GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2004 🢒 Rendezvous - AWD 🢒 DTC 🢒 DTC U2105

Circuit Description

Modules connected to the 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 and 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. When a message that indicates the availability of the transmitter module is not received, the receiver module sets a DTC 21xx where xx is equal to the 2-digit identification number of the transmitter module.

The control module ID number list below provides a method for determining which module is not communicating. A module with a GMLAN serial data circuit malfunction or which loses power during the current ignition cycle will have a Loss of Communication DTC set by other modules that depend on information from that failed module. The module that can communicate will set a DTC indicating the module that can not communicate.

Control Module

ID Number

Engine Control Module (ECM)

05

When more than one Loss of Communication DTC is set in either 1 module or in several modules, diagnose the DTCs in the following order:

  1. Current DTCs before history DTCs unless told otherwise in the diagnostic table.
  2. The DTC which is reported the most times.
  3. From the lowest number DTC to the highest number DTC.

Conditions for Running the DTC
    • Voltage supplied to the modules is in the normal operating voltage range.
    • The vehicle power mode requires serial data communication to occur.
    • The DTC U2100 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 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.

    2.

  2. 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

1
  1. Turn the ignition OFF.
  2. Test the following circuits of the module that is not communicating for an open or a short to ground.
    3. • The battery positive circuits
    • The battery voltage output circuits
    • The ignition positive voltage circuits
    • The ignition voltage output circuits

Refer to Data Link Connector Schematics for the applicable schematic. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 8

Go to Step 2

2

Test the ground circuits of the module that is not communicating for an open. Refer to Data Link Connector Schematics for the applicable schematic. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 8

Go to Step 3

3

Test the GMLAN serial data circuits of the module that is not communicating for an open. Refer to Data Link Connector Schematics for the applicable schematic. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 6

Go to Step 4

4

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 ignition positive voltage circuits
    • The ignition voltage output circuits
    • The ground circuits
    • The GMLAN serial data circuits

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

Did you find and correct the condition?

Go to Step 8

Go to Step 5

5

Important: Perform the module setup procedure if required.

Replace the module that is not communicating. Refer to Control Module References in Body Control Systems.

Did you complete the replacement?

Go to Step 8

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6

  1. Install a scan tool.
  2. Turn the ignition ON, with the engine OFF.
  3. Select the Display DTCs function for the module which was not communicating.

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

Go to the Control Module References for the applicable Diagnostic System Check

Go to Step 7

7

Use the scan tool in order to clear the DTCs.

Did you complete the action?

Go to Step 8

--

8

Select the Display DTCs function for 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 Control Module References for the applicable Diagnostic System Check

Go to Step 9

9
  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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