For 1990-2009 cars

Modules connected to the low speed GMLAN serial data circuits monitor for serial data communications on the low speed GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module detects a bus-off condition, DTC U2100 will be set. These DTCs can be retrieved as history only.

### Diagnostic Aids

The engine will not start when there is a total malfunction of the low speed GMLAN serial data circuits while the engine is not running. The following conditions may cause a total loss of low speed GMLAN data communication:

• | Any of the low speed GMLAN serial data circuits shorted to ground or voltage

• | A module internal malfunction that causes a short to voltage or ground on the low speed GMLAN circuits

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. A partial malfunction in the low speed GMLAN serial data circuits uses a different procedure from a total malfunction of the low speed GMLAN data circuits.

2. The DTC U2100 may be retrieved with a history status, but is not the cause of the present condition.

3. Data link connector terminal 1 provides the connection to the GMLAN serial data low circuit and the GMLAN serial data low circuit respectively.

4. The communication malfunction may have prevented diagnosis of the customer complaint.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Data Communication Schematics Connector End View Reference: Master Electrical Component List
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Important: Make sure the CANdi module is not malfunctioning. When functioning properly, the CANdi module LED will be flashing. In the event of a problem, the LED will be continually illuminated.

1. Turn ON the ignition, with the engine OFF.
2. Perform the Vehicle DTC Information function on the scan tool, under Computer/Integrating Systems.

Does the scan tool communicate with any module on the low speed GMLAN serial data circuit?

Go to Step 3

Go to Step 5

3 | Did you record any DTCs in the range of U2105 to U2177? | Go to DTC U2105-U2199 | Go to Step 4
4 | Important: Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Use the DMM MIN/MAX function to capture intermittent conditions. Test the low speed GMLAN serial data circuits for:

• | An intermittent short to ground

• | An intermittent short to voltage

• | An intermittent open

Refer to the following procedures:

• | Testing for Intermittent Conditions and Poor Connections

• | Circuit Testing

• | Connector Repairs

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 18

Go to Diagnostic Aids

5 | Test the following circuits of the body control module (BCM) for an open or short to ground:

• | The battery positive voltage input circuits

• | The battery positive voltage output circuits

• | The ignition mode switch input circuits

• | The switched battery positive voltage circuits

Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 15

Go to Step 6

6 | Test the ground circuits of the BCM for an open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 15 | Go to Step 7
7 |

1. Turn OFF the ignition.
2. Disconnect the scan tool from the data link connector (DLC).
3. Inspect for poor a connection at the DLC terminal 1. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Diagnostic System Check - Vehicle

Go to Step 8

8 |

1. Disconnect the farthest module from the DLC that is connected to low speed GMLAN serial data circuit. Refer to Data Communication Schematics .
2. Turn ON the ignition, with the engine OFF.
3. Attempt to communicate with the BCM.

Does the scan tool communicate with the BCM?

Go to Step 13

Go to Step 9

9 |

1. Turn OFF the ignition.
2. Disconnect the farthest module from the DLC that is still connected to low speed GMLAN serial data circuit. Refer to Data Communication Schematics .
3. Turn ON the ignition, with the engine OFF.
4. Attempt to communicate with the BCM.

Does the scan tool communicate with the BCM?

Go to Step 10

Go to Step 11

10 | Test the low speed GMLAN serial data circuit between the last two disconnected modules for:

• | A short to ground

• | A short to battery

Did you find and correct the condition?

Go to Step 15

Go to Step 13

11 | Is the BCM the last module still connected to low speed GMLAN data circuit? | Go to Step 12 | Go to Step 9
12 |

1. Turn OFF the ignition.
2. Disconnect the BCM.
3. Test the low speed GMLAN serial data circuit between BCM and the previous disconnected module and between BCM and DLC for:

• | A short to ground

• | A short to battery

• | An open

Did you find and correct the condition?

Go to Step 15

Go to Step 13

13 | Inspect for poor connections at the low speed GMLAN serial data circuit of the last disconnected module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 15 | Go to Step 14
14 | Important: Perform the module setup procedure if required. Replace the last disconnected module. Refer to Control Module References for the appropriate repair instructions. Did you complete the replacement? | Go to Step 15 | --
15 |

1. Reconnect all of the disconnected modules.
2. Install a scan tool.
3. Turn ON the ignition, with the engine OFF.
4. Wait for 10 seconds.

The scan tool may require a power up reset before communication will occur due to a short on the low speed GMLAN serial data circuits. Turn OFF or disconnect the scan tool prior to performing this test.

5. Select the Display DTCs function for each module.
6. Record all of the displayed DTCs and the DTC status.

Did you record any DTCs which begin with a "U" and with a current status?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 16

16 | Did you record any DTCs which do not begin with a "U"? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 17
17 | Did you diagnose all of the DTCs? | Go to Step 18 | Go to Step 15
18 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | --