Diesel Data: Automotive Repair

U1128




DTC U1128

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.

* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC U1128 00

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Lost Communication with Park Brake Control Module

DTC U1128 71

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Lost Communication with Park Brake Control Module Invalid Data

DTC U1128 72

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Lost Communication with Park Brake Control Module Message Counter Incorrect

DTC U1128 74

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Lost Communication with Park Brake Control Module Bus Signal Checksum Error

Diagnostic Fault Information






Circuit/System Description

The serial data circuit is the means by which the control modules in the vehicle communicate with each other. Once the scan tool is connected to the serial data circuit through the data link connector (DLC), the scan tool can be used to monitor each control module for diagnostic purposes and to check for diagnostic trouble codes (DTCs). When the ignition switch is in RUN, each control module communicating on the serial data circuit sends a state of health (SOH) message to ensure that the control module is operating properly. When a control module stops communicating on the serial data circuit, for example if the control module loses power or ground, the SOH message it normally sends on the serial data circuit disappears. Other control modules on the serial data circuit, which expect to receive that SOH message, detect its absence; those control modules in turn set a DTC associated with the loss of SOH of the non-communicating control module. The DTC is unique to the control module which is not communicating and one or more control modules may set the same exact code. A loss of serial data communications DTC does not represent a failure of the control modules that contain the stored code.

Conditions for Running the DTC

* The system voltage is between 9-16 V.

* The vehicle power mode master requires serial data communication to occur.

Conditions for Setting the DTC

A supervised periodic message that includes the transmitter module availability has not been received.

Action Taken When the DTC Sets

The control module(s) is never signaled, therefore the specific subsystem(s) will not function.

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 of 50, without a repeat of the malfunction.

Diagnostic Aids

* Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:

- A control module on the data communication circuit was disconnected while the communication circuit is awake.

- Power to one or more modules was interrupted during diagnosis.

- A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.

- Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.

- If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.

* A control module may have a U-code stored in history that does not require any repairs. Issues with late or corrupted messages between control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a control module based only on a history U-code.

* Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue.

* Some control modules may not have internal protection for specific control circuits and may open a B+ or ignition fuse. If a fuse is open and the B+ or ignition circuit is not shorted to ground, ensure none of the control circuits are shorted to ground before replacing the control module.

Reference Information

Schematic Reference

* Data Communication Schematics Electrical Diagrams

* Control Module References Programming and Relearning

Connector End View Reference

Component Connector End Views 1-2 Shift Solenoid (SS) Valve (MN5)

Description and Operation

Data Link Communications Description and Operation Description and Operation

Electrical Information Reference

* Circuit Testing Circuit Testing

* Connector Repairs Connector Repairs

* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections

* Wiring Repairs Wiring Repairs

Scan Tool Reference

Control Module References Programming and Relearning for scan tool information

Circuit/System Testing

Note:
Use the schematic to identify the following:

* The K83 parking brake control module location on the high speed GMLAN serial data circuits

* The K83 parking brake control modules B+, ignition, ground, serial data communication enable, and serial data circuit terminals

1. Ignition OFF, disconnect the harness connector at the K83 parking brake control module.

2. Ignition OFF and all vehicle systems OFF, key out of the ignition switch, all access doors closed. Test for less than 10 ohms between the ground circuit terminal H and ground.

If greater than the specified range, test the ground circuit for an open/high resistance.

3. Ignition ON, verify that a test lamp illuminates between the B+ circuit terminal A and ground.

If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the K83 parking brake control module for a short to ground. If the circuits test normal, replace the K83 parking brake control module.

4. Ignition ON, verify that a test lamp illuminates between the serial data communication enable circuit terminal P and ground.

If the test lamp does not illuminate, test the entire circuit, including the B+ circuit to the K9 body control module (BCM), for an open/high resistance or short to ground. If the circuits test normal, replace the K9 BCM.

5. Ignition OFF, disconnect the harness connector at the K17 electronic brake control module.

6. Ignition OFF and all vehicle systems OFF, key out of the ignition switch, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 2 ohms in each high speed GMLAN serial data circuit between the K83 parking brake control module and the K17 electronic brake control module:

* K83 parking brake control module serial data circuit terminal F and K17 electronic brake control module serial data circuit terminal 11

* K83 parking brake control module serial data circuit terminal L and K17 electronic brake control module serial data circuit terminal 14

If greater than the specified range, test the serial data circuit for an open/high resistance between the K83 parking brake control module and the K17 electronic brake control module.

7. If all circuits test normal, replace the K83 parking brake control module.

Repair Instructions

Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.

* GMLAN Wiring Repairs GMLAN Wiring Repairs

* Control Module References Programming and Relearning for control module replacement, programming and setup