ISO9141-2 K-Line Network bus communication diagnosis
Part 3 of a 6 part series. Go here to access the full set.
WHO USES IT:
ISO9141-2, also known as K-Line, is used by almost all the OEMs at some point in their network designs. It is primarily used by Asian and Euro brands from model years 1996 to 2006 but is also implemented by GM, FCA and Ford when the system is a joint development with their European engineering teams (think shared platforms or Euro design). Case in point: most pre-CAN Ford systems are J1850PWM but many of their airbag and ABS systems are wired for K-Line, as they were designed by a European supplier and/or Ford of Europe.
HOW IT IS NETWORKED:
ISO9141-2 is wired to DLC pin 7. It is a 12V DC digital signal, but will trick the unaware as the network rests at 12V DC rather than 0VDC. (Aftermarket accessory installers not knowing the electrical signature often tap into it, seeing 12V DC with their test light, and ground the circuit, shutting the bus down. VW/Audi published a TSB about aftermarket radio installs and the related OBD emission test failures).
The bus is active when the ignition is in the ON/RUN position BUT you have to have scan tool connected to the DLC and be requesting something (DTCs, PIDs, etc) to watch any communication activity. It may be active when the BCM is alive but don't count on that. The challenge with ISO9141-2 K-Line is the bus is not a true networked bus. ECUs do not communicate with each other and share data like the J1850 & CAN busses. In other words, they only speak when spoken to. You will NOT get UXXXX DTCs between ECUs as they do not share data. Each is a standalone, orphan, daisy-chained together on a single comm line.
Keep in mind the state change on K-Line. Its either 12V DC when at rest or 0V DC when transmitting. A scope is very helpful here. Set your scope to 5V DC/div and 1ms/div to get you going. Channel 1 lead goes to DLC pin 7. Ground channel goes to DLC pin 4 or 5. DLC breakout box is preferred so you don't compromise (read: spread) the DLC pins
Several OEMs also wire up L-Line to DLC pin 15. The electrical scheme is the same, and testing is identical. Same goes for KWP2000. (bonus material!)
Hey Bob, Thanks for posting this series on communication! Does the scan tool supply the 12V bias voltage or is a module like the BCM doing it? I just glanced at a Mercedes from the early 2000s and it states at ignition on pin 7 should have 12 volts. This isn't always strictly correct information though. I think when K-line is used for OBD2 diagnostics the engine control module must be
Good addendums. The bias voltage comes from any of the ECUs, not the scan tool. The tool grounds the line to initiate communication. An important thing to know is most OEMs that used K-Line on DLC pin 7 also used something else. The main driver for K-Line on DLC pin 7 was so they could meet the US OBDII requirements, specifically CARB. A vehicle had to have one of the 4 specified protocols (pre