For 1990-2009 cars

### Circuit Description

Modules connected to the high speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The modules have prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised; also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. The supervision time-out period is 250 ms. Each message contains the identification number of the transmitter module.

### DTC Descriptors

This diagnostic procedure supports the following DTCs:

• | DTC U0100 Lost Communication With Engine Control Module (ECM) or lost communication with Powertrain Control Module (PCM)

• | DTC U0101 Lost Communication With Transmission Control Module (TCM)

• | DTC U0111 Lost Communication With Battery Energy Control Module (BECM) A

• | DTC U0120 Lost Communication With Starter/Generator Control Module (SGCM)

• | DTC U0121 Lost Communication With Electronic Brake Control Module (EBCM)

• | DTC U0126 Lost communication with Power Steering Control Module (PSCM)

• | DTC U0140 Lost Communication With Body Control Module (BCM)

• | DTC U0155 Lost communication with Instrument Panel Cluster (IPC)

This vehicle has DTCs which include DTC Symptoms. For more information on DTC Symptoms refer to DTC Symptom Description.

DTC Symptom | DTC Symptom Descriptor
--- | ---
00 | No Additional Information

### Conditions for Running the DTCs

• | Voltage supplied to the modules is in the normal operating voltage range.

• | The vehicle power mode requires serial data communication to occur.

• | The DTC U0001 does not have a current status.

### 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

The module uses a default value for the missing parameter.

### 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, without a repeat of the malfunction.

### Diagnostic Aids

• | A poor connection at the inoperative module may cause this code to set.

• | An improperly powered module may cause this code to set.

### Test Description

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

1. The scan tool communicate with OnStar® via class 2, so for high speed GMLAN data link diagnostic the OnStar® will not be considered.

2. The module which was not communicating due to an open in the GMLAN serial data circuits may have set Loss of Communication DTCs for those modules that it was monitoring.

3. The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.

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 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. Attempt to communicate with the body control module (BCM).

Were you able to communicate with BCM?

Go to Step 3

Go to Step 8

3 | Attempt to communicate with the electronic suspension control (ESC) module. Were you able to communicate with ESC module? | Go to Step 8 | Go to Step 4
4 |

1. Turn OFF the ignition.
2. Disconnect the vehicle OnStar®.
3. Bypass the OnStar® installing jumpers between high speed GMLAN serial data circuits at the OnStar® connector:

• | Connect a jumper between high speed GMLAN bus (+) circuit to BCM and high speed GMLAN bus (+) circuit to the ESC module.

• | Connect a jumper between high speed GMLAN bus (-) circuit to BCM and high speed GMLAN bus (-) circuit to the ESC module.

4. Attempt to communicate with ESC module.

Were you able to communicate with ESC module?

Go to Step 5

Go to Step 7

5 | Test the high speed GMLAN serial data circuits of the OnStar® for poor connections. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs. Did you find and correct the condition? | Go to Step 17 | Go to Step 6
6 | Replace the OnStar®. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 17 | --
7 |

1. Turn OFF the ignition.
2. Test all the high speed GMLAN serial data circuits between BCM and ESC module for an open. Refer to the following:

• | Circuit Testing

• | GMLAN Wiring Repairs

Did you find and correct the condition?

Go to Step 17

Go to Step 8

8 | Important: Use the DTC list from DTC Descriptors to determine the closest module to the data link connector (DLC) which is not communicating. Test the following circuits of the module that is not communicating, for an open or a short to ground:

• | The battery positive circuits

• | The battery voltage output circuits

• | The switched battery positive voltage circuits

Did you find and correct the condition?

Go to Step 17

Go to Step 9

9 |

1. Turn OFF the ignition.
2. Test the ground circuits of the module that is not communicating, for an open. Refer to the following:

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

• | Circuit Testing

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 17

Go to Step 10

10 | Test the high speed GMLAN serial data circuits of the module that is not communicating, for an open. Refer to the following:

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

• | Circuit Testing

• | GMLAN Wiring Repairs

Did you find and correct the condition?

Go to Step 15

Go to Step 11

11 | Test the following circuits of the module that is not communicating, for poor connections:

• | The battery positive voltage circuits

• | The battery voltage output circuits

• | The switched battery positive voltage circuits

• | The ground circuits

• | The GMLAN serial data circuits

Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs.

Did you find and correct the condition?

Go to Step 17

Go to Step 12

12 | Replace the module that is not communicating. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 17 | --
13 | Inspect for poor connections at the harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs. Did you find and correct the condition? | Go to Step 17 | Go to Step 14
14 | Replace the BCM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 17 | --
15 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. Retrieve DTCs from the module that was not communicating.

Does the scan tool display any DTCs which do not begin with a "U"?

Go to Diagnostic System Check - Vehicle

Go to Step 16

16 | Use the scan tool in order to clear the DTCs. Did you complete the action? | Go to Step 17 | --
17 | Retrieve DTCs from the modules which had the Loss of Communications DTC set. Does the scan tool display any DTCs which do not begin with a "U"? | Go to the Diagnostic System Check - Vehicle | Go to Step 18
18 |

1. Use the scan tool in order to clear the DTCs.
2. Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared.

Did you complete the action?

System OK

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