Diagnostic Instructions
Diagnostic Fault Information
Circuit
| Short to Ground
| Open/High Resistance
| Short to Voltage
| Signal Performance
|
Class 2 Serial Data
| 1
| 2*
| 1
| --
|
Ground (DLC, terminal 5)
| --
| 1
| --
| --
|
* An open between the data link connector (DLC) and JX 205, or DLC terminal 5 ground will only effect the communication with the scan tool. The vehicle modules will still communicate.
1. No communication on any class 2 serial data module
2. No communication on a specific class 2 serial data module
|
Circuit Description
Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. Connecting a scan tool to the data link connector (DLC)
allows communication with the modules for diagnostic purposes. The class 2 serial data circuit uses JX 205 (splice pack comb) as a common connection between some modules and the DLC. DTCs may be set due to this symptom and during this diagnostic
procedure.
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 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 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 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. |
| • | The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus. |
| • | Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool. |
| • | The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus. |
| | 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
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Testing
- Test for less than 1.0 ohm between the DLC ground circuit terminal 5 and ground.
| ⇒ | If greater than 1.0 ohm, test the ground circuit for open/high resistance. |
- Ignition OFF, disconnect the harness connector (splice pack comb) at JX 205. Install a 3A fused jumper wire between JX 205 terminal A and terminal M, harness side.
- Ignition ON, with a scan tool, attempt to communicate with the body control module (BCM). Communication should be available.
| ⇒ | If no communication, test the serial data circuit for a short to ground, a short to voltage or an open/high resistance between the BCM and the DLC. If the circuit tests normal, replace the BCM. |
- Install another 3A fused jumper wire to JX 205 terminal A, harness side.
- With BCM communications established, use the other end of the jumper wire to connect all other terminals, harness side, one at a time and verify that communication remains available to the BCM.
Important: Allow communications for 5 seconds after connecting a module with the jumper to determine if communication is interrupted or not.
| ⇒ | If communication is interrupted after connecting an individual module at JX 205, test the serial data circuit between JX 205 and the last module connected for a short to ground and short to voltage. If the circuit tests normal, replace
the module that caused no communication. |
Repair Instructions
Perform the
Diagnostic Repair Verification after completing the repair.
