On-board diagnostics

All you want to know about On-board diagnostics

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 1980s 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 was 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 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

History

  • 1970: The United States Congress passes the Clean Air Act and establishes the Environmental Protection Agency.
  • 1975: [Datsun 280z] On-board computers begin appearing on consumer vehicles, largely motivated by their need for real-time 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 a proprietary interface and protocol. The initial ALCL protocol communicates at 160 baud with Pulse-width modulation (PWM) signaling and monitors very few vehicle systems.
  • 1986: An upgraded version of the ALDL protocol appears which communicates at 8192 baud with half-duplex UART signaling. 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 is not applied until the introduction of OBD-II. The data link connector and its position are not standardized, 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 model year 1996 (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: The OBD-II specification is made mandatory for all cars sold in the United States.
  • 2001: The European Union makes EOBD mandatory for all petrol vehicles sold in the European Union, starting in MY2001 (see European emission standards Directive 98/69/EC [1] ).
  • 2008: All cars sold in the United States are required to use the ISO 15765-4 [2] signaling standard (a variant of the Controller Area Network (CAN) bus).

Standard interfaces

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 implement effectively the annual testing program.

OBD 1.5

OBD 1.5 refers to a partial implementation of OBD-II which General Motors used on some vehicles in 1994 and 1995 (GM did not use the term OBD 1.5 in the documentation for these vehicles - they simply have an OBD and an OBD-II section in the service manual.)

For example, the 94-95 Corvettes have one post-catalyst oxygen sensor (although they have two catalytic converters), and have a subset of the OBD-II codes implemented. For a 1994 Corvette the implemented OBD-II codes are P0116-P0118, P0131-P0135, P0151-P0155, P0158, P0160-P0161, P0171-P0175, P0420, P1114-P1115, P1133, P1153 and P1158.[1]

This hybrid system was present on the GM H-body cars in 94-95, W-body cars (Buick Regal, Chevrolet Lumina ('95 only), Chevrolet Monte Carlo ('95 only), Pontiac Grand Prix, Oldsmobile Cutlass Supreme) in 94-95, L-body (Chevrolet Beretta/Corsica) in 94-95, Y-body (Chevrolet Corvette) in 94-95, on the F-body (Chevrolet Camaro and Pontiac Firebird) in 95 and on the J-Body (Chevrolet Cavalier and Pontiac Sunfire) and N-Body (Buick Skylark, Oldsmobile Achieva, Pontiac Grand Am) in 95.

The pinout for the ALDL connection on these cars is as follows:

Female OBD connector pinout
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16

For ALDL connections, pin 9 is the data stream, pins 4 and 5 are ground and pin 16 is battery voltage.

Additional vehicle-specific diagnostic and control circuits are also available on this connector. For instance, on the Corvette there are interfaces for the Class 2 serial data stream from the PCM, the CCM diagnostic terminal, the radio data stream, the airbag system, the selective ride control system, the low tire pressure warning system and the passive keyless entry system.[2]

An OBD1.5 has also been used on Mitsubishi cars of '95 '97 vintage.

An OBD1.5 has been used in some 1995 Volkswagen VR6's (Juice Box's GTI)

An OBD1.5 has also been used in Ford Scorpio 95+ http://www.fordscorpio.co.uk/star.htm

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 OBD-II came in 2 models OBD-IIA and OBD-IIB.

OBD-II Diagnostic connector

Connector

The OBD-II specification provides for a standardized 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 nearly 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. - Ford DCL(+) Argentina, Brasil (pre OBD-II) 1997-2000
  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. - Ford DCL(-) Argentina, Brasil (pre OBD-II) 1997-2000
  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.

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 (pulse-width modulation - 41.6 kbaud, standard of the Ford Motor Company)
    • pin 2: Bus+
    • pin 10: Bus–
    • High voltage is +5 V
    • Message length is restricted to 12 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 +7 V
    • Decision point is +3.5 V
    • Message length is restricted to 12 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 12 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 (250 kbit/s or 500 kbit/s). The CAN protocol is a popular standard outside of the US 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 CAN, 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 the connector shape does not distinguish between the two.

Diagnostic data available

OBD-II provides access to numerous data from the ECU (Electronic Control Unit) 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 manufacturers often enhance the OBD-II code set with additional proprietary DTCs.

EOBD

EOBD is a version of OBD-II required in Europe since Model Year 2004 for diesel vehicles and since 2001 for gasoline vehicles[3]

EOBD2

The term "EOBD2" is a marketing term used by some vehicle manufacturers to refer to manufacturer-specific features that are not actually part of the OBD or EOBD standard.[4] In this case "E" stands for European.

Scan tools

OBD scan tools can be categorized in several ways ranging from whether they are OEM tools or aftermarket tools, whether they require a computer to operate (stand-alone tool vs PC-based software), and the intended market (professional or hobby/consumer use).

The advantages of PC-based scan tools are:

  • Low cost (compared to stand-alone scan tools with similar functionality -if you don't count the cost of a laptop PC).
  • Virtually unlimited storage capacity for data logging and other functions.
  • Higher resolution screen than handheld tools.
  • Availability of multiple software programs.
  • Some are capable of reprogramming

The advantages of stand-alone tools:

  • Wide selection beginning with simple code read/erase tools starting at as low as $79 retail.
  • Simplified operation that requires no computer skills/ PC compatibility issues.
  • Rugged designs, intended for use in and around cars (i.e. no lugging a laptop in and around a car).

