Let's look into the mysterious world of body and chassis controllers and the way they communicate with each other. In this article we will look at the different types of serial data that GM uses right now and what may be coming in the near future and how to go about diagnosing these circuits and understanding why they are used in the first place.

Serial data is made up of high and low voltage pulses strung together, similar to an electronic Morse code. Each string of voltage pulses forms a message. The transmission of data occurs through a wire or "bus" for predetermined time intervals. Devices like modules (also referred to as nodes) connected to the bus can then share information by transmitting and receiving these messages.

Here is a list of data streams that GM uses today;

The data link connector (DLC) allows a scan tool to communicate with the class 2 serial data line. The serial data line is the means by which the microprocessor-controlled modules in the vehicle communicate with each other. Once the scan tool is connected to the class 2 serial data line through the DLC, the scan tool can be used to monitor each module for diagnostic purposes and to check for diagnostic trouble codes (DTCs). Class 2 serial data is transmitted on a single wire at an average of 10.4 Kbps (10,400 bits per second). This value is an average, class 2 uses a variable pulse width modulation to carry data and, depending on the message, it may operate faster or slower. The bus is active at 7.0 volts nominal and inactive at ground potential. When the ignition switch is in RUN, each module communicating on the class 2 serial data line sends a state of health (SOH) message every two seconds to ensure that the module is operating properly. When a module stops communicating on the class 2 serial data line, for example if the module loses power or ground, the SOH message it normally sends on the data line every two seconds disappears. Other modules on the class 2 serial data line, which expect to receive that SOH message, detect its absence; those modules in turn set an internal DTC associated with the loss of SOH of the non-communicating module. The DTC is unique to the module that is not communicating, for example, when the dash integration module (DIM) SOH message disappears, several modules set DTC U1064. Note that a loss of serial data DTC does not normally represent a failure of the module that set it.

The Controller Area Network (CAN) serial data line is high speed serial data bus used to communicate information between the Electronic Brake Control Module (EBCM), the Engine Control Module (ECM) and the Transmission Control Module (TCM). Typical data-transmission speeds must be high enough to ensure that the required real-time response is maintained.

The addressing scheme employed with CAN assigns a label to every message, with each message receiving a unique "identifier". The identifier classifies the content of the message (such as engine speed). Each module processes only those messages whose identifiers are stored in the module's acceptance list. This is CAN's form of message filtering.

The identifier labels both the data content and the priority of the message being sent. Each module can begin transmitting its most important data as soon as the bus is unoccupied. When more than one module starts to transmit simultaneously, the message with the highest priority is assigned first access. A module responds to failure to gain access to the bus by automatically switching to receive mode, that module then repeats the transmission attempt as soon as the bus is free again.
The Next Generation  of serial data may rest with CAN in something called a Local Area Network (LAN). The idea is to have a multiple speed bus of low, medium and fast. The low speed would be for items of driver input (Air Conditioning and Power options like windows and door locks). Medium speed for entertainment and comfort things like remote CD players and DVD players that are out now for 2002. High speed will be for real time items like desired torque and ABS. But this technology will be a while in coming.

The Keyword protocols utilize a single wire bi-directional data line between the modules and the scan tool. The message structure is a request and response arrangement that has some similarities to the UART system. Both Keyword 82 and Keyword 2000 are used for scan tool diagnostics only. The modules do not exchange data on these systems.

UART-based communication systems have a master of the serial data line and remote transceivers. The master controls the message traffic on the serial data line by polling all of the remotes (control modules). The master then waits for the proper response. The communication takes place over a single wire by pulling the voltage low to create a message. As a result, when communication is not occurring, the signal voltage on the data line remains high (usually 5 volts). The data line also has a fixed pulse width limiting its message capabilities.

SBI does not communicate with the scan tool via the DLC. SBI information is interpreted by the driver door module (DDM) and transmitted on the Class 2 data link by the DDM. In this example the DDM acts as a Gateway Module. That is the DDM has the capabilities to talk in two different "languages". The first language is that of the SBI serial data line and the second is the Class 2 serial data line. What happens is the Tech 2 is able to get door module information but it's getting it from the driver door module after it has been translated from the SBI language to Class 2 language. In other words the information is not coming directly from the modules on the SBI data bus.

The simple bus interface (SBI) is used for serial data communication between the door modules as well as the driver door switch (DDS).

