GM Service Manual Online
For 1990-2009 cars only

Circuit Description

Modules connected to the serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for State of Health monitoring (Node Alive messages). A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. Once an identification number is learned by a module, it will monitor for that modules "Node Alive" message. Each module on the serial data circuit which is powered and performing functions that require detection of a communications malfunction is required to send a "Node Alive" message every 2 seconds. When no message is detected from a learned identification number for 5 seconds, a DTC U1xxx , where xxx is equal to the 3 digit identification number, is set.

The Control Module ID Number list provides a method for determining which module is not communicating. A module with an internal serial data circuit malfunction or which loses power during the current ignition cycle would have a Lost Communication DTC set by other modules. The modules that can communicate will set a DTC indicating the module that cannot communicate. When no message is detected from a learned identification number for five seconds, a DTC U1xxx, where xxx is equal to the 3 digit identification number, is set.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

DTC U1XXX Lost Communication With XXX Module

Control Module

ID Number

BCM

064

DRR

137

EBCM

040

PCM

016

Radio

128

SDM

088

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

  1. Current DTCs before history DTCs unless told otherwise in the diagnostic tables.
  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 module is in the normal operating voltage range of 9 to 16 volts.
    • Diagnostic trouble codes U1300, U1301 and U1305 do not have a current status.
    • The vehicle power mode, ignition switch position, requires serial data communication to occur.

Conditions for Setting the DTC

A message from a learned identification number has not been detected for the past 5 seconds.

Test Description

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

  1. A module which loses power during an ignition cycle will cause other modules to set Lost Communication DTCs.

  2. A module which loses ground during an ignition cycle will cause other modules to set Lost Communication DTCs.

  3. The malfunction is due to an open in the serial data circuit or an open in the module.

  4. The module which was not communicating may have set Lost Communication DTCs for those modules that it was monitoring.

  5. The modules which can communicate indicate the module which cannot communicate. You must clear the DTCs 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

Test the battery positive voltage and ignition B+ circuits of the module that is not communicating for an open or a short to ground. Refer to the following:

    •  Circuit Testing
    •  Wiring Repairs

Did you find and correct the condition?

Go to Step 8

Go to Step 2

2

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

Did you find and correct the condition?

Go to Step 8

Go to Step 3

3

Test the serial data circuit of the module that is not communicating for an open or short to ground. Refer to Circuit Testing and Wiring Repairs.

Did you find and correct the condition?

Go to Step 6

Go to Step 4

4

Inspect for poor connections at the battery positive voltage circuits, the ignition B+ circuits, the ground circuits , and the serial data circuit of the module that is not communicating. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs.

Did you find and correct the condition?

Go to Step 6

Go to Step 5

5

Replace the module which is not communicating. Refer to Control Module References for replacement, setup and programming for the appropriate repair instructions.

Did you complete the replacement?

Go to Step 8

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6

  1. Install a scan tool
  2. Turn ON the ignition leaving 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 Diagnostic System Check - Vehicle

Go to Step 7

7

Use the scan tool in order to clear the DTCs.

Did you complete the action?

Go to Step 8

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8

Select the Display DTCs function for the modules which had the Lost Communication with xxx DTC set.

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

Go to Diagnostic System Check - Vehicle

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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