2008 GM Bus Network Protocol
Shown below is a 2008 GM Silverado truck bus network diagram for the DLC connector. When testing NO COMMUNICATION fault message from your scanner, please use this as your guide. With a voltmeter handy, you can prevent garages from charging you expensive rates to fix this problem.
2008 GM Truck Bus Network Description:
Shown above is the DLC data buss network of a 2008 GM Silverado Truck. The government has mandated to use CAN high speed data bus networking
starting this year on all vehicles manufactured and imported in North
America. Not only it is the fastest buss networking, it allows the use of
more modules in the vehicle without major cost. Updates on the vehicle
can be implemented by adding hardware to improve emission, safety and
convenience. For it's part, CAN high speed serial data protocol is called
GMLan bus serial data protocol by GM.
Again this diagram will show both GMLan low speed and high speed bus
serial data wired together. The low speed bus dot connections are shown
on all the low speed data bus modules on the left with Red color
connected to both splice pack 1 bar and splice pack 2 bar connectors.
Both splice bars are connected by way of the green dot connector on top
which originates from DLC terminal pin 1 shown as green wire.
The GMLan high speed serial bus data network comes from DLC terminal pin
6 and pin 14. Notice how each high speed bus wires flow from the DLC
terminals. Starting from the DLC pins, they enter/exit thru the junction
box on top right then thru each group of high speed serial data modules.
Each of this bus wires end in a "termination resistors" shown as box R on
bottom left. These resistors are used to give the network communication a
cleaner signal. One resistor is connected remotely while the second one
is inside the ECM module.
Note also the difference of GMLan Lo Speed and GMLan Hi Speed module
connections. In the Lo Speed, connection is called Linear because the
modules are interconnected by a common bus link (shown as splice bar). If
you disconnect the green dot at the top left S1, all DLC pin 1 bus data
communication is cut off. The Hi Speed modules are called RING connection
because the DLC pin 6 and 14 bus communication cycles to each of the
module. This means, you can disconnect one of the 7 high speed modules on
the right and bus communication continues with all the remaining 6 modules.
Finally, in this GMLan high speed modules, there is an extra 2 wires for
the BCM module on top right. The red bus wire is for serial data enable
circuit and blue bus wire is for WAKE UP serial data bus. These extra
wires are used for ranking or prioritizing the 7 hi speed modules
Troubleshooting 2008 GM truck bus network:
For Lo speed modules on the left, the same method is use for testing the bus wires namely hooking the voltmeter probes at DLC pin 1 and pin 5 at
KOEO. The modules are tested by disconnecting the blue dot connectors on
each of them, one at a time. Any DLC pin 1 voltage reading higher or
lower than 5 volts is the problem. Disconnecting the defective module
will tell you which voltage would return the normal 5 volt reading.
Testing the Hi speed modules need a different approach; you can either
test the voltage at DLC pin 6 against pin 5 and DLC pin 14 against pin 5
at KOEO. Any reading should fluctuate between 1.5 to 3.5 volts depending
which pin you are testing.
Or...you can use an ohmmeter and hook up the ohmmeter leads between DLC
pin 6 and pin 14. Do this with ignition key off or removed. Expected
reading is 60 ohms. Any reading higher or lower than this warrants
Let's say you want to test the TCM module high speed bus wires located
on the right side with BCM above it and ECM below it. Using the ohmmeter
hooked between DLC pin 6 and 14 terminals, disconnect the 2 green dot
connectors at TCM. You will read the resistance upstream (towards the
BCM) whereas if you disconnect the 2 blue dot connectors at TCM, you will
get the resistance towards the ECM. And if you disconnect the red dot
connector at TCM, you will get the resistance of the WAKE UP serial bus wire only.
You will get the exact color of the bus wires and location of the bus
circuit components if you have the actual bus wiring diagram. This
diagram is for demonstration only.
Here below are sample fixes for 2008-2012 GM bus no communication problems.
2008 Silverado No start after connecting defective trailer plug
2011 Avalanche no start stalls mil lamp on
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