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Circuit/System Testing

NOTE:

Each device may need to be disconnected to isolate a circuit fault. Use the schematics and connector end views to identify the following:

  • All low speed GMLAN devices the vehicle is equipped with
  • Low speed GMLAN device and splice (or splice pack) locations on the low speed GMLAN serial data circuit
  • The low speed GMLAN serial data circuit terminals for each device or splice pack

Serial Data Starting Point

  1. Ignition/Vehicle OFF, disconnect the harness connectors at the K56 Serial Data Gateway Module.
  2. Ignition ON/Vehicle In Service Mode.
  3. Test for less than 4.5 V between each of the low speed GMLAN serial data circuit terminals (circuit number 5060) and ground.
    • If 4.5 V or greater 

      Refer to Testing the Serial Data Circuits for a Short to Voltage.

    • Go to next step: If less than 4.5 V 
  4. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
  5. Test for greater than 100 Ω between each of the low speed GMLAN serial data circuit terminals and ground.
    • If 100 Ω or less 

      Refer to Testing the Serial Data Circuits for a Short to Ground.

    • Go to next step: If greater than 100 Ω 
  6. Refer to Testing the Serial Data Circuits for an Open/High Resistance.

Testing the Serial Data Circuits for a Short to Voltage

  1. Ignition/Vehicle OFF, cut the appropriate harness at a low speed GMLAN serial data splice closest to the K56 Serial Data Gateway Module, at least 40 mm (1.57 in) from the splice. Do not cut the wires right at the splice because these wires need to be repaired after the test is done.
  2. Ignition ON/Vehicle In Service Mode.
  3. Test for less than 4.5 V between each of the low speed GMLAN serial data circuits at the splice and ground.
    • If any serial data circuit is 4.5 V or greater 
    1. Ignition/Vehicle OFF, repair all serial data circuits at the tested splice except the faulted serial data circuit, ignition ON/Vehicle In Service Mode.
    2. Verify the low speed GMLAN devices that are still not communicating to identify the faulted serial data circuit.
    3. Ignition/Vehicle OFF, disconnect all devices on the faulted serial data circuit, ignition ON/Vehicle In Service Mode.
    4. Test for less than 1 V between each section of the faulted serial data circuit and ground.
      • If 1 V or greater, repair the short to voltage in the serial data circuit.
      • Go to next step: If less than 1 V
    5. Ignition/Vehicle OFF, repair the faulted serial data circuit to the splice and connect the first device on the faulted serial data circuit, ignition ON/Vehicle In Service Mode.
    6. Verify the scan tool communicates or not with the low speed GMLAN serial data circuit.
      • If the scan tool does not communicate, replace the device that was just connected.
      • If the scan tool communicates and there are more devices to connect, connect the next device and repeat step 3.6.
      • Go to next step: If the scan tool communicates and there are no more devices to connect
    7. All OK.
    • Go to next step: If all serial data circuits are less than 4.5 V 
  4. Inspect all low speed GMLAN splices for damage.

Testing the Serial Data Circuits for a Short to Ground

  1. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down. Cut the appropriate harness at a low speed GMLAN serial data splice closest to the K56 Serial Data Gateway Module, at least 40 mm (1.57 in) from the splice. Do not cut the wires right at the splice because these wires need to be repaired after the test is done.
  2. Test for greater than 100 Ω between each of the low speed GMLAN serial data circuits at the splice and ground.
    • If any serial data circuit is 100 Ω or less 
    1. Repair all serial data circuits at the tested splice except the faulted serial data circuit, ignition ON/Vehicle In Service Mode.
    2. Verify the low speed GMLAN devices that are still not communicating to identify the faulted serial data circuit.
    3. Ignition/Vehicle OFF, disconnect all devices on the faulted serial data circuit.
    4. Test for infinite resistance between each section of the faulted serial data circuit and ground.
      • If less than infinite resistance, repair the short to ground in the circuit.
      • Go to next step: If infinite resistance
    5. Repair the faulted serial data circuit to the splice and connect the first device on the faulted serial data circuit, ignition ON/Vehicle In Service Mode.
    6. Verify the scan tool communicates or not with the low speed GMLAN serial data circuit.
      • If the scan tool does not communicate, replace the device that was just connected.
      • If the scan tool communicates and there are more devices to connect, connect the next device and repeat step 2.6.
      • Go to next step: If the scan tool communicates and there are no more devices to connect
    7. All OK.
    • Go to next step: If each serial data circuit is greater than 100 Ω 
  3. Inspect all low speed GMLAN splices for damage.

