Bulletin No.: PIT5698A
Published date: 07/23/2020

Preliminary Information
PIT5698A LIN Bus Diagnostic Information
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2014 - 2021

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Involved Region or Country North America

Condition

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
raised regarding diagnosis of a LIN bus circuit. This document is intended to help
better understand LIN buses and provide some diagnostics tips.
Note: As always, follow normal SI diagnostics.

Correction:

LIN Bus Information
- The LIN bus consists of a single wire and is used to exchange information between one master control module
(example 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 below. 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 o 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 it is very important we have proper battery voltage
while performing diagnostics. It is recommended to install the GR8 or equivalent battery tender on the vehicle
while diagnosing a LIN bus, or false reading may be read.
- 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 the Fluke 87 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 and 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 Tip Information
Below are some diagnostic tips for the LIN bus circuit, in addition, the LIN bus schematic seen below will be used
as an example in some of the tips. The following is not a ow chart, and as always, follow normal SI diagnostics.
- It is recommended to install the GR8 or equivalent battery tender on the vehicle while diagnosing a LIN bus or
false readings may be obtained. Unlike other GMLAN buses which work at much lower voltages (example 0 to 5
or 1.5/2.5/3.5 volts) the LIN bus operates in a range from 1 to 12 volts, and having proper battery voltage is critical
to get proper readings.
- Inspect for any DTC's.
- If a LIN bus has more than one slave device, check to see if the other devices are working. Using the example
below, 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.
- 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, connectors not fully seated. Using the proper terminal test probes inspect for
these type of failures.
- Testing/monitoring the voltage of the LIN bus can help determine what type of failure to inspect for.
- 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.
Note: Certain types of shorts on a LIN bus circuit can cause the module to shut down and stop outputting the 1112 volts until the fault is no longer present and the ignition/power is cycled or DTC's are cleared.
- 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 device(s) on the same LIN bus. At the master controller, inspect for the
output voltage from the slave device, which will be a steady 11-12 volts (approximately). There are a few things to
keep in mind when testing the output voltage from the slave devices:

1. 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. 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. 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.
2. 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.
3. If there is more than one slave device, such as the example below, keep in mind that each slave device is
sending out an output voltage. In these cases, the other slave device(s) will need to be disconnected along with
the master controller before checking the output voltage from the slave device in question.
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.
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.

Additional SI Keywords
U1343 U1345 U1346 U1347 U1348 U1349 U135D U135E U1501 U1502 U1505 U1509 U150E U150F U1510 U1511
U1512 U1513 U1514 U1515 U1516 U1517 U1518 U1519 U151A U151B U151C U151F U1520 U1521 U1522 U1523
U1524 U1525 U1526 U1528 U152A U152B U152C U152D U1531 U1530 U1532 U1534 U1538 U153A U1540 U1548
U1549 U154A U154B U1550 U1551 U1555 U1556 U1558 U1559 U155D U156D U15E1 U15F0 U15F1 U15F3 U2010
U2011 U2012 U2013 U2022 U2023 U250D

Version History
Version
Modi ed

2
07/23/2019 Created on
07/23/2020 updated to add the 2021 models

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