For 1990-2009 cars

Modules connected to the high speed GMLAN serial data circuits monitor for serial data communications on the high speed GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module detects a bus-off condition a DTC U0001 or U2100 will be set. These DTCs can be retrieved as history only.

### Diagnostic Aids

The high speed GMLAN serial data bus uses two 120 ohms terminating resistors that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is connected at the instrument panel module (IPM) and the other is at the engine control module (ECM). When testing for a short between high speed GMLAN (+) and (-) a reading of 60 ohms is normal. If the high speed GMLAN serial data is open testing the resistance between high speed GMLAN (+) and (-) will read about 120 ohms.

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

• | A short between high speed GMLAN (+) and high speed GMLAN (-) circuits

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

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

• | Any of High Speed GMLAN Serial Data Circuits Open

### Test Description

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

1. A partial malfunction in the high speed GMLAN serial data circuits uses a different procedure from a total malfunction of the high speed GMLAN data circuits. The scan tool communicates with the following modules on the high speed GMLAN serial data circuits:

• | Distance sensing cruise control (DSCC) module

• | Instrument panel module (IPM)

• | Electronic brake control module (EBCM)

• | Electronic suspension control (ESC) module

• | Engine control module (ECM)

• | Transmission control module (TCM)

2. Data link connector (DLC) terminals 6 and 14 provide the connection to the GMLAN serial data high circuit and the GMLAN serial data low circuit respectively.

3. If only one of the high speed GMLAN circuits is open all communication on the high speed bus will be disrupted.

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 works fine. CANdi LED flashes when it is functioning properly. In the event of a problem, the LED will be illuminated continuously.

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 GMLAN serial data circuits?

Go to Step 3

Go to Step 6

3 | Is any DTC set among U2105-2199? | Go to DTC U2105-U2199 | Go to Step 4
4 | Is any DTC set among U0100-0299? | Go to DTC U0100-U0299 | Go to Step 5
5 | Important:: Turn ON the ignition, with the engine OFF to test a short to voltage. Use the DMM MIN/MAX function to capture intermittent conditions.

1. Disconnect the scan tool from the data link connector (DLC).
2. Test the high speed GMLAN serial data circuits for the following conditions:

• | An intermittent short to ground

• | An intermittent short to voltage

• | An intermittent short among circuits

• | An intermittent open on any of the circuits

Refer to the following procedures:

• | Testing for Intermittent Conditions and Poor Connections

• | Circuit Testing

• | Connector Repairs

• | GMLAN Wiring Repairs

Did you find and correct the condition?

Go to Step 21

Go to Diagnostic Aids

6 |

1. Disconnect the instrument panel module (IPM).
2. Test the following circuits of the IPM 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 18

Go to Step 7

7 | Test the ground circuits of the IPM for an open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 18 | Go to Step 8
8 | Inspect for poor connections at the DLC terminals 6 and 14. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 18 | Go to Step 9
9 | Test the high speed GMLAN serial data circuits between the IPM and DLC for:

• | A short to ground

• | A short to battery

• | A short between them

• | An open circuit

Refer to the following procedures:

• | Circuit Testing

• | GMLAN Wiring Repairs

Did you find and correct the condition?

Go to Step 18

Go to Step 10

10 |

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

Does the scan tool communicate with the IPM?

Go to Step 15

Go to Step 11

11 |

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

Does the scan tool communicate with the IPM?

Go to Step 12

Go to Step 13

12 | Test the high speed GMLAN serial data circuits between the last 2 disconnected modules for the following conditions:

• | A short to ground

• | A short to battery

• | A short between them

• | An open circuit

Refer to the following procedures:

• | Circuit Testing

• | GMLAN Wiring Repairs

Did you find and correct the condition?

Go to Step 18

Go to Step 15

13 | Is the IPM the last module still connected to high speed GMLAN data circuits? | Go to Step 14 | Go to Step 11
14 |

1. Turn OFF the ignition.
2. Disconnect the IPM.
3. Test the high speed GMLAN serial data circuits between IPM and the previous disconnected module for:

• | A short to ground

• | A short to battery

• | A short between them

• | An open circuit

Refer to the following procedures:

• | Circuit Testing

• | GMLAN Wiring Repairs

Did you find and correct the condition?

Go to Step 18

Go to Step 16

15 | Test each of the high speed GMLAN circuits of the last disconnected module to the next module closer to DLC for an open. Refer to the following procedures:

• | Circuit Testing

• | GMLAN Wiring Repairs

Did you find and correct the condition?

Go to Step 18

Go to Step 16

16 | Inspect for poor connections at the high speed GMLAN serial data circuits of the suspected module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 18 | Go to Step 17
17 | Replace the suspected module. Refer to Control Module References for replacement, setup, and programming. Is the replacement complete? | Go to Step 18 | --
18 |

1. Reconnect all of the disconnected modules.
2. Install the scan tool.
3. Turn ON the ignition, with the engine OFF.
4. Select the Display DTCs function for each module.
5. Record all of the displayed DTCs and the DTC status.

Is there any DTC which begin with a U and with a current status?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 19

19 | Is there any DTC which does not begin with a U recorded? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 20
20 | Have you any DTCs not diagnosed? | Go to Step 21 | Go to Step 18
21 | Clear the DTCs with the scan tool. Is the operation completed? | System OK | --