As more smart motors, switches and sensors are being used, there are more Local Interconnect Network (LIN) buses on vehicles. Because of this, some questions have been raised regarding the diagnosis of a LIN bus circuit. This tech tip is intended to help better explain LIN buses and provide some diagnostics tips.
LIN Bus Information
The LIN bus consists of a single wire and is used to exchange information between one master control module (the ECM, BCM, etc.) and one or more slave/smart device(s) such as switches, sensors, motors, etc.
The LIN bus is relatively simple and it exchanges data at a slower rate than other GMLAN buses.
LIN buses are not wired to the DLC, thus the scan tool does not communicate directly on a LIN bus. The scan tool will communicate with the master controller of the LIN bus, which is wired to the DLC (High, Low, Chassis, etc), in order to command outputs or read parameters from the slave/smart device(s).
If serial data communication is lost between any of the LIN devices on the LIN bus network, the master control module will set a no communication code against the non-communicating LIN device. Each component on the LIN bus (master and any slave devices) will send out approximately 11-12 volts and then toggle that voltage low when communicating, as shown in the image on this page.
Knowing that each device sends out its own voltage can be very useful when performing diagnostics.
In some cases, this voltage is even present after the ignition is off and the vehicle has completely powered down, because many slave devices are only powered by a hot-at-all-times circuit.
Because the LIN bus voltage ranges between 12 and 1 volt(s), it is very important to have proper battery voltage while performing diagnostics. It is recommended to install a battery tender on the vehicle while diagnosing a LIN bus, or a false reading may be seen.
When a LIN bus is at rest/not communicating, it will read a steady 11-12 volts (approximately).
When monitoring a properly operating LIN bus with a multi-meter set to “peak” min/max (1 ms record), the voltage readings will be approximately 12 volts max and 1 volt min. If the meter is not set to the “peak” min/max setting, it will not capture the true min/max readings.
In most cases, when the ignition is turned on, the master controller will wake up the slave device(s) via the LIN bus. If the slave device does not wake up, the master controller will set a DTC for that device and will continue to try and wake up the slave device. This can be seen by the toggling voltage on the LIN bus.
In some cases the slave device will wake up the master controller, an example of this is the driver’s door-ajar switch. The door-ajar switch is hard-wired to the driver’s window motor. The driver’s window motor has a LIN bus to the BCM. When the driver’s door is opened, the ajar switch closes, thus signaling to the driver’s window motor to send a LIN message to the BCM. This wakes up the BCM. It communicates to the vehicle that the driver’s door is open and the BCM will turn on the dome lights and start waking up the other modules.
Diagnostic Tips
Here are some diagnostic tips for the LIN bus circuit and the LIN bus schematic can be used as an example in some of the tips.
1) Inspect for any DTCs.
2) If a LIN bus has more than one slave device, check to see if the other devices are working. Using the schematic, if the customer’s complaint was that the rear wiper was not operating, check to see if the rear power windows operate. This can help to start narrowing down the area of concern
3) The LIN bus is a single wire, and many faults are basic failures, such as opens, high resistance, shorts to ground/power, poor terminal drag or connectors not fully seated. Using the proper terminal test probes, inspect for these type of failures.
4) Testing/monitoring the voltage of the LIN bus can help determine what type of failure to inspect for.
5) Inspect for the output voltage from the master controller: with the master controller connected, disconnect the slave device and turn on the ignition. At the slave device, inspect for the toggling voltage from the master module on the LIN bus. Remember, if using a Fluke 87 meter set to “peak” min/max (1 ms record), the voltages will be approximately 12-volts max and 1-volt min. If the voltage is present but not toggling, it could indicate a short to power. If the voltage is not present, it could indicate an open or short to ground.
6) Certain types of shorts on a LIN bus circuit can cause the module to shut down and stop outputting the 11- to 12-volts until the fault is no longer present and the ignition/power is cycled, or DTCs are cleared.
7) Inspect for the output voltage from the slave device(s): With the slave device in question plugged in, disconnect the master controller and all other slave devices on the same LIN bus. At the master controller, inspect for the output voltage from the slave device, which will be a steady 11 or 12 volts (approximately). There are a few things to keep in mind when testing the output voltage from the slave devices. First, most slave devices have a hot at all times feed and no switched ignition inputs. In these cases, the ignition does not need to be turned on to test for the output voltage. Second, always check the wiring diagram for the slave device to determine if it has power at all times or if it has a switched ignition.
8) If it has a switched ignition feed, when the master controller is unplugged the slave device may not be powered on. In these cases, the slave device will need to have power applied to the switched ignition input before testing.
9) When testing the LIN bus output voltage from a slave device, which has a switched ignition, some devices will only have the steady 11-12 volts (approximately) output for a few seconds and then it will drop to 0 volts. This is because the testing is done with the master controller disconnected, and when the slave device does not establish communication from the master controller, it can shut down the LIN bus output voltage.
10) If there is more than one slave device, such as the example discussed, keep in mind that each slave device is sending out an output voltage. In these cases, the other slave devices will need to be disconnected along with the master controller before checking the output voltage from the slave device in question.
11) If no voltage is found coming from the slave device, it could indicate poor terminal tension at the LIN bus terminal, the slave device was not powered on (missing power or ground), slave device itself, etc.
12) If the voltage is present check to make sure the correct part number slave device is installed. In many cases, the slave device part numbers change from year to year but they look similar.
