Understanding your vehicle’s diagnostic system is crucial for modern car owners, especially when it comes to troubleshooting and maintenance. For 1999 Honda Prelude owners, knowing about the OBD2 port and its communication protocols is essential for effective vehicle diagnostics. This guide delves into the specifics of the 99 Honda Prelude Obd2 Port, exploring its location, the protocols it uses, and how it fits into the broader context of Honda’s diagnostic evolution.
Honda, like many manufacturers, has evolved its diagnostic systems over the years. Early Honda models utilized proprietary systems before transitioning to the standardized OBD2 (On-Board Diagnostics II) systems. To understand the 99 Honda Prelude’s OBD2 port, it’s helpful to look at the historical progression of Honda’s diagnostic communication.
Before the widespread adoption of OBD2, Honda vehicles, particularly non-OBD compliant models from the early 90s to 2000s, often featured 3-pin or 5-pin Diagnostic Link Connectors (DLCs). These early systems used Honda’s proprietary “92Hm/2” protocol. This protocol was designed for use with the Honda PGM TESTER and allowed communication with up to seven onboard ECUs (Electronic Control Units) via a single wire bus known as “DIAG-H.” This system covered various modules including:
- PGM-FI Engine Control System (for non-OBD regulation compliant models)
- Transmission Control
- Immobilizer Module
- SRS Airbag System
- ABS/VSA (Anti-lock Braking System/Vehicle Stability Assist)
- Electric Power Steering
- Electric Vehicle Powertrain Control Modules
Honda 3-pin DLC connector used in early Honda models for diagnostics.
These pre-OBD2 systems, while functional, were specific to Honda and not compatible with generic scan tools. The communication interface for these systems was primarily through the “DIAG-H” line, a bi-directional serial communication line connected to the 3-pin or 5-pin DLC, transmitting data in the 92Hm/2 protocol to the Honda PGM TESTER.
As automotive industry regulations evolved, particularly with the introduction of CARB (California Air Resources Board) OBD2 mandates, Honda began to incorporate OBD2 compliant systems. This led to the “95Hm-CARB” protocol, which represented a significant step towards OBD2 compatibility while still retaining some Honda-specific diagnostic capabilities.
The 95Hm-CARB protocol was designed to support some OBD2 structures and operated at a higher baud rate (approximately 10.4 kbps + 5bps for ISO 9141-2 Initialization Address Signal). While CARB OBD2 regulations prescribed SAE J1979 for diagnostic communication with OBD2 scan tools, Honda needed to maintain compatibility with their existing PGM TESTER/HDS (Honda Diagnostic System) for more in-depth diagnostics, especially for the PGM-FI (Programmed Fuel Injection) system.
Therefore, the 95Hm-CARB protocol served a dual purpose:
- OBD2 Compliance: To meet CARB OBD2 regulations using SAE J1979 communication for generic OBD2 scan tools.
- Honda Specific Diagnostics: To retain and enhance Honda’s proprietary diagnostic methods via the PGM TESTER/HDS.
The physical interface for OBD2 compliance is the 16-pin DLC, standardized under SAE J1962 / ISO 15031-3. For the 95Hm-CARB protocol, Honda utilized the ISO9141-2 K-LINE communication protocol, connecting it to pin 7 of the 16-pin SAE J1962 DLC. Notably, the ISO9141-2 L-LINE was not used in this implementation.
Standard 16-pin OBD2 DLC connector, as specified by SAE J1962, found in the 1999 Honda Prelude.
Building upon the 95Hm-CARB protocol, Honda introduced the “H/99 Protocol.” This protocol was an extension designed to fully support ISO9141-2 communication controls. A key enhancement of the H/99 protocol was its ability to allow other ECUs, beyond just the PGM-FI ECU, to be connected to the ISO9141-2 K-LINE.
The H/99 protocol maintained two communication modes, ensuring backward compatibility and broad diagnostic capability:
- Mode ‘H’: For Honda PGM TESTER/HDS. This mode uses a Master/Slave, Single Request/Single Response communication method, compatible with the 95Hm-CARB protocol and Honda’s professional diagnostic tools.
- Mode ‘C’: For SAE J1978 OBD Scan Tools. This mode also uses a Master/Slave setup but supports Single Request/Multiple Response, catering to generic OBD2 scanners.
The 1999 Honda Prelude and its OBD2 Port
For owners of a 1999 Honda Prelude, the good news is that it is equipped with a 16-pin OBD2 port. This port is typically located under the dashboard on the driver’s side. Being a 1999 model, the Prelude falls into the era where Honda was utilizing the H/99 protocol. This means the 1999 Prelude’s OBD2 port is designed to be compatible with both generic OBD2 scan tools and Honda-specific diagnostic equipment.
Key Takeaways for the 1999 Honda Prelude OBD2 Port:
- Location: Typically under the dashboard on the driver’s side. Consult your owner’s manual for the precise location if needed.
- Protocol: Utilizes the H/99 protocol, which is based on ISO9141-2.
- Compatibility: Compatible with standard OBD2 scanners (using Mode ‘C’) for basic diagnostics and code reading. Also compatible with Honda PGM TESTER/HDS (using Mode ‘H’) for advanced, Honda-specific diagnostics.
- Functionality: Allows reading of diagnostic trouble codes (DTCs), viewing live data parameters, and potentially performing some actuation tests, depending on the capabilities of the scan tool being used.
In later years, Honda further advanced its diagnostic communication by implementing CAN (Controller Area Network) protocols, known as HDC_CAN (Honda Diagnostic on CAN). CAN is a high-speed communication protocol widely used in modern vehicles. However, for the 1999 Honda Prelude, the diagnostic system is based on the H/99 protocol over ISO9141-2 via the OBD2 port.
For further information and access to repair manuals, resources like Honda Hookup (Honda / Acura Manuals – Hondahookup.com) can be invaluable for 1999 Honda Prelude owners and enthusiasts looking to understand their vehicle’s diagnostics in detail.
In conclusion, the 1999 Honda Prelude features a standard OBD2 port that uses the H/99 protocol, offering a bridge between generic OBD2 diagnostics and Honda’s more in-depth diagnostic systems. Understanding this protocol and the location of your OBD2 port is the first step in effectively diagnosing and maintaining your 1999 Honda Prelude.
