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.

### DTC Descriptors

DTC U0074 00: Control Module Communication Bus B Off

### 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 | U0074 00, 2* | 1 | --
Mid Speed GMLAN Serial Data (-) | 1 | U0074 00, 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.

### Conditions for Running the DTC

The system voltage is between 9-16 volts.

### Conditions for Setting the DTC

A supervised periodic message that includes the transmitter module availability has not been received.

### Action Taken When the DTC Sets

Specific subsystems will not function.

### Conditions for Clearing the DTC

• | A current DTC clears when the malfunction is no longer present.

• | A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

### Diagnostic Aids

• | Use the Data Link References to identify the mid speed GMLAN serial data modules.

• | Some modules may not have internal protection for specific voltage outputs and may open a battery positive voltage or ignition voltage source fuse. If a voltage input fuse is open and no short is found in that circuit, ensure that no module output voltage circuit is shorted to ground before replacing the module.

• | An open in the DLC ground circuit terminal 5 will allow the scan tool to operate to set up the vehicle on the tool and then not communicate with the vehicle. When the scan tool is to the point of communicating with the vehicle, a message on the scan tool will indicate "no CANdi module detected" and will not communicate.

### 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 mid speed module communicates and any DTC beginning with a U exists, refer to Control Module U Code List.

### 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 serial data circuit terminating resistors

• | Control module locations on the serial data circuits

• | Each control modules mid speed serial data circuit terminals

1. Attempt to communicate with all control modules on the mid speed serial data circuit, refer to Data Link References. Communications should not be available with two or more control modules on the mid speed serial data circuit.

⇒ | If only one control module is not communicating, diagnose that control module only; refer to DTC U0100-U0299.

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 3V 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 serial data circuits at an easily accessible control module.
2. Ignition ON, test for greater than 3V between the serial data circuits and ground. Verify that one or more serial data circuits are greater than 3V.

⇒ | If less than the specified range for all serial data circuits, replace the control module that was disconnected.

3. Ignition OFF, disconnect the harness connectors with the mid speed serial data circuits at another control module, in the direction of the circuit shorted to voltage.
4. Ignition ON, test for greater than 3V between the serial data circuits and ground. Verify that one or more serial data circuits are greater than 3V.

⇒ | If less 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 voltage on the serial data circuit between 2 control modules

• | 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 serial data circuits at an easily accessible control module.
2. Ignition OFF for 60 seconds, test for less than 100 Ω between the serial data circuits and ground. Verify that one or more serial data circuits are less than 100 Ω.

⇒ | If greater than the specified range for all serial data circuits, replace the control module that was disconnected.

3. Disconnect the harness connectors with the mid speed 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 100 Ω between the serial data circuits and ground. Verify that one or more serial data circuits are less than 100 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

• | 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 serial data circuits at an easily accessible control module, that is not communicating.
2. Ignition OFF for 60 seconds, test for less than 110 Ω between each pair of serial data circuits. Verify that one pair of serial data circuits are less than 110 Ω.

⇒ | If greater than the specified range for each pair of serial data circuits, 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 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 110 Ω between each pair of serial data circuits. Verify that one pair of serial data circuits are less than 110 Ω.

⇒ | If between the specified range for each pair of serial data circuits, 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

• | Serial data circuits shorted together between a control module and a terminating resistor

• | A shorted terminating resistor

#### Testing the Serial Data Circuits for a Open/High Resistance

1. Ignition OFF, disconnect the harness connectors with the mid speed serial data circuits at an easily accessible control module, that is not communicating.
2. Ignition OFF for 60 seconds, test for greater than 130 Ω between each pair of serial data circuits. Verify that one pair of serial data circuits are greater than 130 Ω.

⇒ | If less than the specified range for each pair of serial data circuits, 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 serial data circuits at another control module, in the direction of the open circuit.
5. Ignition OFF for 60 seconds, test for greater than 130 Ω between each pair of serial data circuits. Verify that one pair of serial data circuits are greater than 130 Ω.
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

• | 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.

• | GMLAN Wiring Repairs

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