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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 Savana - AWD 🢒 Restraints 🢒 SIR 🢒 DTC U1001-U1254

Circuit Description

Each module connected to the class 2 serial data circuit monitors serial data communication 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. Once a module learns another modules identification number, it will monitor the serial data for that modules node alive message. Each module that sends critical operating parameters is required to send a node alive message every two seconds. If no message is detected from a learned module for five seconds, DTC U1001-U1254 will set.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC U1016 Lost Communications With Powertrain Control Module (PCM)
    • DTC U1041 Lost Communications With Electronic Brake Control Module (EBCM)
    • DTC U1064 Lost Communications With Body Control Module (BCM)
    • DTC U1088 Lost Communications With Sensing and Diagnostic Module (SDM)
    • DTC U1096 Lost Communications With Instrument Panel Cluster (IPC)
    • DTC U1128 Lost Communications With Radio
    • DTC U1151 Lost Communications With Onstar (VCIM)
    • DTC U1176 Lost Communications With Remote Function Actuation (RCDLR)

Conditions for Running the DTC
    •  Battery Voltage is 9-16 volts.
    •  DTCs U1300, U1301 or U1305 are not current.
    •  The vehicle power mode requires serial data communication to occur.

Conditions for Setting the DTC

A node alive message has not been received from a module within the last 5 seconds after establishing class 2 serial data communication.

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 ignition is cycled, and the malfunction is no longer present.
    • A history DTC clears when the ignition has been cycled 100 times, without a repeat of the malfunction.

Diagnostic Aids
    • An intermittent open between a module and a star connector may cause this DTC to set.
    • A poor connection at a module or a star connector may cause this DTC to set.
    • An intermittent open in a star connector may cause this DTC to set.
    • An open voltage or ground circuit to a module may cause this DTC to set.
    • An internal module malfunction may cause this DTC to set.

When more than one loss of communication DTC is set in a single module or among multiple modules, diagnose the DTCs in the following order:

  1. Current DTCs before history DTCs, except as specified in the diagnostic tables.
  2. The DTC which is reported the most times.
  3. From the lowest number DTC to the highest number DTC

Test Description

The number below refers to the step number on the diagnostic table.

  1. The module which was not communicating on the class 2 serial data circuit may have set Loss of Communication DTCs for those modules that it was monitoring.

Step

Action

Yes

No

Schematic Reference: Data Link Connector Schematics

Connector End View Reference: Master Electrical Component List and Inline Harness Connector End Views in Wiring Systems

1

Test the following circuits of the module that is not communicating for an open or a short to ground:

    • The battery positive voltage input circuits
    • The battery positive voltage output circuits
    • The ignition voltage input circuits
    • The ignition voltage output circuits
    • The switched battery positive voltage circuits

Refer to the following:

    •  Control Module References in Body Control System for the applicable schematic
    •  Circuit Testing in Wiring Systems
    •  Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 6

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:

    •  Control Module References in Body Control System for the applicable Schematic
    •  Circuit Testing in Wiring Systems
    •  Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 6

Go to Step 3

3

Test the class 2 serial data circuits of the module that is not communicating for an open. 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

Inspect the harness connectors of the module that is not communicating for poor connections and terminal tension at the following circuits:

    • The battery positive voltage input circuits
    • The battery positive voltage output circuits
    • The ignition voltage input circuits
    • The ignition voltage output circuits
    • The switched battery positive voltage supply circuits
    • The ground circuits
    • The class 2 serial data circuits

Refer to the following:

    •  Control Module References in Body Control System for the applicable schematic
    •  Connector Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 6

Go to Step 5

5

Important: Perform the programming or set up procedure for the module, if required.

Replace the module that is not communicating. Refer to Control Module References for the applicable replacement procedure.

Did you complete the replacement?

Go to Step 6

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6

  1. Install a scan tool.
  2. Turn ON the ignition, 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 Control Module References for the applicable Diagnostic System Check

Go to Step 7

7

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 Control Module References for the applicable Diagnostic System Check

Go to Step 8

8
  1. Clear the DTCs with the scan tool.
  2. Continue diagnosing or clearing the DTCs until all of the modules have been diagnosed and all of the DTCs have been cleared.

Did you complete the action?

Go to Control Module References for the applicable Diagnostic System Check

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