Diagnostic Instructions
Diagnostic Fault Information
Circuit
| Short to Ground
| Open/High Resistance
| Short to Voltage
| Signal Performance
|
Low Speed GMLAN Serial Data (+)
| 1
| 2*
| 1
| --
|
Ground (DLC, terminal 5)
| --
| 1*
| --
| --
|
* An open between the data link connector (DLC) and JX 221, or DLC terminal 5 ground will only effect the communication with the scan tool. See Diagnostic Aids.
1. No communication on any low speed GMLAN module
2. No communication on a specific low speed GMLAN module, or group of modules
|
Circuit/System Description
Modules connected to the low speed GMLAN serial data circuit 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 low speed GMLAN serial data circuit uses JX 221, JX 339, and, if equipped, JX 223 as common connection points between some modules and the data link connector (DLC).
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 high speed GMLAN serial data system inoperative. This condition is due to the BCM 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 module will only affect that specific module. This type of failure will set a loss of communication DTC for each module effected, and the other modules will still communicate. |
| • | An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle. When the scan tool attempts to communicate with the vehicle a message "no CANdi module detected" will be displayed. |
| • | An open between the DLC and JX 221 will only effect the communication with the scan tool. The vehicle modules will still communicate, and the vehicle will start. |
| • | The engine will 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 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. |
| - | Tire Pressure Monitor (TPM) system warning lights. |
Reference Information
Schematic Reference
Data Communication Schematics
Connector End View Reference
Description and Operation
Data Link Communications Description and Operation
Electrical Information Reference
Circuit/System Testing
Note: Use the schematic to identify the following:
| • | Control modules the vehicle is equipped with |
| • | Control module locations on the low speed GMLAN serial data circuit |
| • | Each control module's low speed GMLAN serial data circuit terminals |
- Attempt to communicate with all control modules on the low speed GMLAN serial data circuit, refer to
Data Link References. Communications should not be available with two or more control modules on the low
speed GMLAN serial data circuit.
| ⇒ | If only one control module is not communicating, diagnose that control module only, refer to
DTC U0100-U0299. |
| ⇒ | If one or more control modules are communicating but not all, refer to Testing the Serial Data Circuit for an Open/High Resistance. |
- Disconnect the scan tool from the DLC. The following tests will be done at the DLC connector.
- 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. |
- Ignition ON, test for less than 4.5 V between the serial data circuit terminal 1 and ground.
| ⇒ | If greater than the specified range, test the serial data circuit for a short to voltage, refer to Testing the Serial Data Circuit for a Short to Voltage. |
- Ignition OFF for 60 seconds, test for greater than 100 Ω between the serial data circuit terminal 1 and ground.
| ⇒ | 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. |
- Disconnect the harness connector at the JX 221 splice pack.
- Test for less than 2 Ω between the serial data circuit terminal 1 and the JX 221 splice pack terminal D.
| ⇒ | If greater than the specified range, test the serial data circuit for an open/high resistance. If the circuits test normal, replace the splice pack. |
Testing the Serial Data Circuits for a Short to Voltage
- Ignition OFF, disconnect the harness connectors at the JX 221, JX 339, and JX 223 splice packs, as equipped.
- Ignition ON, test for less than 4.5 V between the DLC serial data circuit terminal 1 and ground.
| ⇒ | If greater than the specified range, repair the short to voltage. |
- Test for less than 4.5 V between each serial data circuit and ground.
| ⇒ | If greater than the specified range, perform the appropriate test listed below: |
| • | Serial data circuits with one control module, test the serial data circuit for a short to voltage. If the circuit tests normal replace the control module. |
| • | Serial data circuits with 2 or more control modules, test each section of the serial data circuit for a short to voltage. If the circuits test normal, replace the control module that causes the short to voltage when connected. |
Testing the Serial Data Circuits for a Short to Ground
- Ignition OFF, disconnect the harness connectors at the JX 221, JX 339, and JX 223 splice packs, as equipped.
- Test for infinite resistance between the DLC serial data circuit terminal 1 and ground.
| ⇒ | If not the specified value, repair the short to ground. |
- Ignition OFF for 60 seconds, test for greater than 100 Ω between each serial data circuit and ground.
| ⇒ | If less than the specified range, perform the appropriate test listed below: |
| • | Serial data circuits with one control module, test the serial data circuit for a short to ground. If the circuit tests normal replace the control module. |
| • | Serial data circuits with 2 or more control modules, test each section of the serial data circuit for a short to ground. If the circuits test normal, replace the control module that causes the short to ground when connected. |
Testing the Serial Data Circuit for an Open/High Resistance
- If equipped with a JX 339 splice pack, ignition ON, verify the scan tool communicates with one or more control modules connected to the JX 339 splice pack.
| ⇒ | If all control modules on the JX 339 spice pack do not communicate, test the serial data circuit between the JX 221 and JX 339 splice packs for an open/high resistance. If the circuit tests normal, test each splice pack for an
open/high resistance. |
- Ignition OFF, disconnect one of the harness connectors listed below:
| • | JX 221 if the control modules not communicating are connected to the JX 221 splice pack |
| • | JX 223 if the control modules not communicating are connected to the JX 223 splice pack, if equipped |
| • | JX 339 if the control modules not communicating are connected to the JX 339 splice pack |
- Install a 3A fused jumper wire between the disconnected splice pack connector terminal listed below and a low speed GMLAN serial data circuit that is not communicating:
- Ignition ON, verify the scan tool communicates with the control modules connected to the low speed GMLAN serial data circuit.
| ⇒ | If any control module does not communicate, test each section of the serial data circuit for an open/high resistance. If the circuits test normal, replace the control module that does not communicate when connected. |
Repair Instructions
Perform the
Diagnostic Repair Verification after completing the repair.
