GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 XLR 🢒 Power and Signal Distribution 🢒 Data Communications 🢒 Scan Tool Does Not Communicate with Class 2 Device

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Class 2 Serial Data

1

2¹

1

--

Ground (DLC, terminal 5)

--

1

--

--

¹ An open between the data link connector (DLC) and SP 205, or DLC pin 5 ground will only effect the communication with the scan tool. The vehicle modules will still communicate.

  1. No communication on any class 2 serial data module
  2. No communication on a specific class 2 serial data module

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. The class 2 serial data circuit uses SP 205 (splice pack comb) as a common connection between some modules and the DLC. DTCs may be set due to this symptom and during this diagnostic procedure.

Diagnostic Aids
    • Use the Data Link References to identify the class 2 serial data modules.
    •  This test is used for a total class 2 communication failure. If only 1 module is not communicating and sets no DTC, ensure that the vehicle is equipped with the module, then use DTC U1001-U1254 for diagnostics.
    • The body control module (BCM) detects that the ignition is ON and sends the appropriate power mode message to the other modules. Therefore, the BCM must be connected to the DLC for any other module to communicate with the scan tool.
    • An open in the class 2 serial data circuit between SP 205 and a module(s) will only effect the specific module(s). This type of failure will set a loss of communication DTC for each module effected, and the other modules will still communicate.
    • An open between the data link connector (DLC) and SP 205 will only effect the communication with the scan tool. The vehicle modules will still communicate, and the vehicle will start.
    • The engine may not start when there is a total malfunction of the class 2 serial data circuit.
        The following conditions may cause a total loss of low speed GMLAN data communication:
       - The low speed GMLAN serial data circuit shorted to ground or voltage.
       - A module internal malfunction that causes a short to voltage or ground on the low speed GMLAN circuit.
       - An open in the DLC ground circuit may cause a scan tool loss of communication, while the modules still communicate.
    • Use the DMM MIN/MAX function to capture/locate intermittent conditions.
    • When the class 2 serial data circuit is shorted to ground or to voltage, the following DTCs may set:
       - U1300
       - U1301
       - U1305

Reference Information

Schematic Reference

Data Communication Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Body Control System Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Refer to Control Module References for scan tool information.

Circuit/System Testing
  1. Test for less than 1.0 ohm of resistance between the DLC ground circuit terminal 5 and ground.
    2. ⇒If greater than 1.0 ohm, test the ground circuit for open/high resistance.
  2. Ignition OFF, disconnect the harness connector (splice pack comb) at JX 205 and JX208. Install a 3-amp fused jumper wire between JX  205 terminal A and terminal B harness side.
  3. Ignition ON, with a scan tool, attempt to communicate with the heads up display module (HUD). Communication should be available.
    4. ⇒If no communication, test the serial data circuit for a short to ground, a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.
  4. Install another 3-amp fused jumper wire to JX  205 terminal A, harness side.
  5. With HUD communications established, use the other end of the jumper wire to connect all other pins, harness side, one at a time and verify that communication remains available to the BCM.

    6. Important: Allow communications for 5 seconds after connecting a module with the jumper to determine if communication is interrupted or not.

    ⇒If communication is interrupted after connecting an individual module at JX 205, test the serial data circuit between JX 205 and the last module connected for a short to ground and short to voltage. If the circuit tests normal, replace the module that caused no communication.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

    •  Control Module References for module replacement, setup, and programming
    •  GMLAN Wiring Repairs