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Feature   August 1, 2007   by Mike Duguay, Injectronics Training

Diagnosing General Motors Communications Network Problems

In the June issue of SSGM Magazine, we took at close look at the procedures needed to effectively diagnose and isolate the cause(s) of a network systems failure in Ford motor vehicles. Building on tha...


In the June issue of SSGM Magazine, we took at close look at the procedures needed to effectively diagnose and isolate the cause(s) of a network systems failure in Ford motor vehicles. Building on that experience, let’s move on to the network systems of General Motors vehicles and how we can use the Diagnostic Line Connector (DLC) to find the cause of a communications problem and then to go about fixing that problem.

For years now, GM has been using the following communication systems in their vehicles. While not all are used in one particular model, these communications systems are ones any technicians working on GM vehicles will likely encounter pretty frequently. The communications systems are:

Entertainment and Comfort Bus (E&C)

* Keyword 82 Keyword 2000

* Simple Bus Interface (SBI)

* Serial Peripheral Interface (SPI)

* Universal Asynchronous Receive And Transmit (UART)

* Class 2

Although each network system listed above has its own unique operating features, this article will be focusing on the Class 2 & U.A.R.T. (Universal Asynchronous Receive and Transmit) systems. The U.A.R.T network system uses a master control module which communicates data to other modules. These other modules, then respond by sending the proper message back to the master control module. The important thing to remember about this system is that the voltage is normally high (Five volts) when there is no communication; the voltage is toggled low, when the module(s) communicate; the U.A.R.T. communicates (data bits ) at a fixed rate and the network is located at Pin # 9 at the DLC; and this is a single — wire network system.

To diagnose this network, you will need a lab scope, and your scanner. The lab scope settings I recommend are: Voltage setting = 5V and Time-base setting = 1ms (recommended). Now, hook up the lab scope to Pin # 9 into the back of the DLC, and with the key in the OFF position, you should see Zero Volts. Now turn ON the ignition switch, and you should now have Five Volts on the network. Plug in the scanner, and enter, let’s say, the airbag diagnostic menu and check for codes. You should see the voltage pattern shown below in Diagram C.

As mentioned earlier, U.A.R.T. is the acronym for Universal Asynchronous Receive and Transmit. What exactly does this mean?

By definition, the word Asynchronous carries the idea of a signal that occurs ONLY when a module receives or transmits data. Thus, it is quite possible to be monitoring the network, and seeing the voltage remain steady at five Volts (ie: no communication) for a period of time. If this should occur, simply activate the scanner and request information from a module that uses U.A.R.T. communication.

Let’s move onto the CLASS 2 Serial Data network. Some GM vehicles started using Class 2 Serial Data network system in 1995, which was also the phase-in year for ODB II. Since then, Class 2 is becoming GM’s predominant network system. Modules on this network system share information and each one can transmit and receive messages.

The following summarizes Class 2 Serial Data: The network uses a zero to Seven Volt signal; the voltage rests at Zero volts when there is NO communication; the transmitted data is a variable pulse width signal; Class 2 is found at Pin # 2 of the DLC, and is a single-wire network system; and GM uses what is referred to as a “splice-pack” to connect modules and the DLC to each other for network communication.

The Class 2 bus circuit number and the wire color for this system, each vary, according to the individual module(s). To diagnose this network system, you will require the following: A lab scope and scanner; a high-impedance DVOM; a component locator; a vehicle-specific wiring diagram, which is very important to have; and most importantly, a healthy dose of patience.

To confirm if the network system is operating as it should, hook up the positive lead (Channel # 1) of the lab scope to Pin # 2 at the DLC and ground the lead connected to either Pin # 4 or to the chassis. Next set the voltage setting to Five Volts and the time base setting to 100 ms (recommended). With the ignition switch OFF, there should be NO data being transmitted (NO voltage). Now, turn on the ignition switch and you should see the following signal, as shown in Diagram D.

If the signal you see is not as shown above, you can adjust the time – base setting of the lab scope, to your personal preference.

As with other network systems, this one has some interesting aspects to it which we should spend a little time on. For example, let’s say that a late-model GM pick-up is towed to your shop, with a “no-start” problem. You hook up the scanner, but cannot communicate with the PCM or TBC (Truck Body Computer). Now what?

Using the wiring diagram, locate the ‘splice-pack’ connector. This will vary according to vehicle model and year and thus the need for a component locator as well. Once you have located it, unplug the cap assembly from the connector and try starting the vehicle. If it now runs, you have narrowed down the cause. It is a network system failure. How do you know this? Because removing the cap assembly isolates the PCM and the TBC from the network. The most probable cause of this problem may be one of the following (or a combination of them):

1. Class 2 is shorted to ‘power’

2. Class 2 is shorted to ‘ground’

3. A module is shorted internally and has directly affected the network.

You can now proceed with the necessary diagnostics, to identify the source of the problem(s). As a side point, I spoke with a technician who was trying to source the reason for a 12 Volt signal on the Class 2 network. The vehicle was towed to the shop, as a no-start. The vehicle was a 2002 GM pick-up. In this case, the diagnostic splice-pack was located behind the radio. After he removed the radio, and unplugged it from the harness, he noticed that the 12 Volts now disappeared and the truck started. It turns out that a dime had lodged itself inside the CD player cavity and had caused a 12 Volt feed to transfer onto the Class 2 network. The dime was promptly removed and returned to the customer, along with the diagnostic repair bill.

Happy Motoring!