Testing the Serial Data Circuit for an Open/High Resistance

  1. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
  2. Disconnect the harness connectors at the low speed GMLAN device that is not communicating and closest to the K56 Serial Data Gateway Module.
  3. Disconnect the harness connectors at the K56 Serial Data Gateway Module.
  4. Test for less than 2 Ω in each of the low speed GMLAN serial data circuits (circuit number 5060) end to end between the non-communicating device and the K56 Serial Data Gateway Module.
    • If 2 Ω or greater 

      Repair the open/high resistance in the serial data circuit.

    • Go to next step: If less than 2 Ω 
  5. Inspect all low speed GMLAN splices for damage.
  6. Replace the non-communicating device that was disconnected.
  7. Reconnect the harness connectors at the K56 Serial Data Gateway Module.
  8. Ignition ON/Vehicle In Service Mode.
  9. Verify that all low speed GMLAN devices are communicating.
    • If the devices are not communicating 

      Replace the K56 Serial Data Gateway Module.

    • Go to next step: If the devices are communicating 
  10. All OK.

Testing the Device Circuits

  1. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. Disconnect the harness connectors at the low speed GMLAN device that is not communicating. It may take up to 10 min for all vehicle systems to power down.
  2. Test for less than 10 Ω between each of the ground circuit terminals at the device that was just disconnected and ground.
    • If 10 Ω or greater 
    1. Ignition/Vehicle OFF.
    2. Test for less than 2 Ω in the ground circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, repair the open/high resistance in the ground connection.
    • Go to next step: If less than 10 Ω 
  3. If equipped, verify a test lamp illuminates between each of the B+ circuit terminals and ground.
    • If the test lamp does not illuminate and the circuit fuse is good 
    1. Ignition/Vehicle OFF, remove the test lamp.
    2. Test for less than 2 Ω in the B+ circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, verify the fuse is not open and there is voltage at the fuse. Refer to Power Mode Mismatch .
    • If the test lamp does not illuminate and the circuit fuse is open 
    1. Ignition/Vehicle OFF, remove the test lamp.
    2. Test for infinite resistance between the B+ circuit and ground.
      • If less than infinite resistance, repair the short to ground on the circuit.
      • If infinite resistance, replace the disconnected device.
    • Go to next step: If the test lamp illuminates 
  4. Ignition ON/Vehicle In Service Mode.
  5. If equipped, verify a test lamp illuminates between each of the ignition circuit terminals, which has a fuse in the circuit, and ground.
    • If the test lamp does not illuminate and the circuit fuse is good 
    1. Ignition/Vehicle OFF, remove the test lamp.
    2. Test for less than 2 Ω in the ignition circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • If less than 2 Ω, verify the fuse is OK and there is voltage at the fuse. Refer to Power Mode Mismatch .
    • If the test lamp does not illuminate and the circuit fuse is open 
    1. Ignition/Vehicle OFF, remove the test lamp.
    2. Test for infinite resistance between the ignition circuit and ground.
      • If less than infinite resistance, repair the short to ground on the circuit.
      • If infinite resistance, replace the disconnected device.
    • Go to next step: If the test lamp illuminates 
  6. If equipped, verify a test lamp illuminates between each of the ignition circuit terminals, which is controlled by a control module, and ground.
    • If the test lamp does not illuminate 
    1. Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the control module that controls the ignition circuit and all devices that share the same ignition circuit.
    2. Test for infinite resistance between the ignition circuit and ground.
      • If less than infinite resistance, repair the short to ground on the circuit.
      • Go to next step: If infinite resistance
    3. Test for less than 2 Ω in the ignition circuit end to end.
      • If 2 Ω or greater, repair the open/high resistance in the circuit.
      • Go to next step: If less than 2 Ω
    4. Reconnect the control module that controls the ignition circuit, ignition ON/Vehicle In Service Mode.
    5. Verify a test lamp illuminates between the ignition circuit terminal and ground.
      • If the test lamp does not illuminate, replace the control module that controls the ignition circuit.
      • Go to next step: If the test lamp illuminates
    6. Ignition/Vehicle OFF, reconnect the first device that shares the same ignition circuit, ignition ON/Vehicle In Service Mode.
    7. Verify a test lamp illuminates between the ignition circuit terminal and ground.
      • If the test lamp does not illuminate, replace the device that was just connected.
      • If the test lamp illuminates and there are more devices to connect, connect the next device and repeat step 6.7 until there are no more devices to connect.
    • Go to next step: If the test lamp illuminates 
  7. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. Disconnect the harness connectors at the K56 Serial Data Gateway Module. It may take up to 10 min for all vehicle systems to power down.
  8. Test for less than 2 Ω in each of the low speed GMLAN serial data circuits end to end between the non communicating device and the K56 Serial Data Gateway Module.
    • If 2 Ω or greater 

      Repair the open/high resistance in the serial data circuit.

    • Go to next step: If less than 2 Ω 
  9. Replace the non communicating device that was disconnected.
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