On Board Diagnostics



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On-Board Diagnostics, or OBD, in an automotive context, is a generic term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or a repair technician access to state of health information for various vehicle sub-systems. The amount of diagnostic information available via OBD has varied widely since the introduction in the early 1980's of on-board vehicle computers, which made OBD possible. Early instances of OBD would simply illuminate a malfunction indicator light, or MIL, if a problem were detected—but would not provide any information as to the nature of the problem. Modern OBD implementations use a standardized fast digital communications port to provide myriad realtime data in addition to a standardized series of diagnostic trouble codes, or DTCs, which allow one to rapidly identify and remedy malfunctions within the vehicle.


Contents

Major Milestones in the History of OBD

  • 1970 The United States Congress passes the Clean Air Act and establishes the Environmental Protection Agency.
  • ~1980 On-board computers begin appearing on consumer vehicles, largely motivated by their need for realtime tuning of fuel injection systems. Simple OBD implementations appear, though there is no standardization in what is monitored or how it is reported.
  • 1982 General Motors implements an internal standard for its OBD called the Assembly Line Communications Link (ALCL), later renamed the Assembly Line Diagnostics Link (ALDL). The initial ALCL protocol communicates at 160 baud with PWM signalling and monitors very few vehicle systems.
  • 1986 An upgraded version of the ALDL protocol appears which communicates at 8192 baud with half-duplex UART signalling. This protocol is defined in GM XDE-5024B.
  • ~1987 The California Air Resources Board (CARB) requires that all new vehicles sold in California starting in manufacturer's year 1988 (MY1988) have some basic OBD capability. The requirements they specify are generally referred to as the "OBD-I" standard, though this name isn't applied until the introduction of OBD-II. The data link connector and its position are not standarized, nor is the data protocol.
  • 1988 The Society of Automotive Engineers (SAE) recommends a standardized diagnostic connector and set of diagnostic test signals.
  • ~1994 Motivated by a desire for a state-wide emissions testing program, the CARB issues the OBD-II specification and mandates that it be adopted for all cars sold in California starting in MY1996 (see CCR Title 13 Section 1968.1 and 40 CFR Part 86 Section 86.094). The DTCs and connector suggested by the SAE are incorporated into this specification.
  • 1996 Legislation is passed requiring any component malfunction that causes the MIL to illuminate is the manufacturer's responsibility to repair, provided that the vehicle is within its emissions warranty period. This was done in an effort to persuade manufacturers to produce more robust emissions control equipment.
  • 1998 The OBD-II specification is made mandatory for all cars sold in the United States.
  • 2001 The European Union makes EOBD, a variant of OBD-II, mandatory for all petrol vehicles sold in Europe starting in MY2001 (see European Directive 98/69/EC).
  • 2008 All cars sold in the United States are required to use the ISO15765-4 signalling standard (a variant of the CAN bus).

ALCL/ALDL

The Assembly Line Communications Link (ALCL) was later renamed the Assembly Line Diagnostic Link (ALDL). The two terms are used synonymously. This system was only vaguely standardized and suffered from the fact that specifications for the communications link varied from one model to the next. ALDL was largely used by manufacturers for diagnostics at their dealerships and official maintenance facilities.

The ALCL/ALDL Diagnostic Connector

There were at least three different connectors used with ALDL. General Motors implemented both a 5-pin connector and a 12-pin connector. Lotus implemented a 10-pin connector. The pins are given letter designations in the following layouts (as seen from the front of the vehicle connector):

  5-pin ALDL connector pinout
  
  A B C D E

  10-pin ALDL connector pinout
  
  A B C D E
  K J H G F

  12-pin ALDL connector pinout
  
  F E D C B A
  G H J K L M

Note the difference in pin ordering between the connectors and the fact that the letter I is not used. Unfortunately, the definition of which signals were present on each pin varied between vehicle models. There were generally only three pins used—ground, battery voltage, and a single line for data.

OBD-I

The regulatory intent of OBD-I was to encourage auto manufacturers to design reliable emission control systems that remain effective for the vehicle's "useful life". The hope was that by forcing annual emissions testing for California, and denying registration to vehicles that did not pass, drivers would tend to purchase vehicles that would more reliably pass the test. Along these lines, OBD-I was largely unsuccessful—the means of reporting emissions-specific diagnostic information was not standardized. Technical difficulties with obtaining standardized and reliable emissions information from all vehicles led to an inability to effectively implement the annual testing program.

OBD-II

OBD-II is an improvement over OBD-I in both capability and standardization. The OBD-II standard specifies the type of diagnostic connector and its pinout, the electrical signalling protocols available, and the messaging format. It also provides a candidate list of vehicle parameters to monitor along with how to encode the data for each. Finally, the OBD-II standard provides an extensible list of DTCs. As a result of this standardization, a single device can query the on-board computer(s) in any vehicle. This simplification of reporting diagnostic data led the feasibility of the comprehensive emissions testing program envisioned by the CARB.

