Honda Civic 99 ECU OBD2 Diagram: Your Ultimate Wiring Guide for Engine Swaps

Engine swaps in Honda Civics are a popular modification, breathing new life and performance into these iconic vehicles. If you’re tackling an engine swap in a 1999 Honda Civic and dealing with OBD2 (On-Board Diagnostics II) systems and ECU (Engine Control Unit) wiring, you’ve come to the right place. This comprehensive guide provides the essential wiring information you need, focusing on the “Honda Civic 99 Ecu Obd2 Diagram” and related aspects to ensure a smooth and successful swap.

This guide is designed to be your go-to resource for navigating the complexities of wiring when swapping engines in 1992-2000 Honda Civics. While primarily focused on B and D series engines due to their popularity, the principles discussed can be broadly applied. Let’s dive into the specifics of ECU and OBD2 wiring for your 1999 Honda Civic project.

Understanding OBD1 and OBD2 for Honda Civic Engine Swaps

Before we get into specific diagrams, it’s crucial to understand the difference between OBD1 and OBD2 systems, especially as it relates to Honda Civics around the 1999 model year. OBD2 became standard in vehicles sold in the USA in 1996, including the Honda Civic. 1999 Civics are definitively OBD2, specifically OBD2B. This distinction is vital because OBD1 and OBD2 ECUs and wiring harnesses are not directly interchangeable without modification.

When performing an engine swap, you might encounter situations where you’re mixing OBD generations – for example, installing an older OBD1 engine into your OBD2 1999 Civic, or vice versa. Understanding the wiring differences and having access to accurate diagrams, like a “honda civic 99 ecu obd2 diagram”, is paramount to avoid electrical issues and ensure your engine runs correctly.

Swapping Engines in 1999 Honda Civic: OBD2 Considerations

For 1999 Honda Civics, which are OBD2B, engine swaps often involve considerations for maintaining OBD2 compliance or intentionally converting to OBD1 for tuning flexibility. Let’s explore common swap scenarios and the wiring adjustments you might need to make, referencing diagrams where applicable.

OBD2 Engine into OBD2 Vehicle (1999 Honda Civic – OBD2B)

In theory, swapping an OBD2 engine into an OBD2 1999 Civic should be the most straightforward. However, variations exist within OBD2 itself (OBD2A for 96-98 Civics and OBD2B for 99-00 Civics). It’s critical to note that 96-98 OBD2A and 99-00 OBD2B engine and dash harnesses are NOT interchangeable due to different ECU connector configurations.

D16Y8 (OBD2) into 1999 Civic DX/LX/CX (originally D16Y7)

When swapping a D16Y8 (VTEC) engine into a base model 1999 Civic (DX, LX, CX) originally equipped with a D16Y7 (non-VTEC), you’ll encounter some wiring needs. The D16Y8 utilizes VTEC (Variable Valve Timing and Lift Electronic Control), which the D16Y7 does not.

VTEC Wiring: You will need to wire in the VTEC solenoid and VTEC pressure switch. Refer to Figure 7 (OBD2B hybrid pinouts) for the specific pin locations on your 1999 Civic ECU to add these VTEC wires.
Idle Air Control Valve (IACV): The D16Y7 uses a 3-wire IACV, while some OBD2 ECUs might expect a 2-wire IACV. You may need to wire the IACV from 3-wire to 2-wire configuration as shown in Figure 7 (OBD2B hybrid pinouts).
Intake Air Temperature Sensor (IAT): The D16Y8 intake manifold doesn’t have a provision for the IAT sensor like the D16Y7 manifold. You’ll need to relocate your IAT sensor to the intake arm. Figure 8 (OBD2 IAT into intake arm) visually demonstrates this relocation.
Secondary O2 Sensor: If using a catalytic converter from a VTEC model (like an EX), you might need to extend the secondary O2 sensor wiring using an O2 sensor extension harness.
Knock Sensor: For optimal engine management, consider wiring in the knock sensor if your ECU supports it, especially when using a P2P ECU (D16Y8 ECU). Again, Figure 7 will be crucial for pinout information.

