A brief discussion on the function and pin definition of the OBD interface for automobile fault diagnosis

With the continuous development and popularization of engine electronic control technology and the rise of the Internet of Vehicles, OBD fault diagnosis has become more and more familiar to everyone. The full name of OBD is On Board Diagnostics. Translated into Chinese, it is: On Board Diagnostics System. "OBD Ⅱ" is the abbreviation of "on Board Diagnositics Ⅱ", which is the abbreviation of Type Ⅱ On Board Diagnostics System. In order to standardize the diagnosis of automobile emission and driveability related faults, starting from 1996, all new cars sold in the United States must have similar diagnostic instruments, fault codes and repair steps, that is, comply with OBD Ⅱ procedure regulations. With the increasing degree of economic globalization and automobile internationalization, as the basis for driveability and emission diagnosis, OBD Ⅱ system will be more and more widely implemented and applied.

The OBD Ⅱ program makes automobile fault diagnosis simple and unified, and maintenance personnel do not need to learn each manufacturer's new system. For example, the OBD system will monitor the engine's operating status at any time to see if the car's exhaust exceeds the standard. Once it exceeds the standard, it will immediately issue a warning. When the system fails, the fault (MIL) light or the check engine (Check Engine) warning light will light up, and the powertrain control module (PCM) will store the fault information in the memory. The fault code can be read from the PCM through a certain program. According to the fault code prompt, maintenance personnel can quickly and accurately determine the nature and location of the fault.

The common functions of OBD are as follows:

1. Detect component and system failures at any time to ensure that the vehicle's emissions do not exceed the requirements of OBD regulations during its service life

2. When relevant emission faults are detected, the OBD system can alarm with the MIL light on the dashboard.

3. Faulty vehicles can be repaired promptly, reducing vehicle emissions.

4. The OBD system helps technicians to quickly diagnose and repair the problem, thus reducing maintenance costs.

However, we need to pay attention to the understanding of OBD: the realization of National III and National IV standards must be guaranteed by OBDⅡ, but OBDⅡ is not equal to these emission standards. There are two standards for OBD: SAE's American version of OBD and ISO's European version of EOBD. The time for each country to implement OBD is different. At the same time, OBD has different versions, and the functionality of different versions is also different.

In addition to various power and ground wires, the key ones are CAN line, ISO 0141-2 K line, J1850 bus, and many interfaces. Let's take a look at the OBDⅡ interface pin definition.

The design of the OBD Ⅱ program requires avoiding confusion between systems. This requires not only the use of a standard 16-pin diagnostic interface, but also the use of specific coding and descriptions of components in the manufacturer's documentation. This is to achieve unification and standardization in the following aspects.

For example, the device that provides the computer with crankshaft position and speed information is called a crankshaft position sensor, abbreviated as "CKP", and the computer is uniformly called "PCM".

Each vehicle is equipped with a 16-pin diagnostic interface of standard shape and size, with the same signal allocation for each pin and located in the same position, installed under the instrument panel, somewhere between the left side of the instrument panel and 300mm to the right of the center line of the vehicle. It should be noted that some terminals of the diagnostic interface are designated as specific signals as shown in the attached table. Other terminals are available for use by manufacturers or have not been used on the current model of the vehicle.