For automotive enthusiasts and technicians diving into Honda diagnostics, understanding the evolution of their On-Board Diagnostics (OBD) systems is crucial. From early proprietary systems to OBD2 compliance, Honda has utilized a range of protocols to facilitate vehicle communication and diagnostics. This guide breaks down these protocols, offering a clear picture of Honda’s diagnostic journey and its relevance to modern OBD2 tools, potentially including advanced options from brands like Greddy.
Honda’s Diagnostic Communication Evolution: From Proprietary to OBD2
Honda’s diagnostic systems have undergone significant changes over the years. Initially, Honda vehicles, particularly non-OBD compliant models from the 1990s to early 2000s, employed unique diagnostic communication methods. These early systems were designed primarily for use with Honda’s proprietary tools like the PGM TESTER and HDS (Honda Diagnostic System).
92Hm/2 Protocol: The Foundation
In 1992, Honda introduced the “92Hm/2” protocol. This system was characterized by a 3-pin Data Link Connector (DLC). The pinout for this connector is as follows:
- Pin 1: DIAG-H (Diagnostic High)
- Pin 2: Battery+/NC (Battery Positive or Not Connected)
- Pin 3: Ground
The 92Hm/2 protocol utilized a single-wire bus called “DIAG-H” for communication. This network could connect up to seven Electronic Control Units (ECUs), including:
- PGM-FI (Programmed Fuel Injection) Engine Control System (non-OBD compliant models)
- Transmission Control Module
- Immobilizer Module
- SRS Airbag System
- ABS/VSA (Anti-lock Braking System/Vehicle Stability Assist)
- EPS (Electric Power Steering)
- EV Powertrain Control Modules
This protocol was essential for early Honda diagnostics but was specific to Honda tools and didn’t adhere to emerging OBD standards.
95Hm-CARB Protocol: Bridging the Gap to OBD2
As OBD2 regulations began to take shape, Honda developed the “95Hm-CARB” protocol. This protocol was a significant step towards OBD2 compliance while still accommodating Honda’s diagnostic needs. It incorporated elements of OBD2 structure and increased the baud rate to approximately 10.4 kbps + 5bps (for ISO 9141-2 Initialization Address Signal).
The California Air Resources Board (CARB) OBD2 regulations mandated SAE J1979 for diagnostic communication. While SAE J1979 is crucial for generic OBD2 scan tools, Honda needed to retain functionality for their PGM TESTER/HDS tools, particularly for advanced PGM-FI system diagnostics. The 95Hm-CARB protocol served this dual purpose, allowing both standard OBD2 scan tools (SAE J1978) and Honda’s professional tools to communicate with the vehicle.
This protocol utilized the ISO9141-2 K-LINE for communication, connected to pin 7 of the standardized 16-pin SAE J1962 DLC, which became standard for OBD2 compliant vehicles. The L-LINE in ISO9141-2 was not used in this implementation.
Key aspects of the 95Hm-CARB protocol include:
- ISO9141-2 K-LINE Interface: For OBD2 compliance.
- Dual Communication Modes:
- Mode ‘C’: For SAE J1978 OBD2 Scan Tools (Master/Slave, Single Request/Multiple Response).
- Mode ‘H’: For Honda PGM TESTER/HDS (Master/Slave, Single Request/Single Response).
- PGM-FI ECU Focus: Primarily used for the PGM-FI system on OBD2 Honda vehicles.
H/99 Protocol: Expanding ISO9141-2 Communication
Building upon the 95Hm-CARB protocol, Honda introduced the “H/99” protocol. This evolution was designed to fully support ISO9141-2 communication controls and allowed for additional ECUs beyond just the PGM-FI system to be connected to the ISO9141-2 K-LINE.
The H/99 protocol maintained compatibility with the communication modes and message sets of 95Hm-CARB, ensuring a smooth transition and continued support for both Honda diagnostic tools and generic OBD2 scanners.
HDC_CAN Protocol: Embracing Modern Standards
In later models, Honda adopted Controller Area Network (CAN) for diagnostics, implementing the “HDC_CAN” (Honda Diagnostic on CAN) protocol. CAN is a high-speed communication protocol widely used in modern automotive applications due to its robustness and efficiency.
The shift to CAN protocol reflects the industry-wide move towards faster and more comprehensive diagnostic capabilities. HDC_CAN allows for significantly increased data transfer rates and more complex diagnostic interactions, essential for the advanced systems in contemporary vehicles.
Understanding Honda Protocols for Effective Diagnostics
For anyone working on Honda vehicles, whether using professional-grade tools or aftermarket OBD2 scanners like those from Greddy, understanding these protocol evolutions is vital. Knowing which protocol a vehicle uses helps in selecting the correct diagnostic approach and tools.
While modern OBD2 scanners are designed to be broadly compatible, awareness of Honda’s specific protocol history can be invaluable for:
- Older Honda Vehicles: Diagnosing pre-OBD2 or early OBD2 models may require understanding 92Hm/2 or 95Hm-CARB nuances.
- Advanced Diagnostics: For in-depth troubleshooting, particularly of PGM-FI or chassis/body systems, familiarity with Honda-specific modes within these protocols can be beneficial.
- Tool Compatibility: Ensuring your diagnostic tool, including potentially Greddy Obd2 products, is fully compatible with the Honda protocol in use for the specific model year.
By understanding Honda’s diagnostic protocol journey, technicians and enthusiasts can approach Honda vehicle diagnostics with greater precision and effectiveness, leveraging the right tools and knowledge for optimal results.
