Scan Tool Does Not Communicate with High Speed GMLAN Device
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Scan Tool Does Not Communicate with High Speed GMLAN Device
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Diagnostic Fault Information
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Circuit
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Short to Ground
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Open/High Resistance
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Short to Voltage
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Signal Performance
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High Speed GMLAN Serial Data (+)
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1
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U0100-U02FF*
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1
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-
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High Speed GMLAN Serial Data (-)
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1
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U0100-U02FF*
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1
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-
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Ground (DLC, terminal 5)
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-
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1
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-
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-
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* No communication with one or more high-speed GMLAN modules. An open circuit in only one high-speed GMLAN serial data circuit may allow degraded communication between the two modules. An open between the data link connector (DLC) and the first splice will only effect the communication with the scan tool. The vehicle modules will still communicate.
1. No communication with any high speed GMLAN module
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Circuit/System Description
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The serial data is transmitted on two twisted wires that allow speeds up to 500 Kb/s. The twisted pair is terminated with two 120 ohms resistors, one is internal to the engine control module (ECM) and the other can be a separate resistor in a connector assembly or in another control module. The resistors are used as the load for the High Speed GMLAN buss during normal vehicle operation. The high speed GMLAN is a differential bus. The GMLAN (+) high-speed bus and GMLAN (-) high-speed bus are run to opposite extremes from a standby or idling level of approx. 2.5 V. When the cables are run to their extremes, one volt is added to the GMLAN (+) bus circuit and one volt is subtracted to the GMLAN (-) bus circuit. If data is lost, the control modules generate a DTC for no communication with the control module that is not communicating. Note that a loss of serial data DTC does not represent a failure of the module that set it.
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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. Communication loss U-codes, like these, can be registered for several different reasons. In many cases, they may be obvious to the driver and technician and/or not have any 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:
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A control module on the data communication circuit was disconnected while the communication circuit is awake.
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Power to one or more control modules was interrupted during diagnosis.
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A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.
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Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.
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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.
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Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication task.
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Communication may be available between the body control module (BCM) and the scan tool with the high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems.
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An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
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The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
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Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool.
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These conditions may be caused by a retrofitted navigation radio module (refer to the bulletins). Some customers may comment on one or more of the following problems:
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Vehicle cranks but will not start
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Vehicle stability enhancement system warning lights and messages
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PRNDL gear indicator position errors
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Schematic Reference
Connector End View Reference
Component Connector End Views
Description and Operation
Data Link Communications Description and Operation
Electrical Information Reference
Scan Tool Reference
Control Module References
for scan tool information
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Circuit/System Verification
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Note
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Each control module may need to be disconnected to isolate a circuit fault. Use the schematic to identify the following:
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Control modules the vehicle is equipped with
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High speed GMLAN serial data circuit terminating resistors
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Control module locations on the high speed GMLAN serial data circuits
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Each control module's high speed GMLAN serial data circuit terminals
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1.
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Try to communicate with all control modules in the GMLAN high-speed circuit. See
Data Link References
. It must not be possible to communicate with two or more control modules in the high-speed GMLAN circuit.
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2.
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If only one control module is not communicating, diagnose that control module only. Refer to
DTC U0100-U02FF
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3.
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Disconnect the scan tool from the X84 data link connector (DLC). The following tests will be done at the DLC connector.
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4.
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Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 10 Ω between the ground circuit terminal 5 and ground.
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5.
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If greater than the specified range, test the ground circuit for an open/high resistance.
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6.
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Ignition ON, test for less than 4.5 V between the serial data circuits listed below and ground:
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7.
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If greater than the specified range, test the serial data circuit for a short to voltage. Refer to Testing the Serial Data Circuits for a Short to Voltage.
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8.
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Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 100 Ω between the serial data circuits listed below and ground:
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9.
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If less than the specified range, test the serial data circuit for a short to ground. Refer to Testing the Serial Data Circuits for a Short to ground.
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10.
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Test for 50-70 Ω between the serial data circuits terminal 6 and terminal 14.
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11.
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If less than 35 Ω, test for a short between the serial data circuits. Refer to Testing the Serial Data Circuits for a Short between the Circuits.
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12.
