Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. Connecting a scan tool to the data link connector (DLC) allows communication with the modules for diagnostic purposes.
The engine will not start when there is a malfunction of the class 2 serial data circuit between the vehicle theft deterrent (VTD) system and the powertrain control module (PCM) while the engine is not running. The following conditions will cause a total loss of class 2 serial data communication:
• | A class 2 serial data circuit shorted to ground. |
• | A class 2 serial data circuit shorted to voltage. |
• | An open in the class 2 serial data circuit between the VTD system and the PCM. |
• | An internal malfunction of a module on the class 2 serial data circuit that causes a short to voltage or ground. |
On some vehicles, if the PCM is unable to communicate with the VTD system after the vehicle has started, the PCM will consider the VTD system to be malfunctioning. The PCM will enter a fail enable state and will command the security indicator to illuminate. When the PCM is in a fail enable state the vehicle will NOT stall or stop running. If the PCM is in a fail enable state when the ignition is switched OFF, the PCM will remain fail enable until communications with the VTD system has been restored. When the PCM is in a fail enable state the VTD system is NOT active and the vehicle will start. This feature is NOT available on all GM vehicle lines.
The numbers below refer to the step numbers on the diagnostic table.
A partial malfunction in the class 2 serial data circuit uses a different procedure from a total malfunction of the class 2 serial data circuit. The following modules communicate on the class 2 serial data circuit:
• | Chime Module |
• | electronic brake control module (EBCM) |
• | instrument panel cluster (IPC) |
• | PCM |
• | Radio |
• | serial data gateway module (SDGM) |
• | inflatable restraint sensing and diagnostic module (SDM) |
• | transmission control module (TCM) |
The following DTCs may be retrieved with a history status, but are not the cause of the present condition.
• | U1300 |
• | U1301 |
• | U1305 |
A state of health DTC with a history status may be present along with a U1000 or U1255 code having a current status. This indicates that the malfunction occurred when the ignition was ON.
Data link connector terminals 2 and 5 provide the connection to the class 2 serial data circuit and the signal ground circuit respectively.
A poor connection at star connector terminal A would cause this condition but will not set a DTC.
An open in the class 2 serial data circuit between the DLC and star connector will prevent the scan tool from communicating with any module. This condition will not set a DTC.
The class 2 serial data circuit is shorted to voltage or ground. The condition may be due to the wiring or due to a malfunction in one of the modules. When testing the wire for a short, make sure there is not a module connected to the wire being tested. This test isolates the IPC class 2 serial data circuit.
The IPC detects that the ignition is ON and sends the appropriate power mode message to the other modules. Therefore, the IPC must remain connected to the DLC for any other module to communicate with the scan tool. This test isolates the star connector serial data circuits.
If there are no current DTCs that begin with a "U", the communication malfunction has been repaired.
The communication malfunction may have prevented diagnosis of the customer complaint.
Step | Action | Yes | No |
---|---|---|---|
1 | Does the scan tool power up? | Go to Step 2 | |
Does the scan tool communicate with any module on the class 2 serial data circuit? | Go to Step 3 | Go to Step 7 | |
Did you record any DTCs in the range of U1000 to U1305? | Go to Step 4 | Go to Diagnostic Aids | |
4 | Are the DTCs U1300, U1301 or U1305 retrieved from any module? | Go to Step 5 | Go to Step 6 |
5 |
Important:: Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Use the DMM MIN/MAX function to capture intermittent conditions. Test the class 2 serial data circuit for an intermittent short to ground or an intermittent short to voltage. Refer to the following in Wiring Systems: Did you find and correct the condition? | Go to Step 17 | Go to Step 6 |
Is U1000 or U1255 the only DTC displayed in the previously specified range? | Go to DTC U1000 and U1255 | Go to DTC U1001-U1254 | |
Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | Go to Step 8 | |
8 | Test the DLC ground circuit at terminal 5 for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | Go to Step 9 |
Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | Go to Step 10 | |
Test the class 2 serial data circuit between the DLC and the star connector, terminal A for an open or a short. Turn ON the ignition leaving the engine OFF when testing for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | Go to Step 11 | |
Does the scan tool communicate with the IPC? | Go to Step 13 | Go to Step 12 | |
12 |
Did you find and correct the condition? | Go to Step 17 | Go to Step 15 |
Repeat the following steps on the class 2 serial data circuits connected to the star connector until the concern is located:
Does the scan tool communicate with the IPC? | Go to Step 17 | Go to Step 14 | |
14 |
Did you find and correct the condition? | Go to Step 17 | Go to Step 15 |
15 | Inspect for poor connections at the class 2 serial data circuit of the last disconnected module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 17 | Go to Step 16 |
16 | Replace the last disconnected module. Refer to Control Module References for the appropriate repair instructions. Did you complete the replacement? | Go to Step 17 | -- |
The scan tool may require a power up reset before communication will occur due to a short on the class 2 serial data circuit. Turn OFF or disconnect the scan tool prior to performing this test. Did you record any DTCs which begin with a "U" and with a current status? | Go to Step 19 | Go to Step 18 | |
Did you record any DTCs which do not begin with a "U"? | Go to Step 19 | Go to Step 21 | |
19 | Diagnose the DTCs as directed by the diagnostic procedures for the particular module or malfunction. Refer to Control Module References for the applicable Diagnostic System Check. Did you complete the action? | Go to Step 20 | -- |
20 | Did you diagnose all of the DTCs? | Go to Step 21 | Go to Step 19 |
21 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | -- |