Document # 31-0008
LM2_Manual_1.4.doc
29
4.9 OBD II Basics
OBD stands for “On Board Diagnostics”. It represents a collection of industry and
legislative standards for getting basic diagnostic information from passenger vehicles
sold in the US since Model Year 1996. The “II” means that this is the second attempt at
standardizing across all makes and models.
OBD-II standardized what type of information ECU’s must provide, but did not set a
single standard on how the information should be communicated (typically called a
‘Communication Protocol’ or just ‘Protocol’) or even the electrical characteristics of the
communication link itself (usually referred to as the ‘Physical Communication Link’).
Instead, the various car manufacturers were each allowed to keep using their own
existing protocols and physical links. We can see this compromise on the OBD-II
connector itself:
Pin Assignments:
1. -
9. -
2. J1850+
10. J1850-
3. -
11. -
4. Chassis Ground
12. -
5. Signal Ground
13. -
6. CAN (J2284) High
14. CAN (J2284) Low
7. ISO 9141 K line
15. ISO 9141 L line
8. -
16. Battery Power
Note: Pins marked with ‘-‘ are sometimes used for make/model specific purposes.
Looking closely we can see that there are really three different electrical ‘pairs’, each
representing a different type of physical communication link. J1850 (pins 2&10), CAN
(pins 6&14), and ISO (pins 7&15). On top of these three physical communication links
are six different communication protocols:
•
The J1850 pair uses either J1850pwm (Ford) or J1850vpw (GM)
•
The CAN pair uses either ‘standard’ ISO 15765 or ‘extended’ ISO 15765
•
The ISO pair use either ISO 9141 or ISO 14230 (sometimes referred to as
KWP2000)
So, in order to get OBD-II information from any OBD-II compatible vehicle an interface
needs to ‘speak’ six different languages over three different types of electrical links.
Starting in MY2008, this will drop to one electrical link (CAN) and two protocol variations
