Network - K-Line

The purpose of this test is to verify that data is being continuously exchanged along the K-Line, and it is possible to check that the peak-to-peak voltage levels are correct and that a signal is present during the communication between the scan tool and the ECM.

Figure 1

Figure 2

How to perform the test

Using the CAN Test Box

If you don't have the CAN Test Box, see "Testing Without the CAN Test Box" below.

• First connect the 16-pin plug of the CAN Test Box into the DLC (Diagnostic Link Connector) located on the vehicle as shown in Figure 1. The LEDs on the CAN Test Box will begin to illuminate, notifying you that communication has been established and also to show you which pins are active on the DLC you are connected to. It is important to ensure that the following pins are illuminated as this will indicate that the CAN Test Box is powered up and functioning correctly:

                 Battery V+: Pin 16
                 Chassis GND: Pin 4
                 Signal GND: Pin        5

• Using the leads provided with the CAN Test Box, connect the YELLOW lead to Channel A of the scope.
• Connect the YELLOW banana plug to pin 7.
• Connect the BLACK 4 mm banana plug to pin 4 to provide the ground for the scope.

See Figures 1 and 2.

• Press the spacebar on the PC to make the scope display live data.
• Switch on the vehicle ignition, and ensure that the scan tool is actively communicating with the ECM.

The K-Line waveform will now appear on the screen as shown below.

Important: Once the ECM is communicating with the scan tool, pin 7 LED will begin to flash on the CAN Test Box. If the scan tool shows no communication with the ECM and the pin 7 LED is not flashing, then the scan tool is not sending the command to the ECM to communicate.

If, however, the scan tool shows that it has no communication with the ECM and the pin 7 LED is flashing, then the scan tool is sending the command but the ECM is not completing the communication. Causes of this could be poor connections between the DLC and ECM, incorrect scan tool communication command in software, or a fault within the ECM.

Testing without the CAN Test Box

• Plug one BNC test lead into Channel A of the oscilloscope.
• Connect a clip onto the the black (ground) plug on the BNC test lead and attach it to the vehicle battery negative terminal or a good ground point on the chassis.
• Attach one of the Back-pinning Probe to the colored plug on the BNC test lead.
• Referring to the vehicle technical manual, identify the K-Line pin at an accessible point on the vehicle communication network (usually available at the DLC connector). Carefully probe the back of the DLC connector or another suitable testing location identified by the manufacturer's wiring information.
• Press the spacebar on the PC to start the capture of live data.
• Switch on the vehicle ignition and ensure the scan tool is actively communicating with the ECM.

The K-Line waveform will now appear on the screen as shown below.

Example waveform

Waveform notes

In this display, we can verify that data is being continuously exchanged along the K-Line, and it is possible to check that the peak-to-peak voltage levels are correct and that a signal is present whilst communication is taking place between the scan tool and the ECM. Refer to the vehicle manufacturer's manual for precise waveform parameters.

Figure 3 - Diagram of K-line ISO 9141 network

Figure 4 - K-line ISO 9141 logic table

Technical Information

The K-Line is a very low-speed single-wire serial communication system used on many motor vehicles and commercial vehicles. It is commonly used for the diagnostic connections between the Electronic Control Modules (ECMs) on the vehicle and the diagnostic equipment (scan tools and data loggers). The K-Line is a network based upon the ISO9141 specifications, also known as the 9141 California Air Resources Board (CARB) Standard.

The K-Line is very different to a CAN Bus network and from most communication networks in general. A CAN Bus network, for example, does not have either a central or a primary ECM: all the ECMs are equal as they are all able to transmit messages along the network as well as receive messages.

On the K-Line network or any network compliant with ISO 9141, the direction of message flow is extremely important. The control of the network is dominated by the master ECM, and the message direction and timing depend on which ECM is talking (sending a message) and which ECMs are listening (waiting for a message). Two ECMs therefore cannot send a message at the same time, but have to wait in turn until allowed by the Master ECM. See Figure 3.

The diagram shows that there is only one wire for all communication on the network. The messages therefore need to be sent in binary format and transmitted as a pulsed voltage signal. The voltages on the K-Line are pulsed between two values in binary code (a series of ones and zeroes). The binary code is represented by the voltages shown in Figure 4 below:-

Note: Logic 0 is represented by battery voltage, so may be above 12 V.

Note 1: A K-Line message is different to a CAN message, as CAN always sends an entire message at once while K-Line may send messages split into several parts.

Note 2: A CAN Bus network operates constantly as a communication network and a diagnostic network between the ECMs whilst the vehicle is in operation. The K-Line network is only intended to support diagnostic equipment. However, when a diagnostic machine is not present, the K-Line wiring may be used by other ECMs for communication at different baud rates and with different timing patterns.

Additional information

The 16 pins of the DLC are available on the CAN Test Box and are numbered as follows:

AT166-3