• Perform the Diagnostic System Check prior to using this diagnostic procedure: Diagnostic System Check - Vehicle
• Review the description of Strategy Based Diagnosis: Strategy Based Diagnosis
• An overview of each diagnostic category can be found here: Diagnostic Procedure Instructions

The devices connected to the low speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. The serial data is transmitted over a single wire to the appropriate devices. The transmission speed for GMLAN low speed is up to 83.33 kbit/s. Under normal vehicle operating conditions, the speed of the bus is 33.33 kbit/s. The devices toggle the serial data circuit between 0–5 V during normal communications. To wake the devices connected to the low speed GMLAN serial data circuit, a voltage wake up pulse of 10 V is sent out. If serial data is lost, devices will set a no communication code against the non-communicating device. A loss of serial data communications DTC does not represent a failure of the device that set it.

• Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U code present. However, there is no associated “current” or “active” status. Loss of communication U codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:
• A device on the data communication circuit was disconnected while the communication circuit is awake.
• Power to one or more devices was interrupted during diagnosis
• A low battery condition was present, so some devices stop communicating when battery voltage drops below a certain threshold.
• Battery power was restored to the vehicle and devices on the communication circuit did not all re-initialize at the same time.
• If a loss of communication U code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.
• Do not replace a device reporting a U code. The U code identifies which device needs to be diagnosed for a communication issue.
• Communication may be available between the device and the scan tool with the low speed GMLAN serial data system inoperative. This condition is due to the device using both the high and low speed GMLAN systems.
• An open in the low speed GMLAN serial data circuit between the splice pack and a device will only affect that specific device. This type of failure will set a loss of communication DTC for each device affected, and the other devices will still communicate.
• An open in the X84 Data Link Connector ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
• The engine may not start when there is a total malfunction of the low speed GMLAN serial data circuit.
• Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed LAN communications with the scan tool.
• These conditions may be caused by the installation of an aftermarket navigation radio device (see bulletins). Some customers may comment of one or more of the following concerns:
• Vehicle will not crank
• Vehicle cranks but will not start
• Vehicle stability enhancement system warning lights and messages
• PRNDL gear indicator position errors
• Some devices may not have internal protection for specific voltage outputs and may open a battery positive voltage or ignition voltage source fuse. If a voltage input fuse is open and no short is found in that circuit, ensure that no device output voltage circuit is shorted to ground before replacing the device.

• Data Communication Schematics
• Control Module References

Master Electrical Component List

Data Link Communications Description and Operation

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

Control Module References for scan tool information

1. Ignition ON/Vehicle In Service Mode.
2. Verify that the scan tool is communicating with vehicle.
• If the scan tool is not communicating with vehicle

  Refer to Scan Tool Does Not Communicate with Vehicle.

• If the scan tool is communicating with vehicle
3. Attempt to communicate with all devices on the low speed GMLAN bus. Refer to Data Link References to determine how many devices should be communicating on the low speed GMLAN bus.
4. Verify two or more devices are not communicating on the low speed GMLAN bus.
• If only one device is not communicating

  Refer to Circuit/System Testing – Testing the Device Circuits.

• If two or more devices are not communicating
5. Ignition/Vehicle OFF, disconnect the scan tool from the X84 Data Link Connector. The following tests will be done at the X84 Data Link Connector.
6. Ignition ON/Vehicle In Service Mode.
7. Test for less than 4.5 V between the serial data circuit terminal 1 and ground.
• If 4.5 V or greater

  Repair the short to voltage in the serial data circuit between the X84 Data Link Connector and the K56 Serial Data Gateway Module.

• If less than 4.5 V
8. 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.
9. Test for greater than 100 Ω between the serial data circuit terminal 1 and ground.
• If 100 Ω or less

  Repair the short to ground in the serial data circuit between the X84 Data Link Connector and the K56 Serial Data Gateway Module.

• If greater than 100 Ω
10. Test for less than infinite resistance between the serial data circuit terminal 1 and ground.
• If infinite resistance

  Repair the open/high resistance in the serial data circuit between the X84 Data Link Connector and the K56 Serial Data Gateway Module.

• If less than infinite resistance
11. Refer to Circuit/System Testing – 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.

• 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.

• If greater than 100 Ω
6. Test for less than infinite resistance between each of the low speed GMLAN serial data circuit terminals and ground.
• If infinite resistance

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

• If less than infinite resistance
7. Connect a jumper wire between the low speed GMLAN serial data circuit terminal (circuit number 5060) and the low speed GMLAN serial data #3 circuit terminal (circuit number 2100). It is recommended to use a pair of diagnostic test probes EL-35616-58 to avoid damage to the harness connectors and terminals.
8. Ignition ON/Vehicle In Service Mode.
9. Using Data Bus Diagnostic Tool or scan tool, verify that all low speed GMLAN devices are communicating:
• If the devices are not communicating

  Refer to Testing the Serial Data Circuits for an Open/High Resistance.

• If the devices are communicating
10. Replace the K56 Serial Data Gateway Module.

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.
• 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.
• If the scan tool communicates and there are no more devices to connect
7. All OK.
• If all serial data circuits are less than 4.5 V
4. Inspect all low speed GMLAN splices for damage.

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.
• 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.
• If the scan tool communicates and there are no more devices to connect
7. All OK.
• If each serial data circuit is greater than 100 Ω
3. Inspect all low speed GMLAN splices for damage.

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.

• If less than 2 Ω
5. Inspect all low speed GMLAN splices for damage.
6. Replace the non-communicating device that was disconnected.

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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.

• If less than 2 Ω
9. Replace the non communicating device that was disconnected.

Perform the Diagnostic Repair Verification after completing the repair: Diagnostic Repair Verification.

For control module replacement, programming, and setup, refer to: Control Module References.