To keep pace with increasingly sophisticated safety, fuel economy, emissions, vehicle handling, comfort and convenience, and driver information systems, the number of electronically controlled circuits used in GM vehicles grows every model year. In order for a current GM vehicle to start, run and operate as designed, it relies on clear communications among an ever-increasing number of electronic control modules. The methods these electronic controllers use to communicate among themselves (and the way we diagnose them) have also evolved. This video covers (at a high level) the tests commonly found under Circuit System Verification and Circuit System Testing in many Service Information data communication diagnostic procedures. It will explain the purpose of each test, demonstrate how the test is performed, and describe the possible outcomes. Since data communication technologies are constantly evolving and diagnostic procedures change frequently, when diagnosing a vehicle concern, always: refer to Service Information (or SI) for the vehicle you are working on, use Strategy Based Diagnosis to guide the diagnostic process, and perform tests in the order stated in GM diagnostic procedures. The initial steps in a typical GMLAN diagnostic procedure include verifying the customer s concern, searching for applicable bulletins, inspecting for mechanical concerns, and verifying that the vehicle powers up. With those checks complete, connect the scan tool and view the Vehicle DTC Display on GDS2. Look for modules identified as having No Communication. These are modules that do not communicate with the scan tool. A module may be identified as having No Communication either because it has experienced a fault that prevents communication or because the vehicle is not equipped with the option the module supports. Determine if the vehicle is equipped with each module identified as having No Communication by either checking the vehicle's RPO codes or visually checking for the option. For example, if GDS2 reports that the Park Brake Control Module fails to communicate the presence of park brake pedal indicates that the vehicle is not equipped with this feature. For modules supporting features that are not immediately obvious, such as an Active Safety Control Module, refer to RPO information.
According to the schematic shown, vehicles with RPO UGN will be equipped with an Active Safety Control Module. If UGN isn t listed in a vehicle s RPO codes, then it is normal for GDS2 to show this module as not communicating. Once you have determined which modules the vehicle is equipped with, determine the number of modules not communicating. If a single module fails to communicate, diagnostics will focus on that specific module. Tests include checking for an open or high resistance in a battery positive, ignition, or ground circuit, and checking the network connection to that module. If all these circuits are ok, a fault inside the module is the likely cause of the communication concern. If at least two modules fail to communicate, further testing is needed to narrow the scope of your diagnosis. Begin by verifying that there is good ground to the Data Link Connector or DLC. There are two ground terminals at the DLC. The ground at terminal 4, along with battery positive voltage at terminal 16, provide ground and power to the scan tool. A fault with the ground circuit to terminal 4 can prevent the scan tool from powering up. The scan tool receives data by reading the difference in voltage between the ground at terminal 5 and each network terminal. If there is a fault with the ground circuit to terminal 5, the scan tool may power up but may not be able to communicate with modules on the vehicle. Use a DMM to check for less than 10 Ohms resistance between terminal 5 of the DLC and ground. If resistance is greater than 10 Ohms, repair the cause of the open or high resistance in the circuit connecting terminal 5. If resistance is less than 10 Ohms, the next step is to check the network circuits for higher than normal voltage. During normal operation, network voltage toggles between a low state and a high state. For example, the Class 2 network normally toggles between a low state of zero volts and a high state of about 7 volts. Low Speed GMLAN normally toggles between a low state of zero volts and a high state of about 5 volts. The wires in dual-wire networks at the DLC are designated as either bus positive or bus negative. The bus positive wire carries a voltage that toggles from about 2.5 volts to about 3.5 volts. The bus negative wire carries a voltage that toggles from about 2.5 volts to about 1.5 volts. Network voltage changes very quickly. Even networks designated as low speed carry voltage signals switching tens of thousands of times a second. When measuring network voltage, a DMM displays an average reading that should fall between the high and low state.
