For Honda enthusiasts looking to boost the performance of their third-generation Prelude Si, engine swaps are a popular and effective upgrade. Specifically, swapping the original B21 engine with a more powerful H22A VTEC engine is a well-trodden path. This guide focuses on the crucial aspect of this modification: adapting the wiring of your 1990-1991 Honda Prelude Si, which utilizes an OBD1 (On-Board Diagnostics 1) system, to seamlessly integrate the JDM (Japanese Domestic Market) H22A engine. While the process might seem daunting, understanding the nuances of your Prelude’s OBD1 system and the H22A wiring is key to a successful and rewarding engine swap.
This article serves as your expert resource, providing a detailed breakdown of the wiring modifications necessary for this upgrade. We’ll guide you through leveraging your existing B21 OBD1 wiring harness and adapting it for the H22A, ensuring a smooth transition and optimal performance.
Understanding Your 90-91 Prelude Si and OBD1
The 1990 and 1991 Honda Prelude Si models are significant as they feature Honda’s OBD1 engine management system. This is a crucial advantage compared to the earlier 88-89 Preludes (OBD0) when undertaking an engine swap. The OBD1 system in your 90-91 Prelude simplifies the electrical integration process, making it significantly less complex to adapt to a newer engine like the H22A.
A key benefit of the 90-91 OBD1 system is its standardized wiring architecture. This means that the sensor functions and ECU (Engine Control Unit) pin assignments are consistent with other OBD1 Honda systems from the early to mid-1990s. For instance, a P13 ECU from a 1994 Honda (OBD1) can be connected to your 90-91 Prelude’s wiring harness with minimal to no connector modifications.
Image showcasing an OBD1 ECU connection, highlighting the compatibility and ease of integration.
Image displaying OBD1 ECU connectors, emphasizing the standardized interface.
For 88-89 Prelude Owners (OBD0):
If you own an 88-89 Prelude with the older OBD0 system and potentially a carburetor setup, the process involves additional steps. Converting to PGM-FI (Programmed Fuel Injection) requires either swapping to an OBD0 to OBD1 wiring harness or sourcing OBD1 ECU connectors from a 92-95 Honda at a junkyard and performing a solder-based connector swap, following a wiring diagram. For carbureted models, a more extensive overhaul is needed, including replacing the entire fuel and electrical wiring system, along with upgrading the fuel tank and fuel pump to support fuel injection. While manageable with a Honda service manual, it’s a more involved undertaking compared to working with a 90-91 OBD1 Prelude.
Engine Wiring: Focus on Coil and Igniter Connections
When swapping to a JDM H22A engine, one of the most intricate parts is dealing with the coil and igniter unit connections. This guide will emphasize these aspects to ensure clarity and accuracy in your wiring process.
It’s important to note the variations in H22 engine configurations. US market H22 engines may feature external coil ignition systems, while JDM H22A engines, which are commonly used in swaps, typically come with internal coil ignition systems, integrated within the distributor. European 3rd generation Preludes with external coil ignition systems offer a point of reference, but modifications are still needed when integrating a JDM H22A.
To assist with this process, refer to these wiring diagrams:
- H22A OBD-1 92-95 Wiring Diagram: http://www.we-todd-did-racing….D.jpg and http://www.we-todd-did-racing….D.jpg
- B20 OBD-1 90-91 Wiring Diagram: http://www.we-todd-did-racing….D.jpg
While these diagrams show similarities, remember that sensor positions on the engine and within the car can differ between 3rd and 4th generation Preludes. Sensor types and connectors also vary, but approximately 95% of the wiring is conceptually similar, simplifying the adaptation process.
Connecting the H22A Engine to Your Prelude’s ECU
The optimal approach for wiring is to utilize your original 90-91 Prelude (B20A) engine wiring harness as the foundation and adapt it for the H22A engine. This strategy is generally more straightforward than trying to adapt the H22A engine harness to your car’s wiring.
Therefore, retain the original wiring harness from your B20A engine. Likewise, keep the H22A engine wiring harness; it will serve as a donor source for connectors and wires needed to create your hybrid harness.
Key Wiring Differences Between H22A and B20A
Here’s a visual comparison highlighting the main differences between the JDM H22A and B20A wiring harnesses:
Image contrasting the H22A and B20A wiring harnesses, pointing out connector junctions and alternator cable routing.
Key points of divergence illustrated in the image:
- H22A Main Connector Junction: This is where sensor wires converge towards the ECU in a JDM H22A setup.
- B20A Main Connector Junction: Similar to the H22A, this junction handles sensor wires going to the ECU in a 3rd generation Prelude.
- H22A Alternator Cable: The power cable running from the alternator to the relay box.
- B20A Alternator Cable: The corresponding alternator power cable in the B20A harness.
Here’s a closer look at connectors #2 and #4 of the B20A wiring harness:
Image detailing connectors #2 and #4 on the B20A wiring harness, focusing on relay box and alternator connections.
Connector #5 and #6 in the earlier comparison image refer to the injector and idle valve connector assemblies. Crucially, B20A connectors are compatible with H22A injectors, simplifying this aspect of the swap:
Image showing injector connectors, emphasizing the compatibility between B20A and H22A connectors.
Similarly, the alternator connectors (#7 and #8 in the main comparison image) are also compatible between the B20A and H22A harnesses:
Image highlighting alternator connector compatibility, showcasing the interchangeable nature of these connectors.
However, distributor connectors (#9 and #10) are different. The image below clearly shows the variance, with grey connectors belonging to the H22A:
Image displaying the different distributor connectors of H22A (grey) and B20A, emphasizing the need for modification.
Oil pressure sensor connectors (#11 and #12) also differ. The B20A utilizes a plug-type connector, while the H22A often uses a bolt-connected sensor.
Connector #13 is for the knock sensor, and #14 is for the resistor box connector. The resistor box connector may remain in its original position, but if using an OBD1 resistor box, you’ll need to switch to an OBD1-type connector.
MAP Sensor Considerations:
Neither the H22A nor B20A wiring harnesses typically include a dedicated MAP (Manifold Absolute Pressure) sensor connector. The MAP sensor is usually wired separately from the ECU. For this swap, you can utilize the stock MAP sensor from your 3rd generation Prelude. It can be sourced from the black vacuum box that houses EGR solenoids. Mount the MAP sensor in the engine bay and connect it to a vacuum line on the intake manifold. Importantly, the MAP sensor wiring to the ECU is compatible across OBD1 generations, eliminating the need for wiring modifications and ensuring proper function with the P13 ECU.
Shared Connector Types (Simplifying the Swap)
Both the B20A and H22A wiring harnesses utilize the same connector types for several critical sensors, which greatly simplifies the wiring adaptation:
- Speed sensor
- Lambda (O2) sensor
- Injectors
- Throttle Position (TP) sensor
- Intake Air Temperature (IAT) sensor
- Idle valve sensor
- Water temperature sensor (to ECU)
- Water coolant fan sensor
- Water temperature sensor (to dashboard)
- Starter signal sensor
- Shift position sensor
- Alternator sensor
Different Connector Types Requiring Adaptation
The key connector differences that you will need to address in your hybrid harness are:
- EGR valve lift sensor
- Distributor connectors
- Oil pressure sensor
- Main ECU connectors
Sensors Present on H22A Wiring but Absent on B20A
The B20A wiring harness lacks connectors for the following sensors that are present on the H22A engine and wiring:
- VTEC oil pressure sensor
- VTEC oil pressure switch sensor
- Knock sensor
- ECT (Engine Coolant Temperature) sensor near the thermostat on the H22A
Using the provided wiring diagrams and the component information detailed above, you can construct a hybrid wiring harness tailored for your H22A engine swap, based on your original B20A harness. The modified harness skeleton will resemble this:
Image depicting a hybrid wiring harness skeleton, illustrating the adapted B20A harness for H22A.
After creating the hybrid harness, install it on the engine and meticulously verify wire lengths and connector placements. Ensure all connections are secure and properly seated. All soldered connections must be robust and well-insulated, particularly those for the distributor and knock sensor, which are sensitive to signal integrity.
The finalized, modified wiring harness should resemble this:
Image of the final modified wiring harness, showcasing the completed adaptation for the H22A engine.
Distributor Connection: Addressing Key Differences
The distributor connection presents some of the most crucial wiring differences. As highlighted earlier, the connectors are not directly compatible:
Reiterating the image of distributor connector differences, emphasizing the need for connector replacement.
When modifying the wiring, replace the distributor connectors entirely, extending the wire replacements all the way to the main connectors near the ECU. Perform soldering work near the large white main connectors, away from the engine bay’s heat. Avoid soldering close to the distributor connectors as the engine bay’s high temperatures and repeated handling of distributor connectors during the swap can compromise solder joints.
JDM H22A distributors incorporate three essential sensors, an ignition coil, and an igniter unit. Consult the wiring diagrams for pin assignments and connections for:
- CYL – Cylinder sensor
- TDC – Top Dead Center sensor (Piston position)
- CKP – Crankshaft Position sensor
- ICM – Igniter Control Module (IGNITER UNIT)
This image shows the distributor connector and the igniter unit cable (Yellow/Green), which needs to be connected to ECU pin A21:
Image showing the distributor connector and the igniter unit cable, specifically pointing out the Yellow/Green wire.
Detailed view of the distributor connector, highlighting pin locations and wire colors.
Another detailed view of the distributor connector from a different angle, further clarifying pin assignments.
As shown in the wiring diagrams and these images, the ignition unit is integrated within the H22 distributor. The igniter unit cable is the primary wiring difference in terms of ignition compared to the original B20 engine setup.
Coil Connection: Adapting for the Internal Coil H22A
On 3rd generation Preludes with the B20 engine, the ignition coil is mounted externally in the engine bay, near the main connectors. The red circle in the image below indicates the original location of the external coil:
Image illustrating the original location of the external ignition coil in a 3rd generation Honda Prelude engine bay.
The external coil has two smaller connectors (pink and grey), both with similar wiring (Blue and Black/Yellow cables). Remove these connectors as they are no longer needed with the internal coil H22A distributor. Isolate the smaller gauge wires, as they will not be reused. You will only need the larger gauge coil wires.
This image shows the modified wiring after removing the unnecessary connectors:
Image depicting the modified coil wiring, showing the removal of smaller connectors and preparation for direct connection.
For a secure and reliable connection, use a robust connector in your wiring harness:
Image displaying a secure connector used for the coil wiring, emphasizing reliable electrical connection.
You can often source suitable connectors from junkyards, particularly from 4th generation Preludes or Accords, which may utilize similar connector types.
Conclusion
This guide provides a comprehensive overview of the electronic modifications required when performing an H22A engine swap into a 1990-1991 Honda Prelude Si. By focusing on the OBD1 system compatibility and meticulously adapting your existing B20A wiring harness, you can successfully integrate the more powerful H22A engine. Pay close attention to the distributor and coil wiring differences, utilize the provided diagrams, and ensure all connections are robust and well-insulated for a reliable and high-performing engine swap. Always consult the official Honda service manuals for detailed wiring diagrams and specifications relevant to your specific Prelude model and engine. With careful planning and execution, you can significantly enhance the performance of your 90-91 Prelude Si with this engine upgrade.
