ECU - Electronic Control Unit Connector.
The 1992–1995 Honda Prelude (BB1/BB4 chassis) uses an OBD1 Engine Control Module (ECM) mounted in the passenger-compartment kick panel area. Two engine variants were offered: the H22A 2.2-litre DOHC VTEC and the H23A 2.3-litre DOHC non-VTEC, each requiring a different ECM calibration. The ECM manages Honda’s PGM-FI multi-point fuel injection system, controlling fuel injectors, ignition timing, and — on H22A models — the VTEC solenoid valve. Diagnostic trouble codes are retrieved via the two-pin service check connector using MIL blink codes, as this era predates the OBD2 16-pin DLC introduced on 1996-onwards Preludes. ECM connectors follow Honda’s lettered A/B/D block scheme; all connectors are read from the wire side with the locking tab at the top.
Connector A
| Pin | name | Signal |
|---|---|---|
| A4 | VTEC Solenoid Valve | Analog |
| A21 | Ignition Control Module | RPM |
| A24 | Power Ground | Ground |
Connector B
| Pin | name | Signal |
|---|---|---|
| B10 | Vehicle Speed Sensor | Speed |
Connector D
| Pin | name | Signal |
|---|---|---|
| D2 | Brake Switch | Analog |
| D6 | VTEC Pressure Switch | Analog |
| D11 | Throttle Position Sensor | Analog |
| D13 | Engine Coolant Temperature Sensor | Analog |
| D14 | Primary 02 Sensor | Analog |
| D15 | Intake Air Temperature Sensor | Analog |
| D17 | MAP Sensor | Analog 0.32V @ -13.9 PSI 4.84V @ 10.94 PSI |
Notes
- The VTEC solenoid valve output (A4) and VTEC pressure switch input (D6) are present only on H22A VTEC-equipped models; the H23A non-VTEC ECM omits these signals.
- ECM reset procedure: disconnect the negative battery terminal for at least 30 seconds, or remove the ECM backup fuse, to clear stored DTCs; idle relearn is required afterwards.
- All connector pins are numbered from the wire (harness) side with the locking tab at the top — not from the component (pin) side.
- The service check connector (two-pin SCS connector) is used to retrieve OBD1 blink codes via the MIL; no 16-pin DLC is fitted on 1992–1995 models.
Signals
- VTEC Solenoid Valve (A4) — ECM output that energises the VTEC solenoid to switch oil pressure and engage the high-lift cam profile on H22A engines.
- Ignition Control Module (A21) — RPM signal exchanged between the ECM and ignition control module for timing advance control.
- Power Ground (A24) — chassis/power ground reference for the ECM.
- Vehicle Speed Sensor (B10) — pulse signal from the vehicle speed sensor (VSS) used by the ECM to monitor road speed.
- Brake Switch (D2) — input from the brake pedal switch; used by the ECM to modify fuel and idle strategies.
- VTEC Pressure Switch (D6) — confirms oil pressure has risen sufficiently for VTEC engagement; ECM monitors this signal after commanding the VTEC solenoid.
- Throttle Position Sensor (D11) — analogue voltage input indicating throttle plate angle for load calculation and enrichment.
- Engine Coolant Temperature Sensor (D13) — negative-temperature-coefficient (NTC) thermistor input used for cold-start enrichment and fan control.
- Primary O2 Sensor (D14) — upstream heated oxygen sensor signal used for closed-loop fuel trim.
- Intake Air Temperature Sensor (D15) — NTC thermistor measuring inlet air temperature for charge-density correction.
- MAP Sensor (D17) — manifold absolute pressure sensor; output ranges from approximately 0.32 V at −13.9 PSI to 4.84 V at 10.94 PSI; primary load input for PGM-FI fuelling calculations.