Many Honda and Acura D and B series engines are equipped with a 2 wire Idle Air Control Valve (IACV). This simpler system uses one wire for power and another for ECU-controlled idle regulation. However, certain 1999-2000 Honda Civic models, specifically the DX, LX, and EX (automatic transmission), deviated from this standard by incorporating a 3 wire IACV. This 3 wire IACV setup includes a power wire, a ground wire, and a third wire for ECU idle control.
If you are upgrading your 1999-2000 Honda Civic DX, LX, or automatic EX to an OBD1 ECU using a chipped or tunable ECU solution, you’ll encounter an incompatibility. These aftermarket OBD1 ECUs are typically designed for the 2 wire IACV system. Therefore, to ensure proper idle control with your new OBD1 ECU, you will need to convert your existing 3 wire IACV wiring to a 2 wire configuration.
This guide will walk you through the process of converting your 3 wire IACV wiring to a 2 wire setup, allowing seamless integration with your OBD1 ECU in your 99-00 Civic.
Why Convert to a 2 Wire IACV?
The necessity for this conversion arises from the design of most aftermarket OBD1 ECUs. These performance-oriented ECUs are engineered to work with the more common 2 wire IACV system found on earlier Honda models and across most of the Honda/Acura range. When installing an OBD1 ECU in a 99-00 Civic DX, LX, or automatic EX, the 3 wire IACV wiring becomes incompatible, leading to idle control issues. By rewiring to a 2 wire configuration, you ensure that your new OBD1 ECU can correctly manage the idle air control valve, resulting in stable and predictable engine idling.
Parts Required for the Conversion
Before starting the wiring conversion, gather the necessary parts:
- 2 Wire IACV Plug: You will need a 2 wire IACV connector to replace the existing 3 wire plug. This can often be sourced from a parts car or purchased as a new replacement part.
- Wiring Tools: Standard wiring tools such as wire strippers, crimpers (if using crimp connectors), soldering iron and solder (if soldering), and heat shrink tubing or electrical tape for insulation are essential.
In some cases, depending on your engine setup, you might also need to consider:
- 2 Wire IACV Compatible Intake Manifold/Throttle Body: If your engine still retains the original intake manifold and throttle body designed for a 3 wire IACV, you might need to swap to a 2 wire IACV compatible intake manifold and throttle body. A common solution is using the intake manifold and throttle body from a 96-00 Civic EX with a manual transmission, as these models utilized a 2 wire IACV. However, if you have already performed an engine swap to a B series or a D series engine that natively uses a 2 wire IACV, this step may not be necessary.
Wiring the 3 Wire IACV to 2 Wire Configuration
The conversion process involves modifying both the wiring at the IACV plug and at the ECU. Follow these steps carefully:
At the IACV Plug:
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Identify and Disconnect the Orange Wire (Ground): Locate the orange wire on your 3 wire IACV plug. This wire is the ground wire and will no longer be used in the 2 wire configuration. Carefully cut this orange wire and tape off the end to insulate it. Ensure it is safely tucked away and cannot short circuit.
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Connect Yellow/Black Wire to Green/Black Wire: Identify the yellow/black wire on your engine harness (this is the power wire). Connect this yellow/black wire to the green/black wire on your replacement 2 wire IACV plug. You can use solder and heat shrink tubing for a robust and insulated connection, or high-quality crimp connectors if soldering is not preferred.
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Connect Black/Blue Wire to Black Wire: Locate the black/blue wire on your engine harness (this is the idle control signal wire from the ECU). Connect this black/blue wire to the black wire on your replacement 2 wire IACV plug. Again, use soldering and heat shrink or crimp connectors for a secure and insulated connection.
At the ECU (Connector “B”):
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Locate ECU Connector “B”: Identify connector “B” at your ECU. Refer to your Civic’s wiring diagrams if needed to confirm the correct connector.
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Move Black/Blue Wire from Pin B6 to Pin B23: Find the black wire with a blue stripe currently located at pin B6 of connector “B”. Carefully depin this wire from pin B6 and re-pin it into pin B23 of the same connector “B”. Pin B23 is the correct location for the IACV signal wire for a 2 wire IACV setup on OBD1 ECUs in this application.
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Remove and Disregard the Orange Wire at Pin B15: Locate the orange wire at pin B15 of connector “B”. Depin this orange wire and remove it completely. This wire is no longer needed and should be taped off and safely tucked away, just like the orange wire at the IACV plug.
Completion and Next Steps
After completing these wiring modifications, your 1999-2000 Honda Civic DX, LX, or automatic EX is now wired for a 2 wire IACV system. You can now proceed with installing your OBD2B to OBD1 ECU Jumper Harness and connect your OBD1 ECU. With the wiring correctly converted, your OBD1 ECU will be able to properly control the 2 wire IACV, ensuring correct idle behavior for your engine.
Remember to double-check all wiring connections and pin locations before starting your vehicle to avoid any electrical issues. Properly converting your 3 wire IACV wiring is a crucial step for a successful OBD1 ECU conversion in these specific Honda Civic models.