For example, with the ignition ON measuring the voltage between ground and the High Speed GMLAN Bus Positive circuit, the DMM should display a reading that fluctuates around 3 volts. A higher than normal voltage indicates the network circuit is shorted to voltage. Diagnostic procedures specify that if voltage is greater than 4.5 volts, the next steps are to disconnect network connections and test individual sections of network wiring to isolate the cause of the short to voltage. A voltage that is too low gives you a clue that the network circuit may have high resistance, an open, or may be shorted to ground. Data communication diagnostic procedures will advise testing for those conditions next To test for a short to ground, turn the ignition OFF and wait two minutes for vehicle systems to power down and network activity to stop. If you measure resistance while the network is active, the network voltage signals will make resistance readings inaccurate. To be certain that network activity has stopped, check the voltage between ground and a network terminal. If voltage is zero, then you can be assured there is no network activity and resistance measurements will be accurate. Several conditions may keep the network active. If network activity persists, disconnecting the negative battery terminal will ensure that resistance measurements will not be disrupted by network voltage. Measure the resistance between ground and each network terminal. Normal resistance is typically very high. Diagnostic procedures specify that if resistance is less than 100 Ohms, the wiring should be checked for a short to ground by disconnecting network connections and testing individual sections of network wiring to find the source of the low resistance. Testing dual-wire GMLAN networks for an open is different from testing single wire networks such as Class 2 or Low Speed GMLAN for an open. Dual-wire networks incorporate terminating resistors at each end of the network, giving a total resistance of about 60 Ohms as measured between the two network terminals at the DLC. The network resistance can give you a clue as to the type of network fault exists: A resistance of less than 35 ohms can indicate that there is a short between the two network circuits. A resistance of between 35 and 50 Ohms may indicate a short between network circuits, but it may also indicate that the network has an extra terminating resistor. An extra terminating resistor can be added accidentally if an incorrect module was installed. Some modules are available with and without the terminating resistors installed to reduce the need for terminating resistors in the wiring harness. Study the vehicle service history for evidence that of a recent module replacements.
A resistance of 120 Ohms indicates that an open exists in one or both network wires, such as if a module were disconnected from the network. Single wire networks can incorporate splice packs which provide a way to separate the individual branches of a network to test for circuit faults. In schematics, the letters JX are used to identify a splice pack. Removing the cover from a splice pack separates the branches of the network. The individual branches can then be tested for a short to power or ground. The module connected by each branch can be disconnected and the branch tested end-to-end for high resistance or an open. Some data communication diagnostic procedures support using the Data Bus Diagnostic Tool during the Circuit System Verification Procedure to quickly identify a fault with a network for further Diagnosis using a DMM. To use the Data Bus Diagnostic Tool, first determine which modules the vehicle is equipped with using network schematics and the vehicle s RPO information. Start the tool. Connect the MDI you are using, if necessary. Select the DLC terminals of the network you need to test. For example, select DLC Terminals 6 and 14 to test the High Speed GMLAN. In some instances, you may be prompted to select the Baud Rate or network speed for a network. Network speed can be found in the Data Link Communications Description and Operation document in SI and is sometimes expressed in Kilobits per second. In some instances, selecting the incorrect Baud Rate will prevent the Data Bus Diagnostic Tool from working and may set DTCs in vehicle modules. After clicking the check mark symbol, the Detected State displays the condition of the network. In this example, High Speed GMLAN is selected. The Detected State displays Control Module Ground Connection Malfunction. This fault prevents a module from communicating. All of the modules capable of responding are listed below. Compare these modules to the network schematic to identify which module fails to communicate. The next step would be to check the ground circuit connecting the module that does not communicate. Additional fault conditions include: Control Module Not Awake CAN Bus Circuit Shorted to High Voltage CAN Bus Circuit Shorted to Low Voltage CAN Bus [-] Shorted to CAN Bus [+] and CAN Bus Open Diagnosing a single wire network (such as Low Speed GMLAN) with the Data Bus Diagnostic Tool is different from diagnosing dual-wire networks. The Data Bus Diagnostic Tool cannot distinguish a fault