B16A2/B18C1/B18C5 (OBD2) into 1999 Civic EX/HX/DX/LX/CX

Swapping in performance B-series engines like the B16A2, B18C1 (GSR), or B18C5 (Type R) into a 1999 Civic also requires attention to wiring, even within OBD2 systems.

IAT Relocation: Similar to the D16Y8 swap, B-series intake manifolds typically don’t have an IAT sensor location. Relocate the IAT to the intake arm as shown in Figure 8. Alternatively, you can convert to a round style IAT sensor that can be placed on the manifold itself, as depicted in Figure 9 (Round style IAT).
IACV Wiring (DX/LX/CX): If swapping into a non-VTEC model (DX, LX, CX), you will likely need to address the IACV wiring, potentially converting from 3-wire to 2-wire as with the D16Y8 swap (refer to Figure 7).
IAB Wiring (B18C1 with P72 ECU): For B18C1 swaps utilizing a P72 ECU (GSR ECU), you’ll need to wire in the IAB (Intake Air Bypass) system. Figure 7 provides the necessary pinouts for OBD2B systems.
Secondary O2 Sensor Extension: As with the D16Y8, ensure the secondary O2 sensor wiring is sufficient length, especially if using a different catalytic converter.

OBD1 Engine into OBD2 Vehicle (1999 Honda Civic – OBD2B) Using OBD1 ECU

A common modification strategy is to use an OBD1 ECU (like the P28, P30, P72, P75) in an OBD2 Civic. This is often done for tuning purposes as OBD1 ECUs are generally considered more easily tunable. When installing an OBD1 engine and ECU into your 1999 OBD2 Civic, more significant wiring modifications are needed.

D16Z6 (OBD1 VTEC) into 1999 Civic DX/LX/CX (originally D16Y7)

VTEC Wiring: You’ll need to wire the VTEC solenoid and pressure switch to the OBD1 P28 ECU. Figure 1 (OBD1 ECU Wiring) provides the pinout diagram for OBD1 ECU wiring, including VTEC.
IACV Wiring: As before, the IACV wiring might need conversion from 3-wire (OBD2 Civic) to 2-wire (OBD1 ECU). Figure 6 (OBD2A hybrid pinouts) & Figure 7 (OBD2B hybrid pinouts), though labeled OBD2A/B, illustrate the 3-wire to 2-wire IACV conversion concept which is relevant even when using an OBD1 ECU.
IAT Relocation & potentially OBD1 IAT Sensor: Relocate the IAT sensor to the intake arm (Figure 8). You might also consider rewiring to use an OBD1 style IAT sensor (Figure 9) for direct compatibility with the OBD1 ECU.
Distributor Adapter: The D16Z6 distributor is OBD1, and your 1999 Civic harness is OBD2. You will need an OBD2 to OBD1 distributor adapter harness to connect the distributor correctly.
Injector Clips: OBD1 and OBD2 injectors might have different connector clips. You may need to remove the injector “clips” as shown in Figure 10 for plug-and-play injector connection, or use OBD1 injectors.
Eliminate OBD2 Sensors (CKF, EVAP, Secondary O2): When running an OBD1 ECU, you will typically eliminate the Crankshaft Fluctuation Sensor (CKF), EVAP purge system, and secondary O2 sensor as these are not monitored or used by OBD1 ECUs.

