For 1990-2009 cars

### 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
--- | --- | --- | --- | ---
Battery Positive Voltage (B+) | 1 | 1 | -- | --
Ground (DLC, terminal 5) | -- | 1 | -- | --
Mid Speed GMLAN Serial Data (+) | 1 | 2* | 1 | --
Mid Speed GMLAN Serial Data (-) | 1 | 2* | 1 | --
* 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. 2. An open in only one mid speed GMLAN serial data circuit may allow degraded communication between the modules.

### 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 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

Component Connector End Views

#### Description and Operation

Data Link Communications Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

Attempt to communicate with all modules on the mid speed GMLAN bus.

⇒ | If any high speed module communicates and any DTC beginning with a U exists, refer to Diagnostic Trouble Code (DTC) List - Vehicle.

### Circuit/System Testing

1. Ignition OFF, test for less than 1.0 ohm between the data link connector (DLC) ground circuit terminal 5 and ground.

⇒ | If greater than the specified range, test the ground circuit for an open/high resistance.

2. Ignition OFF, disconnect the X1 harness connector at the infotainment display.
3. Ignition OFF, test for less than 2 ohms between the DLC serial data circuit terminals and the CIM serial data circuit terminals listed below:

• | DLC terminal 3 and CIM terminal 22

• | DLC terminal 11 and CIM terminal 23

⇒ | If greater than the specified range, test the serial data circuit for open/high resistance.

4. Ignition OFF, test for infinite resistance between the following serial data circuits of the DLC and ground.

• | Terminal 3

• | Terminal 11

⇒ | If less than the specified value, test the serial data circuits for a short to ground.

5. Ignition ON, test for less than 4.5 volts between the following serial data circuits of the DLC and ground.

• | Terminal 3

• | Terminal 11

⇒ | If greater than the specified range, test the serial data circuit for a short to voltage.

6. Ignition OFF, test for infinite resistance between the DLC serial data circuit terminal 3 and terminal 11.

⇒ | If less than the specified value, test for a short between the serial data circuits.

7. Connect the X1 harness connector at the infotainment display.
8. Ignition OFF, disconnect the X1 harness connector of the radio.
9. Ignition ON, attempt to communicate with the display unit. Communication should not be available.

⇒ | If communication is established, replace the radio.

10. Ignition OFF, disconnect the X1 harness connector of the infotainment display.
11. Ignition OFF, test for infinite resistance between the following serial data circuits and ground.

• | Terminal 19

• | Terminal 20

⇒ | If less than the specified value, test the serial data circuit for a short to ground.

12. Ignition ON, test for less than 1-volt between the following serial data circuits and ground.

• | Terminal 19

• | Terminal 20

⇒ | If greater than the specified range, test the serial data circuit for a short to voltage.

13. Ignition OFF, test for infinite resistance between the serial data circuit terminal 19 and terminal 20.

⇒ | If less than the specified value, test for a short between the serial data circuits.

14. If the circuits test normal, replace the infotainment display.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

• | GMLAN Wiring Repairs

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