The OBD-II Diagnostic Connector

The OBD-II specification provides for a standarized hardware interface—the female 16-pin (2x8) J1962 connector. Unlike the OBD-I connector, which was sometimes found under the hood of the vehicle, the OBD-II connector is always located on the driver's side of the passenger compartment near the center console. SAE J1962 defines the pinout of the connector as:

  1. -
  2. Bus positive Line of SAE-J1850
  3. -
  4. Chassis ground
  5. Signal ground
  6. CAN high (ISO 15765-4 and SAE-J2234)
  7. K line of ISO 9141-2 and ISO 14230-4
  8. -
  9. -
  10. Bus negative Line of SAE-J1850
  11. -
  12. -
  13. -
  14. CAN low (ISO 15765-4 and SAE-J2234)
  15. L line of ISO 9141-2 and ISO 14230-4
  16. Battery voltage

The assignment of unspecified pins is left to the vehicle manufacturer's discretion.

OBD-II Signal Protocols

There are five signalling protocols currently in use with the OBD-II interface. Any given vehicle will likely only implement one of the protocols. Often it is possible to make an educated guess about the protocol in use based on which pins are present on the J1962 connector:

  • SAE J1850 PWM (41.6 kbaud, standard of the Ford Motor Company)
    • pin 2: Bus-
    • pin 10: Bus+
    • High voltage is +5V
    • Message length is restricted to 11 bytes, including CRC
    • Employs a multi-master arbitration scheme called 'Carrier Sense Multiple Access with Non-Destructive Arbitration' (CSMA/NDA)
  • SAE J1850 VPW (Variable Pulse Width) (10.4/41.6 kbaud, standard of General Motors)
    • pin 2: Bus+
    • Bus idles low
    • High voltage is +7V
    • Decision point is +3.5V
    • Message length is restricted to 11 bytes, including CRC
    • Employs CSMA/NDA
  • ISO 9141-2. This protocol has a data rate of 10.4 kbaud, and is similar to RS-232. ISO 9141-2 is primarily used in Chrysler, European, and Asian vehicles.
    • pin 7: K-line
    • pin 15: L-line (optional)
    • UART signaling (though not RS-232 voltage levels)
    • K-line idles high
    • High voltage is Vbatt
    • Message length is restricted to 11 bytes, including CRC
  • ISO 14230 KWP2000 (Keyword Protocol 2000)
    • pin 7: K-line
    • pin 15: L-line (optional)
    • Physical layer identical to ISO 9141-2
    • Data rate 1.2 to 10.4 kbaud
    • Message may contain up to 255 bytes in the data field
  • ISO 15765 CAN (250kbit/sec or 500kbit/sec). The CAN protocol is a popular standard outside of the automotive industry and is making significant in-roads into the OBD-II market share. By 2008, all vehicles sold in the US will be required to implement the CAN bus, thus eliminating the ambiguity of the existing five signalling protocols.
    • pin 6: CAN High
    • pin 14: CAN Low

Note that pins 4 (battery ground) and 16 (battery positive) are present in all configurations. Also, ISO 9141 and ISO 14230 use the same pinout, thus you cannot distinguish between the two simply by examining the connector.

Diagnostic data available via OBD-II

OBD-II provides access to numerous data from the ECU and offers a valuable source of information when troubleshooting problems inside a vehicle. The SAE J1979 standard defines a method for requesting various diagnostic data and a list of standard parameters that might be available from the ECU. The various parameters that are available are addressed by "parameter identification numbers" or PIDs which are defined in J1979. For a list of basic PIDs, their definitions, and the formulae to convert raw OBD-II output to meaningful diagnostic units, see OBD-II PIDs. Manufacturers are not required to implement all PIDs listed in J1979 and they are allowed to include proprietary PIDs that are not listed. The PID request and data retrieval system gives access to real time performance data as well as flagged DTCs. For a list of generic OBD-II DTCs suggested by the SAE, see Table of OBD-II Codes. Individual manufactures often enhance the OBD-II code set with additional proprietary DTCs.

OBDII Scan Tools

OBDII scan tools can be categorized in two ways, based on whether they require a computer to operate (stand-alone vs PC-based), and the intended market (professional or hobby/consumer use). Thus, all scan tools fall into one of the following four categories:

  • Stand-alone
    • Professional
    • Hobby/Consumer
  • PC-based
    • Professional
    • Hobby/Consumer

PC-Based Scan Tools

The advantages of PC-based scan tools are:

  • Low cost (compared to stand-alone scan tools with similar functionality)
  • Virtually unlimited storage capacity for data logging and other functions
  • Higher resolution screen
  • Availability of multiple software programs
  • Some are capable of reprogramming

Professional

Hobby/Consumer

Software

Standalone

Professional

  • Autologic - Factory-quality scan tools with outstanding BMW coverage.
  • Launch X-431 - The world's best-selling diagnostic tool.

Hobby/Consumer

Hobby Resources

General OBDII Information

Scan Tool Parts


SAE standards documents relating to OBD-II

  • J1962 - Defines he physical connector used for the OBD-II interface.
  • J1850 - Defines the signaling and timings for "GM-style" VPW communication
  • J1978 - Defines minimal operating standards for OBD-II scan tools
  • J1979 - Defines standards for diagnostic test modes
  • J2012 - Defines standards for EPA emission test report format.
  • J2178-1 - Defines standards for network message header formats and physical address assignments
  • J2178-2 - Gives data parameter definitions
  • J2178-3 - Defines standards for network message frame IDs for single byte headers
  • J2178-4 - Defines standards for network messages with three byte headers

The Future of On-Board Diagnostics

An OBD-III specification is in the regulatory development phase. Information on the content of this specification is limited. Some have speculated that OBD-III will include the capability for a vehicle to report emissions violations automatically using some sort of radio transmitter.

References

External Links

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