What’s the On board diagnostics ?

What’s the On board diagnostics ?

On-board diagnostics (OBD) is an automotive term referring to a vehicle’s self-diagnostic and reporting capability. OBD systems give the vehicle owner or repair technician access to the status of the various vehicle subsystems. The amount of diagnostic information available via OBD has varied widely since its introduction in the early 1980s versions of on-board vehicle computers. Early versions of OBD would simply illuminate a malfunction indicator light or “idiot light” if a problem was detected but would not provide any information as to the nature of the problem. Modern OBD implementations use a standardized digital communications port to provide real-time 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.


OBD2 diagnostic connector

The SAE J1962 specification provides for two standardized hardware interfaces, called type A and type B. Both are female, 16-pin (2×8), D-shaped connectors, and both have a groove between the two rows of pins. But type B has the groove interrupted in the middle, so you can’t plug a type A male connector into a type B socket. You can, however, mate a type B male plug in a type A female socket.

The type A connector is used for vehicles that use 12V supply voltage, whereas type B is used for 24V vehicles and it is required to mark the front of the D-shaped area in blue color.

SAE J1962 defines the pinout of the connector as:

1 Manufacturer discretion:

  • GM: J2411 GMLAN/SWC/Single-Wire CAN
  • VW/Audi/BMW: Switched +12V to tell a scan tool whether the ignition is on.
  • Ford, FIAT: Infotainment CAN High
9 Manufacturer discretion:

  • BMW: TD (Tachometer Display) signal aka engine RPM signal.
  • GM: 8192 bit/s ALDL where fitted.
  • Ford: Infotainment CAN-Low
2 Bus Positive Line of SAE J1850 PWM and VPW 10 Bus Negative Line of SAE J1850 PWM only (not SAE J1850 VPW)
3 Manufacturer discretion:

  • Ford: DCL(+) Argentina, Brazil (pre OBD-II) 1997-2000, USA, Europe, etc.
  • Ford: Medium Speed CAN-High
  • Chrysler: CCD Bus(+)
11 Manufacturer Discretion:

  • Ford: DCL(-) Argentina, Brazil (pre OBD-II) 1997-2000, USA, Europe, etc.
  • Ford: Medium Speed CAN-Low
    Chrysler: CCD Bus(-)
4 Chassis ground 12 Manufacturer discretion:
5 Signal ground 13 Manufacturer discretion:

  • Ford: FEPS – Programming PCM voltage
6 CAN-High (ISO 15765-4 and SAE J2284) 14 CAN-Low (ISO 15765-4 and SAE J2284)
7 K-Line of ISO 9141-2 and ISO 14230-4 15 L-Line of ISO 9141-2 and ISO 14230-4
8 Manufacturer discretion:

  • BMW: Second K-Line for non OBD-II (Body/Chassis/Infotainment) systems.
  • FIAT: Infotainment CAN-Low.
16 Battery voltage:

  • Type “A” 12V/4A
  • Type “B” 24V/2A


Source: https://en.wikipedia.org/wiki/On-board_diagnostics

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