B16A2/B18C1/B18C5 (OBD2 VTEC) into 1999 Civic DX/LX/CX Using OBD1 P30/P72

VTEC and Knock Sensor Wiring: Wire in the VTEC solenoid, pressure switch, and knock sensor to your OBD1 P30/P72 ECU using Figure 1 as a guide.
IACV and IAT Considerations: Address the IACV wiring and IAT sensor relocation/conversion as described in the D16Z6 OBD1 swap section.
Distributor Adapter and OBD2 VTEC Distributor: For 1999-00 Civics, you’ll need an OBD2B to OBD1/2 distributor adapter and an OBD1/2 B-series DOHC VTEC distributor to ensure compatibility.
Injector Clips and OBD2 Alternator: Similar to the D-series OBD1 swap, address injector clips (Figure 10). You might consider using an OBD2 B-series alternator for easier plug-and-play or use an OBD2-to-OBD1 alternator adapter.
Eliminate OBD2 Sensors: Eliminate CKF, EVAP, and secondary O2 sensor functions as they are not utilized by OBD1 ECUs.

OBD0 Engine into OBD2 Vehicle (1999 Honda Civic – OBD2B) Using OBD1 ECU

While less common, you might consider installing an even older OBD0 engine (like a B18A OBD0) into your 1999 Civic and using an OBD1 ECU (P75 in this example). This requires similar wiring considerations as the OBD1 engine swap, but with a few unique points.

B18A (OBD0) into 1999 Civic EX/SI/DX/CX/VX using OBD1 P75 ECU

Fan Switch Rewiring: OBD0 engines often have the fan switch located on the back of the block, unlike OBD1/OBD2 engines where it’s on the thermostat housing. You will need to rewire the fan switch from the thermostat housing of your Civic to the back of the OBD0 block. Figure 4 (OBD0 fan switch/OBD1 fan switch) visually highlights the difference in fan switch locations.
Injector and Alternator Compatibility: Reuse EX/SI/DX injectors or source them for CX/VX models. OBD0 and OBD1 alternators generally have the same plugs, simplifying alternator wiring.
O2 Sensor Wiring (CX/VX): For 1-wire O2 sensor CX models or 7-wire O2 sensor VX models, you’ll need to rewire to a 4-wire O2 sensor configuration as detailed earlier and potentially using the diagram at http://www.hybridautomotive.com/tech/o2/.
Eliminate OBD2 Sensors: As with other OBD1 ECU swaps in an OBD2 vehicle, eliminate the CKF, EVAP, and secondary O2 sensor functions.

Utilizing Honda Civic 99 ECU OBD2 Diagrams Effectively

Throughout your engine swap, having a clear “honda civic 99 ecu obd2 diagram” is essential. Figure 7 (OBD2B hybrid pinouts) is the most relevant diagram for a 1999 Honda Civic. It provides crucial pinout information for the OBD2B ECU connectors, which is necessary for:

Identifying Pin Locations: Locating the correct pins for adding VTEC, knock sensor, or modifying IACV wiring.
Understanding Wire Colors: Matching wire colors between your engine harness and the ECU pinouts to ensure correct connections.
Pin Removal and Insertion: Figure 1 (OBD1 ECU Wiring), Figure 6 (OBD2A hybrid pinouts) and Figure 7 (OBD2B hybrid pinouts) include diagrams showing how to remove pins from Honda ECU connectors, which is often necessary for clean wiring modifications.

Figure 11 (OBD2 Harness Comparison) is also valuable for understanding the differences between OBD2A and OBD2B ECU connectors, reinforcing why 96-98 and 99-00 harnesses are not directly interchangeable.

Conclusion: Wiring Success for Your 1999 Honda Civic Engine Swap

Engine swaps in 1999 Honda Civics, while rewarding, demand meticulous attention to wiring, particularly concerning the OBD2 system and ECU. By understanding the nuances of OBD1 vs. OBD2, utilizing accurate diagrams like the “honda civic 99 ecu obd2 diagram” (Figure 7), and carefully following the wiring modifications outlined in this guide, you can confidently navigate the wiring aspects of your swap.

Remember to always double-check your wiring against diagrams before making connections, and when in doubt, consult with experienced Honda technicians or online communities specializing in Honda engine swaps. With careful planning and execution, your 1999 Honda Civic engine swap will be a resounding success, delivering enhanced performance and driving enjoyment.