The Serial Peripheral Interface (also known as the Serial Parallel Interface or SPI) is a three wired data transfer system. Usually found on some IPC circuits and Heads Up Displays it has two lines for data and a third line for a clock signal to time the rate of the data. On the Malibu and Cutlass the major data transfer between the BCM and the IPC takes place over the SPI data line as well. On these vehicles the IPC is not a Class 2 module but an SPI module instead. The BCM is the master of the bus and the IPC is a remote module. Data is sent and received by toggling a voltage to ground. The module grounding the signal is transmitting and the sending the voltage to be toggled is receiving. The BCM also toggles a voltage sent from the IPC to maintain a Clock line that keeps track of the highs and lows ( ones and zeros ) so data makes sense.

The DLC is a standardized 16 way connector located below the instrument panel and close to the steering column. Both the connector design and location are dictated by industry wide agreements. All OBD-II compliant DLCs are required to provide a power supply, B+ at all times, to Pin 16.This positive supply is used to power the scan tool. All DLCs supply a power ground at Pin 4 and a clean signal ground at Pin 5. The Class 2 serial data circuit is provided at pin 2 of the DLC.

Other devices that have there own serial data circuits are On Star and the Cadillac Navigation system. A problem with these serial data circuits will set U codes. If you have a problem of communications between the VCU and the VIU in a 3 button On Star system a U1500 may set. I f a Cadillac has a communication problem with the Navigation Control Head and the Navigation Control Module a U2150, U2151 or U2153. In these systems knowing how the serial data systems talk is your best defense for getting to the root of the problem.

Power Moding

Another innovation that has taken place is Power Moding. To learn more about it check out the  Power Moding Article

We can't have a discussion about diagnosing serial data without having to bring up those nasty U codes. When we break down how they work we'll get a better understanding of the whole picture. First of all the "U" indicates that the code is a communications error. Next the "1" says it's a GM code (manufacturer specific) and the next three digits tell us what module is giving us fits. You see GM assigns a number (an I.D.) to each module. For example if you get a DTC U1040, the 040 is the ID number for the EBTCM. If it were the PCM that clammed up you'd see a U1016, the 016 is the PCM ID number. These numbers are the same whether you are working on a Cavalier or Deville. If you wanted to get a list of the ID numbers that pertain to the vehicle you are working on in SI 2000 got -Body and Accessories-Data Link Communications-Diagnostic Procedures. Under the Diagnostic Procedures heading you'll see a group of DTCs from U1001 to U1254. The diagnostic charts for these codes are the same because every module on Class 2 needs the same three things to talk. Power, ground and a serial data line. So when you get into a diagnostic routine where one of these codes is set get a schematic and a connector end view of the offending module and make your checks with a DMM. The charts will have you check for power first then ground and then the data line. If all three are present then the module is ready for the bone yard! A lot easier than some of the DTCs involved with other automotive systems. Just remember the BIG THREE!

But by now you may be saying "Hold on there sport, what about the codes U1000 or U1255?" (generic loss of communication). Easier than doin' a 30K service. What these codes mean is that there is a problem with a module in the Class 2 data line but we're not sure which one. So with the Tech 2 go to the Diagnostic Circuit Check and then Message Monitor to see a list of active modules, if they all show as active take a closer look- Time-out! One thing to remember about Class 2 is that if a module is down before you turn on the ignition, it may not show up on the list of modules in "Message Monitor". You'll take a look at the list, see all modules as active and think all is well but make sure all the modules have reported for duty. How do you know what modules should be on the list? Simple, the list is under "Description and Operation" in "Data Link Communications" or on some newer vehicles the list is actually put on the same page with the U1000 or U1255 trouble charts. O.K. Time back in! Armed with your list, take it over to the vehicle and compare. If you find a module that is supposed to be there but has decided to sleep in, what do we do next? C'mon, clocks ticking, you guessed it the BIG THREE! Get the schematics or connector end view, find the module and well you know the rest.

But you may now be asking "What happens if I get a short to power or ground on that Class 2 line?" We mentioned earlier that the serial data line for Class 2 is terminal 2 on the DLC, get out the trusty DMM set it to voltage and see where your at. If there is a steady showing of close to B+ the problem is a short to voltage. If the DMM shows 0 volts on the DMM a short to ground may be the culprit. If so set DMM to ohms and check for continuity to ground. Now what do I do. I know!!! If the modules are on a star pattern, go to the star connector hub and check each module line from there. If it's a loop pattern, unplug each module one at a time until the problem goes away. If the problem is still there, get a schematic and check your Class 2 wiring one section at a time to isolate the cause.