GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Safari - 2WD 🢒 DTC 🢒 DTC U1001-U1254
Table 1: Control Module ID Numbers

Circuit Description

Modules connected to the class 2 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 . These messages are also known as 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 class 2 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 two seconds. When no message is detected from a learned identification number for five seconds, a DTC U1xxx is set.

The control module ID number list provides a method for determining which module is not communicating. A module with an internal class 2 serial data circuit malfunction or which loses power during the current ignition cycle would have a lost communication DTC set by other modules. Use the control module ID number list in order to determine which module is not communicating and the DTC U1001-U1254 diagnostic table in order to diagnose the malfunction.

Control Module ID Numbers

Control Module

ID Number

PCM

016

TCSCM

026

EBCM

041

BCM

064

SDM

088

IPC

096

DIC

097

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

Conditions for Running the DTC
    •  Voltage supplied to the module is in the normal operating voltage range of 9-16 volts.
    •  DTCs U1300, U1301 or U1305 do not have a current status.
    •  The vehicle power mode 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.

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.

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 DTC from these modules to avoid future misdiagnosis.

Step

Action

Yes

No

Schematic Reference: Data Link Connector Schematics

1

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

    • Battery voltage
    • Ignition voltage
         Refer to the following procedures:
      •  Control Module References in Body Control Systems 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 procedures:

    •  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 gStep 3

3

  1. Remove the comb from SP261.
  2. Inspect for poor connections at SP261. 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 splice pack SP261. 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 5

5

Inspect for poor connections/terminal tension at the circuits of the module that is not communicating:

    • The battery voltage
    • The ignition voltage
    • The ground
    • The class 2
        Refer to the following procedures:
       -  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 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 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 Control Module References in Body Control System for the applicable Diagnostic System Check

Go to Step 8

8

Use the scan tool in order to clear the DTCs

Did you clear all DTCs?

Go to Step 9

--

9

Select the Display DTCs function for the modules which had the DTC set between U1001 and U1254.

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. 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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