About Opentopia
Opentopia Directory Encyclopedia Tools

On Board Diagnostics

Encyclopedia : O : ON : ONB : 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 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

ALDL/ALCL

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 ALDL/ALCL 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, with the 12 pin connector being used in the vast majority of GM cars.  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):



12-pin ALDL connector pinout


F E D C B A
G H J K L M



10-pin ALDL connector pinout


A B C D E
K J H G F



5-pin ALDL connector pinout


A B C D E


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.  No battery voltage is present in the 12 pin ALDL connector.



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 1.5

OBD 1.5 is an OBD I system with the 16 pin OBD II diagnostic connector. An OBD I code reader must be used. It is only believed to be used by GM in 1994 and 1995.



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:

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).



PC-Based Scan Tools

The advantages of PC-based scan tools are:







SAE standards documents relating to OBD-II



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

Birnbaum, Ralph and Truglia, Jerry.  Getting to Know OBDII.  New York, 2000.



External Links
















 




From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.















Search Titles









0123456789
ABCDEFGHIJ
KLMNOPQRST
UVWXYZ?
















E-mail this article to:



Personal Message: