Many 5th generation Honda Prelude owners looking to enhance engine performance and tuning capabilities explore converting their factory OBD2 (On-Board Diagnostics II) distributor system to an OBD1 (On-Board Diagnostics I) setup. This modification primarily applies to USDM (United States Domestic Market) H22A4 engines and allows for significant advantages in engine management. This guide will outline the process for converting your OBD2 distributor to OBD1, focusing on clarity and accuracy for a successful modification.
The primary reasons for undertaking a 5th Gen Prelude Obd2 To Obd1 distributor conversion revolve around gaining greater control over engine timing and taking advantage of aftermarket performance parts. Specifically, OBD2 H22 engines have ECU-controlled ignition timing, meaning the distributor itself is not mechanically adjustable. This limitation can hinder precise tuning, especially when modifications are made to the engine. Converting to OBD1, which utilizes an externally coiled distributor with an 8-pin connector, unlocks the ability to manually adjust base timing. This is crucial for optimizing engine performance under various conditions and modifications. Furthermore, OBD1 conversion becomes essential when installing aftermarket adjustable cam gears. On H22A4 engines with the stock OBD2 setup, attempting to adjust the exhaust cam gear will typically trigger a CYP (Cylinder Position Sensor) check engine light, limiting tuning flexibility.
To perform this conversion, you will need to adapt the wiring harness from your engine’s CKP (Crankshaft Position) plug to accommodate the 8-pin female connector of the OBD1 distributor. The OBD2 distributor uses a 4-pin connector which integrates the CYP P (Cylinder Position Sensor Positive), CYP M (Cylinder Position Sensor Negative), and ICM (Ignition Control Module) terminals.
Sourcing the necessary connectors can be straightforward. A practical approach is to acquire them from a junkyard, targeting any 1992-1995 Honda vehicle, as these models commonly used OBD1 systems and the required connectors. When sourcing, ensure you obtain both the 8-pin female connector (for the OBD1 distributor) and a male 4-pin connector (often found on 4-wire O2 sensors). It’s recommended to cut the connectors with a sufficient length of wire attached to facilitate soldering during the wiring process.
Below is the distributor conversion wiring schematic that details the connections between the OBD2 harness and the OBD1 distributor:
| OBD2 Connector | OBDI Connector | Function |
|---|---|---|
| Black | Blue/Yellow | CKP M (Crankshaft Position Sensor Negative) |
| Red | White/Blue | TDC M (Top Dead Center Sensor Negative) |
| White | WHITE | CYP M (Cylinder Position Sensor Negative) |
| Yellow | Blue/Green | CKP P (Crankshaft Position Sensor Positive) |
| Green | Orange/Blue | TDC P (Top Dead Center Sensor Positive) |
| Blue | Orange | CYP P (Cylinder Position Sensor Positive) |
| Yellow/Green | Yellow/Green | ICM (Ignition Control Module) |
| Green | Green | ICM (Ignition Control Module) |
This table provides a clear mapping for each wire, ensuring a correct and functional 5th gen prelude OBD2 to OBD1 distributor conversion. By carefully following these wiring instructions, you can successfully convert your 5th gen Prelude to an OBD1 distributor system, gaining valuable control over engine timing and enabling the use of aftermarket performance upgrades. This modification is a popular and effective way to enhance the tuning potential of your H22A4 engine.
