Scan Tool Does Not Communicate With Class 2 Device

SCAN TOOL DOES NOT COMMUNICATE WITH CLASS 2 DEVICE

CIRCUIT DESCRIPTION
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. Diagnostic trouble codes (DTCs) may be set due to this symptom and during this diagnostic procedure. Complete the diagnostic procedure in order to ensure all the DTCs are diagnosed and cleared from memory.

DIAGNOSTIC AIDS
- The body control module (BCM) detects that the ignition is ON and sends the appropriate power mode message to the other modules. The BCM must be connected to the class 2 serial data circuit for many other modules to communicate with the scan tool.
- If the class 2 serial data circuit was shorted to ground, U1300 or U1305 will be set as history DTCs. Refer to 68817.
- If the class 2 serial data circuit was shorted to B+ U1301 or U1305 will be set as history DTCs. Refer to 68817.

TEST DESCRIPTION

Steps 1 - 11:

Steps 12 - 18:

Steps 19 - 26:

Steps 27 - 35:

The numbers below refer to the step numbers on the diagnostic table.
2. 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:
- Auxiliary power drop connector
- BCM
- Driver information center (DIC)
- Electronic brake control module (EBCM)
- Instrument panel cluster (IPC)
- Powertrain control module (PCM)
- Radio
- OnStar (UE1)
- Sensing and diagnostic module (SDM)
3. The following DTCs may be retrieved with a history status, but are not the cause of the present condition:
- U1300
- U1301
- U1305
6. A State of Health DTC with a history status may be present along with a U1000 or U1255 having a current status. This indicates that the malfunction occurred when the ignition was ON.
7. Data link connector terminals 2 and 5 provide the connection to the class 2 serial data circuit and the signal ground circuit respectively.
9. A poor connection at the DLC terminal of the star connector would cause this condition but will not set a DTC.
10. An open in the class 2 serial data circuit between the DLC and star connector will prevent the scan tool from communicating with the modules. This condition will not set a DTC.
12. If communication cannot be established the class 2 serial data circuit may be shorted to voltage or ground. The condition may be due to the wiring or due to a malfunction in the BCM. When testing wires for a short, make sure there are no modules connected to the wires being tested.
13. The BCM detects that the ignition is ON and sends the appropriate power mode message to the other modules. Therefore, the BCM must remain connected to the DLC for the other modules to communicate with the scan tool. This test isolates the PCM class 2 serial data circuit.
31. If there are no current DTCs that begin with a "U", the communication malfunction has been repaired.
32. The communication malfunction may have prevented diagnosis of the customer complaint.