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 buss 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 data link connector (DLC) end of the link and the other is at the engine management module, engine control module (ECM) or powertrain control module (PCM), end of the link. The DLC terminating resistor resides internally in the BCM. When testing for a short between high speed GMLAN (+) and (-) with the engine management module removed, a reading off 120 ohms is normal. However, to completely analyze the buss the body control module (BCM) should be temporally removed when testing for shorts between the high speed GMLAN (+) and (-) while also making sure that all modules and the scan tool are disconnected from the buss. A normal reading across the high speed GMLAN buss with the terminating resistor at the DLC remove and all module and scan tool disconnected is infinite ohms. Make certain to reconnect the BCM after testing, it is necessary for proper vehicle powermoding.

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

### 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.

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

3. Data link connector terminals 6 and 14 provide the connection to the GMLAN serial data high 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 GMLAN serial data circuits?

Go to Step 3

Go to Step 6

3 | Did you record any DTCs in the range of U2105 to U2177? | DTC U0100-U0299 or U2105-U2199 | Go to Step 4
4 | Did you record DTC U0001? | Go to DTC U0001 | Go to Step 5
5 | 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 them

Refer to the following:

• | Testing for Intermittent Conditions and Poor Connections

• | Circuit Testing

• | Connector Repairs

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 19

Go to Diagnostic Aids

6 | 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 16

Go to Step 7

7 | 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 16 | Go to Step 8
8 |

1. Turn OFF the ignition.
2. Disconnect the scan tool from the data link connector (DLC).
3. 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 Diagnostic System Check - Vehicle

Go to Step 9

9 |

1. Disconnect the farthest module from the DLC that is connected to high speed GMLAN serial data circuits.
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 14

Go to Step 10

10 |

1. Turn OFF the ignition.
2. Disconnect the farthest module from the DLC that is still connected to high speed GMLAN serial data circuits.
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 11

Go to Step 12

11 | Important: There are two 120 ohms terminating resistors that connect between the (+) and (-) high speed GMLAN buss. Refer to Diagnostic Aids. Test the high speed GMLAN (+) and (-) serial data circuits between the last 2 disconnected modules for:

• | A short to ground

• | A short to battery

• | A short between GMLAN (+) and GMLAN (-)--Refer to Diagnostic Aids.

Did you find and correct the condition?

Go to Step 16

Go to Step 14

12 | Is the BCM the last module still connected to high speed GMLAN data circuits? | Go to Step 13 | Go to Step 10
13 |

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 and between BCM and DLC for the following:

• | Short to ground

• | Short to battery

• | Short between high speed GMLAN (+) and high speed GMLAN (-)--Refer to Diagnostic Aids.

• | An open

Did you find and correct the condition?

Go to Step 16

Go to Step 14

14 | Inspect for poor connections at the high speed GMLAN serial data circuits 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 16 | Go to Step 15
15 | 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 16 | --
16 |

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 17

17 | Did you record any DTCs which do not begin with a "U"? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 18
18 | Did you diagnose all of the DTCs? | Go to Step 19 | Go to Step 16
19 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | --