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For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 XLR 🢒 Vehicle Control Systems 🢒 Computer/Integrating Systems 🢒 DTC U1001-U1254

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 (SOH) monitoring. 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 module's 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 2 seconds. When no message is detected from a learned identification number for 5-10 seconds, a DTC U1xxx where xxx is equal to the 3-digit identification number is set.

The control module ID number list below provides a method for determining which module is not communicating. A module with a class 2 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 modules that can communicate will set a DTC indicating the module that cannot communicate. The following modules communicate on class 2 serial data circuit:

Control Module

ID Number

Body Control Module (BCM)

064

Inflatable Restraint Sensing and Diagnostic Module (SDM)

088

Instrument Panel Cluster (IPC)

096

Head Up Display (HUD)

098

Radio

128

Audio Amplifier

129

CD Changer (CDX)

130

Digital Radio Receiver (DRR)

137

Communication Interface Module (OnStar®)

151

HVAC Control Module

153

Driver Door Module (DDM)

160

Passenger Door Module (PDM)

161

Driver Door Switch (DDS)

164

Driver Position Module (DPM)

166

Fold Top Controller (FTC)

177

Remote Control Door Lock Receiver (RCDLR)

193

Steering Column Lock Control Module (SCLCM)

194

The body control module (BCM) emulates some devices that are on high speed GMLAN serial data circuit to the modules on class 2 serial data circuit. The BCM transmits multiple Node Alive messages using source IDs that correspond to each of those high speed GMLAN nodes. This is in addition to the BCM behaving itself as a real node. The following module communicate on high speed GMLAN serial data circuit and are emulated by BCM on class 2 serial data circuit:

Control Module

ID Number

Engine Control Module (ECM)

017

Transmission Control Module (TCM)

024

Distance Sensing Cruise Control (DSCC) Module

033

Electronic Brake Control Module (EBCM)

040

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 table.
  2. The DTC which is reported the most times.
  3. From the lowest number DTC to the highest number DTC.

DTC Descriptor

This diagnostic procedure supports the DTCs U1xxx, where xxx is equal to the 3-digit identification number from the tables above.

DTC Symptom

DTC Symptom Descriptor

00

No Additional Information

Conditions for Running the DTC
    • The following diagnostic trouble codes do not have a current status:
       - U1300
       - U1301
       - U1305
    • 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.

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
    • An intermittent open between the inoperative module and a star connector may cause these codes.
    • A poor connection at the inoperative module or a star connector may cause these codes.
    • An intermittent open in a star connector may cause these codes.
    • An improperly powered module may cause these codes.
    • An improper communication on the high speed GMLAN serial data circuit may cause these codes.

Test Description

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

  1. A module that does not communicate in high speed GMLAN link may cause the setting of the class 2 DTCs.

    2.

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

    3.

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

    4.

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

    5.

  5. The module which was not communicating may have set lost communication DTCs for those modules that it was monitoring.

    6.

  6. 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

Connector End View Reference: Master Electrical Component List

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Record the DTCs set in by each module.

Did you record any DTCs in the range of U2105-U2199 or U0100-U0299?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3

Important:: Use the control module ID number list in order to determine which module is not communicating.

  1. Test the following circuits of the module that is not communicating for an open or short to ground:
    2. • The battery positive voltage circuits
    • The ignition voltage circuits
    • Battery positive voltage output circuits
    • Switched battery positive voltage circuits
  2. If the module that doesn't communicate is the fold top control (FTC), a short to ground on any of the following circuits opens the battery supply voltage circuit fuse. Test the following circuits for short to ground, if necessary:
    3. • Valve 1 control circuit
    • Valve 2 control circuit
    • Valve 3 control circuit
    • Valve 4 control circuit
    • Valve 5 control circuit
    • Pump direction A circuit
    • Pump direction B circuit

Refer to the following:

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

Did you find and correct the condition?

Go to Step 11

Go to Step 4

4

  1. Turn OFF the ignition.
  2. Test the ground circuits of the module that is not communicating for an open. Refer to the following:
    3. •  Control Module References for the applicable schematic

Did you find and correct the condition?

Go to Step 11

Go to Step 5

5

  1. Disconnect the star connector that is connected to the module that is not communicating.
  2. Inspect for poor connections at the star connectors. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Step 6

6

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

Did you find and correct the condition?

Go to Step 9

Go to Step 7

7

Test the following circuits of the module that is not communicating for poor connections:

    • The battery positive voltage circuits
    • The ignition voltage circuits
    • Battery positive voltage output circuits
    • Switched battery positive voltage circuits
    • Ground circuits
    • Class 2 serial data circuit

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

Did you find and correct the condition?

Go to Step 9

Go to Step 8

8

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

Did you complete the replacement?

Go to Step 11

--

9

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Retrieve DTCs from the module which was not communicating.

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

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 10

10

Use the scan tool in order to clear the DTCs.

Did you complete the action?

Go to Step 11

--

11

Retrieve DTCs from 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 Trouble Code (DTC) List - Vehicle

Go to Step 12

12
  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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