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 Silverado truck bus network wiring

2008 bus wiring for GM

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 operation.

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 further investigation.

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.

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