See List of Standalone OBD-II Scan Tools, List of OBD-II Cables & Scanning Software, and List of OBD-II Gauges & Performance Monitors.

Mode of Operation

Here is a basic introduction to the OBD communication protocol:

Mode $01 is used to identify what Powertrain information is available to the scan tool.

Mode $02 displays Freeze Frame data.

Mode $03 lists the total number of powertrain or emission related DTC stored. It also displays exact numeric, 5 digit codes identifying the faults.

Mode $04 is used to clear DTCs and Freeze Frame.

Mode $05 displays the oxygen sensor monitor screen and the test results gathered about the oxygen sensor.

There are ten numbers available for diagnostics:

  1. $01 Rich-to-Lean O2 sensor threshold voltage
  2. $02 Lean-to-Rich O2 sensor threshold voltage
  3. $03 Low sensor voltage threshold for switch time measurement
  4. $04 High sensor voltage threshold for switch time measurement
  5. $05 Rich-to-Lean switch time in ms
  6. $06 Lean-to Rich switch time in ms
  7. $07 Minimum voltage for test
  8. $08 Maximum voltage for test
  9. $09 Time between voltage transitions in ms

Mode $06 is a Request for On-Board Monitoring Test Results for Non-Continuously Monitored System. There are typically a minimum value, a maximum value, and a current value for each non-continuous monitor.

Mode $07 is a Request for continuously Monitored Systems. This is used by service technicians after a vehicle repair, and after clearing diagnostic information to see test results after a single driving cycle to determine if the repair has fixed the problem.

There are only three continuous monitors to be identified: fuel, misfire, and the comprehensive component.

Mode $08 could enable the off-board test device to control the operation of an on-board system, test, or component.

Mode $09 is used to deliver IUMPR. (In Use Monitor Performance Ratio)

Mode $0A is required to store Permanent DTCs as per CARB.

Software Required

Many of the scan tools require a "host" computer, such as a laptop computer. Some come with proprietary software for Microsoft Windows. Two open source programs are available: Opendiag,[5] and Freediag or ScanTool.[6][7]

Standards documents

SAE standards documents on OBD-II

  • J1962 - Defines the physical connector used for the OBD-II interface.
  • J1850 - Defines a serial data protocol. There are 2 variants- 10.4 kbit/s (single wire, VPW) and 41.6 kbit/s (2 wire, PWM). Mainly used by US manufacturers, also known as PCI (Chrysler, 10.4K), Class 2 (GM, 10.4K), and SCP (Ford, 41.6K)
  • J1978 - Defines minimal operating standards for OBD-II scan tools
  • J1979 - Defines standards for diagnostic test modes
  • J2012 - Defines standards trouble codes and definitions.
  • 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*
  • J2284-3 - Defines 500K CAN Physical and Data Link Layer

ISO standards

  • ISO 9141: Road vehicles — Diagnostic systems. International Organization for Standardization, 1989.
    • Part 1: Requirements for interchange of digital information
    • Part 2: CARB requirements for interchange of digital information
    • Part 3: Verification of the communication between vehicle and OBD II scan tool
  • ISO 11898: Road vehicles — Controller area network (CAN). International Organization for Standardization, 2003.
    • Part 1: Data link layer and physical signalling
    • Part 2: High-speed medium access unit
    • Part 3: Low-speed, fault-tolerant, medium-dependent interface
    • Part 4: Time-triggered communication
  • ISO 14230: Road vehicles — Diagnostic systems — Keyword Protocol 2000, International Organization for Standardization, 1999.
    • Part 1: Physical layer
    • Part 2: Data link layer
    • Part 3: Application layer
    • Part 4: Requirements for emission-related systems
  • ISO 15765: Road vehicles — Diagnostics on Controller Area Networks (CAN). International Organization for Standardization, 2004.
    • Part 1: General information
    • Part 2: Network layer services
    • Part 3: Implementation of unified diagnostic services (UDS on CAN)
    • Part 4: Requirements for emissions-related systems

References

 This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (August 2007)
  1. ^ (Dec. 1993) 1994 Corvette Service Manual, Book 2. General Motors Corporation, pp. 6E3-A-166 : 6E3-A-223. 
  2. ^ (Dec., 1993) 1994 Corvette Service Manual, Book 2. General Motors Corporation, p. 6E3-A-11. 
  3. ^ KBM Systems - OBD Specifications :: OBD Introduction
  4. ^ KBM Systems - OBD Specifications :: OBD Introduction
  5. ^ Opendiag home page
  6. ^ ScanTool home page
  7. ^ Freediag home page
  • Birnbaum, Ralph and Truglia, Jerry. Getting to Know OBD II. New York, 2000. ISBN 0-9706711-0-5.
  • SAE International. On-Board Diagnostics for Light and Medium Duty Vehicles Standards Manual. Pennsylvania, 2003. ISBN 0-7680-1145-0.

See also

External links

Retrieved from "http://en.wikipedia.org/wiki/On-board_diagnostics"

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