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Scan Tool Does Not Communicate with Low Speed GMLAN Device > < DTC U18B9-U18BF

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Scan Tool Does Not Communicate with Mid Speed GMLAN 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

Mid Speed GMLAN Serial Data (+)
1
U0100-U0299*
1
-

Mid Speed GMLAN Serial Data (-)
1
U0100-U0299*
1
-

Ground (DLC, terminal 5)
-
1
-
-

* No communications with one or more mid speed GMLAN modules. An open in only one mid speed GMLAN serial data circuit may allow degraded communication between the 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 on any mid speed GMLAN module.
 

Circuit/System Description

Modules connected to the mid speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules when the ignition switch is in any position other than OFF. The mid speed GMLAN serial data bus uses a terminating resistor that is in parallel with the mid speed GMLAN (+) and (-) circuits.

Diagnostic Aids

•  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're 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 control module on the data communication circuit was disconnected while the communication circuit is awake.
  -  Power to one or more modules was interrupted during diagnosis.
  -  A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.
  -  Battery power was restored to the vehicle and control modules 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 control module reporting a U code. The U code identifies which control module needs to be diagnosed for a communication issue.
•  Communication may be available between the BCM and the scan tool with the mid speed GMLAN serial data system inoperative. This condition is due to the BCM using the high, mid, and low speed GMLAN systems.
•  An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
•  The engine will not start when there is a total malfunction of the mid speed GMLAN serial data bus.
•  Technicians may find various Local Area Network communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool.
  These conditions may be caused by the installation of an aftermarket navigation radio module (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

Reference Information


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

Circuit/System Testing


Note:  Each control module may need to be disconnected to isolate a circuit fault. Use the schematic to identify the following:
•  Control modules the vehicle is equipped with
•  Mid speed GMLAN serial data circuit terminating resistors
•  Control module locations on the mid speed GMLAN serial data circuits
•  Each control module's mid speed GMLAN serial data circuit terminals

1. Attempt to communicate with all control modules on the mid speed GMLAN serial data circuit. Refer to Data Link References . Communications should not be available with two or more control modules on the mid speed GMLAN serial data circuit.
⇒ If only one control module is not communicating, diagnose that control module only. Refer to DTC U0100-U02FF .
2. Disconnect the scan tool from the DLC. The following tests will be done at the DLC connector.
3. All access doors closed, ignition OFF for 60 seconds, test for less than 10 Ω between the ground circuit terminal 5 and ground.
⇒ If greater than the specified range, test the ground circuit for an open/high resistance.
4. Ignition ON, test for less than 4.5 V between the serial data circuits listed below and ground:
  •  Terminal 3
  •  Terminal 11
⇒ 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.
5. Ignition OFF for 60 seconds, test for greater than 100 Ω between the serial data circuits listed below and ground:
  •  Terminal 3
  •  Terminal 11
⇒ If not the specified value, test the serial data circuit for a short to ground. Refer to Testing the Serial Data Circuits for a Short to ground.
6. Test for 50-70 Ω between the serial data circuits terminal 3 and terminal 11.
⇒ 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.
⇒ 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.
⇒ 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.
⇒ If infinite, test the serial data circuits between the DLC and the first connection to the serial data circuit for an open/high resistance.

Testing the Serial Data Circuits for a Short to Voltage

1. Ignition OFF, disconnect the harness connectors with the mid speed GMLAN serial data circuits at an easily accessible control module.
2. Ignition ON, test for greater than 4.5 V 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 greater than 4.5 V.
⇒ If all serial data circuits are less than the specified range, replace the control module that was disconnected.
3. Ignition OFF, disconnect the harness connectors with the mid speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to voltage.
4. Ignition ON, test for greater than 4.5 V 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 greater than 4.5 V.
⇒ If all serial data circuits are less than the specified range, replace the control module that was just disconnected.
5. Repeat step 3 until one of the follow conditions are isolated:
  •  A short to voltage on the serial data circuit between 2 control modules or splice packs, if equipped
  •  A short to voltage on the serial data circuit between a control module and a terminating resistor

Testing the Serial Data Circuits for a Short to Ground

1. Ignition OFF, disconnect the harness connectors with the mid speed GMLAN serial data circuits at an easily accessible control module.
2. Ignition OFF for 60 seconds, 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.
⇒ If all serial data circuits are greater than the specified range, replace the control module that was disconnected.
3. Disconnect the harness connectors with the mid speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to ground.
4. Ignition OFF for 60 seconds, 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.
⇒ If greater than the specified range for all serial data circuits, replace the control module that was just disconnected.
5. Repeat step 3 until one of the follow conditions are isolated:
  •  A short to ground on the serial data circuit between 2 control modules or splice packs, if equipped
  •  A short to ground on the serial data circuit between a control module and a terminating resistor

Testing the Serial Data Circuits for a Short between the Circuits

1. Ignition OFF, disconnect the harness connectors with the mid speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
2. Ignition OFF for 60 seconds, test for less than 35 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 35 Ω.
⇒ If each pair of serial data circuits is greater than the specified range, replace the control module that was disconnected.
3. Connect the harness connectors at the control module that was disconnected.
4. Disconnect the harness connectors with the mid speed GMLAN serial data circuits at another control module, in the direction of the circuits shorted together.
5. Ignition OFF for 60 seconds, test for less than 35 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 35 Ω.
⇒ If each pair of serial data circuits is within the specified range, replace the control module that was just disconnected.
6. Repeat step 3 until one of the following conditions are isolated:
  •  Serial data circuits shorted together between 2 control modules or splice packs, if equipped
  •  Serial data circuits shorted together between a control module and a terminating resistor
  •  A shorted terminating resistor

Testing the Serial Data Circuits for an Open/High Resistance

1. Ignition OFF, disconnect the harness connectors with the mid speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
2. Ignition OFF for 60 seconds, test for greater than 100k 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 is greater than 100k Ω.
⇒ If each pair of serial data circuits is less than the specified range, replace the control module that was disconnected.
3. Connect the harness connectors at the control module that was disconnected.
4. Disconnect the harness connectors with the mid speed GMLAN serial data circuits at another control module, in the direction of the open circuit.
5. Ignition OFF for 60 seconds, test for greater than 100k 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 is greater than 100k Ω.
⇒ If each pair of serial data circuits is less than the specified range, replace the control module that was just disconnected.
6. Repeat step 3 until one of the following conditions are isolated:
  •  An open/high resistance on the serial data circuit between 2 control modules or splice packs, if equipped
  •  An open/high resistance on the serial data circuit between a control module and a terminating resistor
  •  An open/high resistance terminating resistor

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

•  Control Module References for control module replacement, setup, and programming

Scan Tool Does Not Communicate with Low Speed GMLAN Device > < DTC U18B9-U18BF