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If 35-50 Ω there may be a third terminating resistor between the serial data circuits. This can happen if the incorrect control module is installed. Some control modules are available with and without the terminating resistors installed to reduce the need of terminating resistors in the wiring harness.
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13.
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If greater than 70 Ω but less than infinite, test the serial data circuit for an open/high resistance. Refer to Testing the Serial Data Circuits for an Open/High Resistance.
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14.
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If infinite, test the serial data circuits between the DLC and the first connection to the serial data circuit for an open/high resistance.
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Note
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Some control modules with an internal terminating resistor have a loop in the harness that connects the internal terminating resistor to the serial data circuit. When wired this way, test these loop circuits for the appropriate failure mode short to voltage, short to ground, or open/high resistance prior to replacing the control module for each of the following tests.
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Testing the Serial Data Circuits for a Short to Voltage
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1.
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Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module.
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2.
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Ignition ON, test the voltage between each data circuit of the connector of the control module that was just disconnected and ground (must be higher than 4.5 V). Confirm that the voltage of one or more data circuits are over 4.5 V.
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3.
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If all serial data circuits are less than the specified range, replace the control module that was disconnected.
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4.
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Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to voltage.
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5.
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Ignition ON, test the voltage between the data circuits of the connector of the control module that was just disconnected and ground (must be higher than 4.5 V). Confirm that the voltage of one or more data circuits are over 4.5 V.
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6.
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If all serial data circuits are less than the specified range, replace the control module that was just disconnected.
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7.
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Repeat step 3 until one of the follow conditions are isolated:
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A short to voltage on the serial data circuit between 2 control modules or splice packs, if equipped.
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A short to voltage on the serial data circuit between a control module and a terminating resistor.
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Testing the Serial Data Circuits for a Short to Ground
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1.
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Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module.
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2.
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Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 1k Ω between each serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are less than 1k Ω.
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3.
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If all serial data circuits are greater than the specified range, replace the control module that was disconnected.
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4.
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Disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to ground.
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5.
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Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 1k Ω between the serial data circuits at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are less than 1k Ω.
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6.
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If greater than the specified range for all serial data circuits, replace the control module that was just disconnected.
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7.
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Repeat step 3 until one of the follow conditions are isolated:
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A short to ground on the serial data circuit between 2 control modules or splice packs, if equipped.
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A short to ground on the serial data circuit between a control module and a terminating resistor.
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A short to ground on the serial data circuit between the DLC and the first control module or splice pack.
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Testing the Serial Data Circuits for a Short between the Circuits
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1.
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Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
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2.
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Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 110 Ω between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are less than 110 Ω.
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3.
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If each pair of serial data circuits is greater than the specified range, replace the control module that was disconnected.
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4.
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Connect the harness connectors at the control module that was disconnected.
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5.
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Disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the circuits shorted together.
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6.
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Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 110 Ω between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are less than 110 Ω.
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7.
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If each pair of serial data circuits has resistance greater than the specified range, replace the control module that was just disconnected.
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8.
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Repeat step 3 until one of the following conditions are isolated:
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Serial data circuits shorted together between 2 control modules or splice packs, if equipped.
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Serial data circuits shorted together between a control module and a terminating resistor.
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Serial data circuits shorted together between the DLC and the first control module or splice pack.
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A shorted terminating resistor.
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Testing the Serial Data Circuits for an Open/High Resistance
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1.
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Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
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2.
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Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 130 Ω between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are greater than 130 Ω.
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3.
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If each pair of serial data circuits is less than the specified range, replace the control module that was disconnected.
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4.
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Connect the harness connectors at the control module that was disconnected.
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5.
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Disconnect the harness connectors with the high speed GMLAN serial data circuits at another control module, in the direction of the open circuit.
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6.
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Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 130 Ω between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are greater than 130 Ω.
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7.
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If each pair of serial data circuits is less than the specified range, replace the control module that was just disconnected.
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8.
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Repeat step 3 until one of the following conditions are isolated:
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An open/high resistance on the serial data circuit between 2 control modules or splice packs, if equipped.
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An open/high resistance on the serial data circuit between a control module and a terminating resistor.
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An open/high resistance terminating resistor.
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Perform the
Diagnostic Repair Verification
after completing the repair.