condition from normal operation on a single wire network, so when a single wire network is selected, the Detected State will read Not Available. Note that the Body Control Module will stop responding intermittently. This is normal behavior for this module. When diagnosing single wire networks, study the list of responding modules to determine the type of fault affecting the network. If no modules respond, diagnostics will direct you to test for a short to ground, a short to voltage, or an open in the branch of Low Speed GMLAN that connects the DLC to the nearest splice pack. If one or more Low Speed GMLAN modules do not respond diagnostics will direct you to test for an open between the splice pack and the module that is not responding. Diagnostic procedures have changed over time for the Media Oriented Systems Transport (or MOST) network. Always refer to Service Information for the latest diagnostic procedures. DTC U0028 setting in the radio as Current indicates that a fault exists with the Media Oriented Systems Transport or MOST network. MOST diagnostics begins by verifying the operation of the media disc player by inserting and ejecting a disc. If the media disc player is able to load and eject a disc, it is less likely to be the cause of the break. If the media disc player ejects a disc or if the vehicle is not equipped with a media disc player, the next step is to check if any modules that fail to communicate with the scan tool. Several modules connected to the MOST network are also connected to other networks, such as High Speed GMLAN or Low Speed GMLAN. A fault with a module may affect how it communicates over both networks. If this is the case, it can be quicker to diagnose the concern as a loss of communication with a High Speed GMLAN or Low Speed GMLAN module. In this case, all modules are communicating, so the next step is to check for any DTCs set against the MOST control line. The MOST control line is also referred to as the MOST Communication Enable Circuit, or the Electronic Control Line (or ECL.) The MOST control line carries a signal that activates modules connected to the MOST network. A fault with the MOST control line may cause one or more modules to remain asleep. If there are no DTCs for the MOST control line, the next step is to identify where in the MOST network the fault is located. This is done using a function of GDS2 called the MOST Bus Diagnostic Starting Point. Navigate to the MOST Bus Diagnostic Starting Point by selecting Module Diagnostics, Radio, then Control Functions. Selecting MOST Bus Diagnostic Staring Point brings you to a new screen that displays several parameters including one called Node Locations of MOST Bus Communication Break. This parameter displays two
numbers that match the Node numbers of the modules listed below. In the example shown here, we see that Node 3 is the Instrument Cluster and Node 4 is the Human Machine Interface or HMI Module. Diagnostics will focus on these modules and the network wiring between them. The next steps use the test connectors in the MOST Bus Diagnostic Tool Kit. Connect the appropriate test connector in place of the first module identified in the Node Locations of MOST Bus Communication Break parameter. In this example, the instrument cluster is disconnected and the test connector installed. Next, turn the ignition On and perform a quick check to ensure that MOST control line voltage is between 9 and 13 Volts. A voltage higher or lower indicates a fault with the MOST control line. Next, view the status of DTC U0028. If the Status changes from Current to History, focus your diagnosis on the Instrument Cluster; testing its power, ground, and MOST network connections. If the Status of DTC U0028 remains Current turn the ignition Off, remove the test connector, and reconnect the instrument cluster. Move on to the next module identified in the Node Locations of MOST Bus Communication Break parameter which, in this example is the HMI module. Disconnect the HMI module and connect the appropriate test connector from the MOST Bus Diagnostic Tool Kit. Turn the ignition on and check the status of the DTC U0028 again. If the Status changes from Current to History, focus your diagnosis on the HMI Module; testing its power, ground, and MOST network connections. If the Status of DTC U0028 remains Current there is likely a fault in the MOST network circuits connecting the Instrument Cluster to the HMI module. The MOST signal is a very sensitive signal. Very good electrical connections and circuit integrity are needed to carry the MOST signal from one module to another. With the ignition Off, remove the test connector and carefully inspect the MOST network circuit terminals for good terminal tension using the appropriate test probes from the Terminal Test Kit. If the MOST terminals have appropriate tension, test each MOST network wire for a short to voltage, a short to ground, or high resistance. In this video, we covered at a high level the tests commonly found under Circuit System Verification and Circuit System Testing in many data communication diagnostic procedures. Remember: Diagnostic procedures change frequently and more information is needed to diagnose network concerns for a specific vehicle than what is covered in this video. When diagnosing a vehicle concern, always refer to Service Information for the vehicle you are working on, use Strategy Based Diagnosis to guide the diagnostic process, and always perform tests in the order stated in GM diagnostic procedures.
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