 models follow pre-OBD conventions, whilst 1992–1994 models comply with OBD1, using a two-pin Service Check Connector for diagnostic trouble code retrieval via MIL blink codes rather than a standardised scan-tool port. The unit controls multi-point fuel injection, ignition timing, idle air control, and the VTEC solenoid valve, monitoring inputs from MAP, throttle position, coolant temperature, intake air temperature, and primary and secondary oxygen sensors. The ECU is located within the passenger compartment and uses lettered multi-pin connector blocks consistent with Honda’s contemporary ECU architecture.
Connector C
| Pin | name | Signal |
|---|---|---|
| C2 | Vehicle Speed Sensor | Speed |
| C4 | Engine Speed Pulse | RPM |
Connector D
| Pin | name | Signal |
|---|---|---|
| D11 | Throttle Position Sensor | Analog |
| D13 | Engine Coolant Temperature Sensor | Analog |
| D14 | Primary O2 Sensor | Analog |
| D15 | Intake Air Temperature Sensor | Analog |
| D16 | Secondary O2 Sensor | Analog |
| D17 | MAP Sensor | Analog 0.32V @ -13.9 PSI 4.84V @ 10.94 PSI |
Notes
- Connectors are read from the wire side (back of the connector), not the pin side, with the locking tab at the top — this determines the pin numbering orientation.
- The NSX uses Honda’s OBD1-era Service Check Connector (two-pin) for DTC retrieval; a jumper wire between the terminals causes the MIL to flash stored fault codes.
- VTEC solenoid valve activation is commanded by the ECU; an oil pressure switch feeds a confirmation signal back to the ECU.
- ECM reset procedure: disconnect the negative battery terminal for at least 30 seconds, or remove the ECM backup fuse, to clear stored DTCs. This also clears idle learn data, requiring an idle relearn procedure afterwards.
Signals
- Vehicle Speed Sensor (C2) — vehicle speed pulse input to the ECU, used for speed-dependent fuel and ignition corrections.
- Engine Speed Pulse (C4) — RPM signal input derived from the crankshaft or distributor, used for engine load and timing calculations.
- Throttle Position Sensor (D11) — analogue voltage input indicating throttle plate angle; used for acceleration enrichment and VTEC engagement logic.
- Engine Coolant Temperature Sensor (D13) — analogue NTC thermistor input; used for cold-start enrichment and fan control.
- Primary O2 Sensor (D14) — analogue input from the upstream (pre-catalyst) oxygen sensor; used for closed-loop fuel trim.
- Intake Air Temperature Sensor (D15) — analogue NTC thermistor input for inlet air density compensation.
- Secondary O2 Sensor (D16) — analogue input from the downstream (post-catalyst) oxygen sensor; used for catalyst efficiency monitoring.
- MAP Sensor (D17) — manifold absolute pressure analogue input; 0.32 V at −13.9 PSI vacuum, 4.84 V at 10.94 PSI boost reference; primary engine load signal for PGM-FI fuelling calculations.
References
- Acura NSX 1991 Service Manual — Honda Motor Co. / NSX Technical Center