Converting your Honda from OBD2 to OBD1 can seem daunting, but with the correct information and a bit of DIY spirit, it’s a manageable task, especially for those looking to tune older Honda vehicles or perform engine swaps. This guide provides a detailed Honda Obd2 To Obd1 Pinout diagram, essential for anyone undertaking this conversion.
For Honda enthusiasts and DIY mechanics working on a budget, understanding the wiring differences between OBD2 and OBD1 systems is crucial. This article breaks down the pin connections required to adapt an OBD2 Honda to run an OBD1 ECU, such as the popular P28, P30, and P72 ECUs, often favored for their tuning capabilities.
Understanding OBD2 and OBD1 Systems in Honda Vehicles
Before diving into the pinout, it’s important to understand the basics of OBD systems. OBD stands for On-Board Diagnostics, and these systems are used in vehicles to monitor and control various engine and emissions components.
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OBD1 (First Generation On-Board Diagnostics): Used in vehicles typically manufactured before the mid-1990s, OBD1 systems in Hondas are simpler and provide basic diagnostic information. They are favored by tuners for their ease of modification and chip tuning capabilities.
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OBD2 (Second Generation On-Board Diagnostics): Mandatory in most vehicles sold in the USA from 1996 onwards, OBD2 is a more advanced system offering standardized diagnostic codes, enhanced monitoring, and greater complexity. While more comprehensive, OBD2 ECUs are often less tunable compared to their OBD1 counterparts, especially for performance modifications on older Honda engines.
The conversion from OBD2 to OBD1 is often performed when installing an older Honda engine into a newer OBD2 chassis, or when swapping to an OBD1 ECU for tuning purposes in an originally OBD2 car. This requires adapting the wiring harness to match the different pin configurations between the two systems.
Honda OBD2 to OBD1 ECU Conversion Pinout Diagram
Below is the comprehensive pinout diagram for converting a Honda OBD2 system to OBD1. This diagram is based on a popular conversion harness configuration used with ECUs like the P28 Uberdata, P30, and P72.
| **OBD1 Pinout (Left) | OBD2 Pinout (Right)** |
|---|---|
| Connector A (ECU Connector A) | Connector A (OBD2 ECU Connector A) |
| A1 to A4 | A2 to A1 |
| A3 to A3 | A4 to A8 |
| A5 to A2 | A6 to A6 |
| A7 to A16 | A8 No Use |
| A9 to A12 | A10 No Use |
| A11 No Use | A12 to A27 |
| A13 to A18 | A14 No Use |
| A15 to A17 | A16 to A19 |
| A17 to A26 | A18 No Use |
| A19 No Use | A20 to A15 |
| A21 to A20 | A22 No Use |
| A23 to A10 | A24 to A23 |
| A25 to A11 | A26 to A9 |
Connector B (ECU Connector B) | Connector B (OBD2 ECU Connector B)
B1 to A24 | B2 to A22
B3 No Use | B4 Blank
B5 to C5 | B6 Blank
B7 to C29 | B8 to C16
B9 to C6 | B10 to C18
B11 to C4 | B12 to C14
B13 to C3 | B14 to C13
B15 to C2 | B16 to C12
Connector D (ECU Connector C – OBD1) | Connector D (OBD2 ECU Connector D)
D1 to C10 | D2 to D5
D3 to D6 | D4 to C7
D5 No Use | D6 to C15
D7 to C8 | D8 No Use
D9 to C17 | D10 to D16
D11 to D1 | D12 No Use
D13 to D2 | D14 to D7
D15 to D8 | D16 No Use
D17 to D3 | D18 No Use
D19 to D4 | D20 to D10
D21 to D12 | D22 to D11
This pinout is compatible with various OBD1 ECUs including P28, P30, P72, and other OBD1 Honda ECUs.
It’s a common practice to salvage OBD2 plugs from automatic transmission Honda vehicles. These often have extra pins that can be used to replace any short or missing pins required for your conversion harness, ensuring you have all the necessary terminals for a complete and reliable connection.
Important Notes:
- Verify Compatibility: While this pinout is widely used, always double-check the specific wiring diagrams for your vehicle model and ECU to ensure compatibility.
- Professional Installation: If you are not comfortable working with vehicle wiring, it’s highly recommended to seek professional installation to avoid damaging your ECU or vehicle’s electrical system.
- ECU Selection: Ensure the OBD1 ECU you choose (P28, P30, P72, etc.) is appropriate for your engine and intended modifications.
- DIY at Your Own Risk: Performing this conversion is a DIY task and should be undertaken with caution. Mistakes in wiring can lead to serious issues.
By carefully following this Honda OBD2 to OBD1 pinout guide, you can successfully convert your Honda’s ECU system, opening up possibilities for tuning and engine management customization. Always prioritize safety and accuracy when working with automotive electrical systems.
