Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. 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:
   • BCM
   • Electronic brake control module (EBCM)
   • Heads up display (HUD)
   • Instrument panel cluster (IPC)
   • PASS-Key® lll
   • Powertrain control module (PCM)
   • Radio
   • SDM
   • Devices installed at the Auxiliary Power Drop Connector
2. 3.  The following DTCs may be retrieved with a history status, but are not the cause of the present condition.
   • U1300
   • U1301
   • U1305
3. 6.  A state of health DTC with a history status may be present along with a DTC U1000 or U1255 code having a current status. This indicates that the malfunction occurred when the ignition was ON.
4. 7.  Data link connector terminals 2 and 5 provide the connection to the class 2 serial data circuit and the signal ground circuit respectively.
5. 9.  A poor connection at terminal A would cause this condition but will not set a DTC.
6. 10.  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.
7. 11.  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 BCM class 2 serial data circuit.
8. 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 any other module to communicate with the scan tool. This test isolates the PCM class 2 serial data circuit.
9. 15.  This test isolates the radio class 2 serial data circuit.
10. 17.  This test isolates the EBCM class 2 serial data circuit.
11. 19.  This test isolates the SDM class 2 serial data circuit.
12. 21.  This test isolates the IPC class 2 serial data circuit.
13. 23.  This test isolates the PASS-Key® III class 2 serial data circuit.
14. 25.  This test isolates the auxiliary power drop connector class 2 serial data circuit.
15. 27.  This test isolates the HUD class 2 serial data circuit.
16. 31.  If there are no current DTCs that begin with a "U", the communication malfunction has been repaired.
17. 32.  The communication malfunction may have prevented diagnosis of the customer complaint.

Fig 1: Scan Tool Does Not Communicate with Class 2 Device (1 Of 5)

Courtesy of GENERAL MOTORS CORP.

Fig 2: Scan Tool Does Not Communicate with Class 2 Device (2 Of 5)

Courtesy of GENERAL MOTORS CORP.

Fig 3: Scan Tool Does Not Communicate with Class 2 Device (3 Of 5)

Courtesy of GENERAL MOTORS CORP.

Fig 4: Scan Tool Does Not Communicate with Class 2 Device (4 Of 5)

Courtesy of GENERAL MOTORS CORP.

Fig 5: Scan Tool Does Not Communicate with Class 2 Device (5 Of 5)

Courtesy of GENERAL MOTORS CORP.