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OBDII Bus Description - On Board Diagnostics II

2019/09 12

OBDII Bus Description(General Motors (GM OBD II Dianostic Interface)
- Engine Diagnostic Products

OBD2 [On-Board Diagnostics II] defines a communications protocol and a standard connector to acquire data from passenger cars. OBD was required by U.S. EPA on all gasoline powered cars and light duty trucks manufactured for the U.S. after 1996 to help monitor/inspect vehicle emissions [as required by the Clean Air Act Amendments of 1990]. OBDII is the second generation of the OBD specification. The first generation OBD monitored fewer emission related components, was not calibrated to a specific level of emission performance, and was not as fell defined. Any passenger cars and truck produced after 1996 uses the OBD II standard. The OBD-II standard allows for multiple electrical interfaces, which complicates the hardware used to interface with the vehicle.

OBDII will light a warning lamp called a MIL (malfunction indicator lamp), also known as the "check engine" light on the dash. A scan-tool may also be used to probe the OBDII connector OBDII data as defined by the SAE J1979 standard. The warning light may come on for any number of reasons and manufacturers recommend having the vehicle serviced as soon as possible. However; the Check Engine light could also come on for such simple reasons as filling the tank while the vehicle is running, or leaving the gas cap off. It may take up to three days for the light to go back off, after coming on for a missing gas cap. The OBD II interface is located in the cab, and must be located with in a certain area within the cab as defined by the standard.

J1962 - describes the standardized 16-pin trapezoidal connector.
The male plug is use to connect to your car computer and the female is for making an extension cable or some other device

The J1850 VPW single wire protocol, used by GM may be found on an OBDII bus, if so the connector will have contacts in pins 2, 4, 5, and 16, with no contact in pin 10. The J1850 PWM two wire protocol, used by Ford may be found on an OBDII bus, if so the connector will have contacts in pins 2, 4, 5, and 10, with no contact in pin 16. The ISO 9141-2 single wire protocol, used by Chrysler may be found on an OBDII bus, if so the connector will have contacts in pins 4, 5, 7, 15 and 16. The protocol and command set is fixed by SAE J1979, so they are the same for all three protocols, only the electrical layers are changed. The CAN Bus may also be found on the OBDII bus

J1850 bus Description
The J1850 bus is used for diagnostics and data sharing applications in vehicles. The J1850 bus takes two forms;
A 41.6Kbps Pulse Width Modulated (PWM) two wire differential approach, or a 10.4Kbps Variable Pulse Width (VPW) single wire approach. The single wire approach may have a bus length up to 35 meters (with 32 nodes). A high resides between 4.25 volts and 20 volts, a low is any thing below 3.5 volts. High and low values are sent as bit symbols (not single bits). Symbols times are 64uS and 128uS for the single wire approach. The ISO 9141-2 single-wire asynchronous interface operates at 10.4kbps

OBDII Related Standards
J1962 - SAE (Society of Automotive Engineers) standard defining the physical connector used for the OBDII interface.
J1850 - SAE standard for the Class B Communications Network Interface (standard defines the actual J1850 signaling and timings)
J1939, ISO 11898
J1978 - SAE standard for OBD II scan tools
J1979 - SAE standard for diagnostic test modes
J2012 - SAE standard for EPA emission test report format.
J2178-1 - SAE standard for Class B Communications Network Message: Detailed Header Formats and Physical Address Assignments
J2178-2 - SAE standard for Class B Communications Network Message: Data Parameter Definitions
J2178-3 - SAE standard for Class B Communications Network Message: Frame IDs for Single Byte Forms of Headers
J2178-4 - SAE standard for Class B Communications Network Message: Message Definitions for Three Byte Headers
Code of Federal Regulations [CFR]: 40 CFR; 86.094-17h