GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 Safari - 2WD 🢒 DTC 🢒 DTC U1001-U1254

Circuit Description

Each module connected to the class 2 serial data circuit monitors serial data communication during normal vehicle operation. Operating parameters and other information 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 U1026 Lost Communications With Transfer Case Control Module (TCCM)
    • 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 U1097 Lost Communications With Driver Information Center (DIC)

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 with a learned identification number within the last 5 seconds.

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

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 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.

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

  1. The malfunction is due to an open in the class 2 serial data circuit or an open in the module.

  1. The module which was not communicating due to an open in the class 2 serial data circuit may have set lost communication DTCs for those modules that it was monitoring.

  1. 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

1

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

    • Switched battery input voltage
    • Ignition input voltage
    • Ignition output voltage

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 9

Go to Step 2

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 9

Go to Step 3

3

Inspect for poor connections at the star connector. 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 7

Go to Step 4

4

Test the class 2 circuit of the module that is not communicating for an open between the module and the star connector. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 7

Go to Step 5

5

Inspect for poor connections at the following locations, for the module that is not communicating:

    • Switched battery input voltage circuits
    • Ignition input voltage circuits
    • Ignition output voltage circuits
    • Ground circuits
    • Class 2 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 7

Go to Step 6

6

Important: Replace and reprogram the module which is not communicating. Refer to Control Module References in Body Control System for the appropriate repair instructions.

Did you complete the replacement?

Go to Step 9

--

7
  1. Connect 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 in Body Control System for the applicable Diagnostic System Check

Go to Step 8

8

Clear the DTCs with the scan tool.

Did you clear all DTCs?

Go to Step 9

--

9

Select the Display DTCs function for the modules which had the Lost Communications with XXX DTC set.

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

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

Go to Step 10

10
  1. Clear the DTCs with the scan tool.
  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

--