For 1990-2009 cars

### Circuit Description

Modules connected to the GMLAN serial data circuit monitor for serial data communications on the GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. The body control module (BCM) maintains a transmit error counter (TEC) and a receive error counter (REC). The counter values increase with detected errors and will decrease with error free messages.

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC U2100 CAN Bus Communication Malfunction (Bus Off)

### Conditions for Running the DTC

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

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

### Conditions for Setting the DTC

A certain number of not valid transmitted messages on the GMLAN serial data circuits are detected by the module. 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 the high speed GMLAN serial data circuits open.

### Action Taken When the DTC Sets

• | The module suspends all message transmission.

• | The module uses default values for all parameters received on the high speed GMLAN serial data circuits.

• | The module inhibits the setting of all other high speed GMLAN communication DTCs.

### 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

The BCM can set this DTC as current because it is the gateway between the class 2 and high speed GMLAN serial data circuits, and the BCM diagnostics is performed on class 2 data link.

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 BCM 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.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Data Communication Schematics or Body Control System Schematics Connector End View Reference: Component Connector End Views
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Turn the ignition ON, with the engine OFF. Did you record DTC U2100 as current? | Go to Step 4 | Go to Step 3
3 | 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 high speed GMLAN serial data circuits for:

• | An intermittent short to ground

• | An intermittent short to voltage

• | An intermittent short between ground and voltage

• | An intermittent open on any of the circuits

Did you find and correct the condition?

Go to Step 15

Go to Step 6

4 |

1. Turn OFF the ignition.
2. Disconnect the farthest module from the DLC that is connected to high speed GMLAN serial data circuits.
3. Turn ON the ignition, with the engine OFF.
4. Retrieve DTCs from BCM.

Does the DTC still set as current?

Go to Step 5

Go to Step 9

5 |

1. Turn OFF the ignition.
2. Disconnect the farthest module from the DLC that is connected to high speed GMLAN serial data circuits.
3. Turn ON the ignition, with the engine OFF.
4. Retrieve DTCs from BCM.

Does the DTC still set as current?

Go to Step 7

Go to Step 6

6 | Test the high speed GMLAN serial data circuits between the last two disconnected modules for:

• | A short to ground

• | A short to battery

• | A short between ground and battery

• | An Open

Did you find and correct the condition?

Go to Step 12

Go to Step 9

7 | Is the BCM the last module still connected to high speed GMLAN data circuits? | Go to Step 8 | Go to Step 5
8 |

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

• | A short to ground

• | A short to battery

• | A short between ground and battery

• | An open

Did you find and correct the condition?

Go to Step 12

Go to Step 10

9 | Test each of the high speed GMLAN circuits of the last disconnected module to the next module closer to DLC for an open. Did you find and correct the condition? | Go to Step 12 | Go to Step 10
10 | Inspect for poor connections at the high speed GMLAN serial data circuits of the suspected module. Did you find and correct the condition? | Go to Step 12 | Go to Step 11
11 | Replace the suspected module. Did you complete the replacement? | Go to Step 12 | --
12 |

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 high 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 13

13 | Did you record and DTCs which do not begin with a "U"? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 14
14 | Did you diagnose all of the DTCs? | Go to Step 15 | Go to Step 12
15 | Use the scan too in order to clear the DTCs. Did you complete the action? | System OK | --