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Home >> Cadillac >> 2006 >> STS V >> Repair and Diagnosis >> External Pages >> Different car >> Section 227 (Data Communication System) >> Diagnostic Information and Procedures >> Scan Tool Does Not Communicate with Low Speed GMLAN Device >> Circuit/System Testing
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Circuit/System Testing

WARNING: This page is about a different car, the 2007 Cadillac STS. However, it is still accessible from the selected car via links, so may be relevant.
  1. Test for less than 1.0 ohm of resistance between the DLC ground circuit terminal 5 and ground.
    • 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 SP 201 SP 300 SP 303.
  3. Ignition ON, with a scan tool, attempt to communicate with the instrument panel cluster (IPC). Communication should be available.
    • If no communication, disconnect the modules still connected to the DLC one at a time, starting with the furthest from the DLC and attempt to communicate after each disconnect. When communication occurs, test the serial data circuit for a short to ground and a short to voltage between the last module disconnected and the previous module disconnected. If the circuit tests normal, replace the last module disconnected. If no communication is possible, test the serial data circuit for a short to ground, a short to voltage or an open/high resistance between the last module disconnected and the DLC. If the circuit tests normal, replace the last module disconnected.
  4. Install a 3-amp fused jumper wire to pin 13 of SP 201, harness side.
  5. With IPC communications established, use the other end of the jumper wire to connect all other pins, harness side, one at a time and verify that low speed communication remains available to the IPC.
    IMPORTANT: Allow communications for 5 seconds after connecting a module with the jumper to determine if communication is interrupted or not.
    • If low speed communication is interrupted after connecting an individual module at SP 201, 300, or 303, test the serial data circuit between the splice pack 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.
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