Converting your 5th generation Honda Prelude from an OBD2 (On-Board Diagnostics II) to an OBD1 (On-Board Diagnostics I) distributor might seem like a complex modification, but for enthusiasts looking to unlock more control over their engine’s performance, it’s a worthwhile endeavor. Specifically for USDM (United States Domestic Market) H22A4 engines, opting for an OBD1 distributor, typically a USDM H22A1 or H23A1 distributor, opens up tuning capabilities that are restricted in the stock OBD2 setup. This guide provides a clear walkthrough of the process, focusing on the wiring aspects of this conversion.
One of the primary motivations for this conversion is to gain the ability to adjust the base ignition timing. In OBD2 H22A4 Preludes, ignition timing is managed entirely by the ECU (Engine Control Unit), and the distributor itself offers no mechanical adjustment. Furthermore, for those looking to fine-tune their engine’s camshaft timing, the OBD2 system in these Preludes presents limitations, particularly with aftermarket adjustable exhaust cam gears, often triggering a CYP (Cylinder Position Sensor) check engine light when adjustments are made. Switching to an OBD1 distributor circumvents these limitations, granting tuners greater flexibility.
To perform this conversion, you’ll be working with different connector types. The OBD1 external coil distributor uses an 8-pin male connector, while the OBD2 distributor utilizes a 4-pin connector. This 4-pin OBD2 connector incorporates the terminals for the CYP P (Cylinder Position Sensor Positive), CYP M (Cylinder Position Sensor Negative), and ICM (Ignition Control Module).
The key to a clean and reliable conversion lies in creating a conversion harness. This involves repurposing the CKP (Crankshaft Position Sensor) plug from your engine harness and wiring it to a new 8-pin female connector that will interface with the OBD1 distributor. Sourcing these connectors can be done by salvaging them from 1992-1995 Honda vehicles at a junkyard. Cut the connectors with a sufficient length of wire to facilitate soldering. You will also need a male 4-pin connector, commonly found on 4-wire oxygen sensors in many vehicles.
The wiring for the distributor conversion is detailed in the table below. Carefully follow these connections to ensure proper function:
| 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) |
By meticulously following this wiring guide, you can successfully convert your 5th gen Prelude’s distributor system from OBD2 to OBD1. This modification provides enhanced tuning capabilities, particularly for ignition timing and camshaft adjustments, making it a popular upgrade for performance-oriented Prelude owners. Remember to double-check all connections and ensure secure soldering for a reliable and long-lasting conversion.
