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U185B




DTC U185B

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 Descriptor
DTC U185B 00
- Battery Energy Control Module Lost Communication with Hybrid Powertrain Control Module 2 on High Voltage Energy Management Communication Bus

Diagnostic Fault Information






Circuit/System Description

Control modules connected to the serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the control modules. The control modules have prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. Each message contains the identification number of the transmitter module.

This diagnostic is used to check the loss of communication with the hybrid powertrain control module 2 on high voltage energy high speed GMLAN bus. The battery energy control module supervises a periodic serial data message to check the communication status of the hybrid powertrain control module 2. If this message is not received within 1 second, this DTC is set.

Conditions for Running the DTC

* The control module is awake and communicating.
* The system voltage is at least 9 V.

Conditions for Setting the DTC

A supervised periodic message that includes the transmitter module availability has not been received within 1 second.

Action Taken When the DTC Sets

* No Cell Balancing function. However, cell balancing switch diagnostics still run.
* DTC U185B is a type B DTC.

Conditions for Clearing the DTC

DTC U185B is a type B DTC.

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 control 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.
* Communication may be available between the control module and the scan tool with one or more GMLAN serial data systems inoperative. This condition is due to the control module using multiple GMLAN busses.
* Use Data Link References Initial Inspection and Diagnostic Overview to determine what GMLAN serial data communications the non communicating control module uses.
* The control module 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 [1][2]Electrical Diagrams
* Control Module References Programming and Relearning

Connector End View Reference

Component Connector End Views Hybrid/EV Battery Pack X1
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
Special Tools

EL-48900 - HEV Safety Kit

For equivalent regional tools, refer to Special Tools Tools and Equipment.

Circuit/System Verification

Verify that DTC U007A, U1814, B1428, B1370, B1380, B1440, B1441, B1325, or P0562 is not set.

If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle Diagnostic Trouble Code (DTC) List - Vehicle to diagnose those DTCs prior to this diagnostic.

Circuit/System Testing

Danger: Always perform the High Voltage Disabling procedure prior to servicing any High Voltage component or connection. Personal Protection Equipment (PPE) and proper procedures must be followed.


The High Voltage Disabling procedure will perform the following tasks:


* Identify how to disable high voltage.

* Identify how to test for the presence of high voltage.

* Identify condition under which high voltage is always present and personal protection equipment (PPE) and proper procedures must be followed.


Before working on any high voltage system, be sure to wear the following Personal Protection Equipment:


* Safety glasses with appropriate side shields when within 15 meters (50 feet) of the vehicle, either indoors or outdoors.

* Certified and up-to-date Class "0" Insulation gloves rated at 1000V with leather protectors.


- Visually and functionally inspect the gloves before use.

- Wear the Insulation gloves with leather protectors at all times when working with the high voltage battery assembly, whether the system is energized or not.


Failure to follow the procedures exactly as written may result in serious injury or death.


Danger: The Volt Battery Pack will utilize an exchange program. Please consult the most recent revision of bulletin/PI #PIP4841, available in Service Information (SI), for a list of approved Volt Battery Pack service procedures. Components that may be removed and serviced without exchanging the complete battery pack are identified in the bulletin/PI. Please contact the GM Technical Assistance Center (1-877-446-8227) if you have any questions.


Note: Use the schematic to identify the following:

* The control module locations on high voltage energy management high speed GMLAN serial data circuits
* The control module B+, ignition, ground, accessory wake up serial data, and serial data circuit terminals

1. Vehicle OFF, disconnect the harness connector at the K114B hybrid powertrain control module 2.
2. Test for less than 10 ohm between each ground circuit terminal and ground.

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

3. If equipped, vehicle in Service Mode, verify that a test lamp illuminates between each B+ circuit terminal 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 K114B hybrid powertrain control module 2 for a short to ground. If the circuits test normal, replace the K114B hybrid powertrain control module 2.

4. If equipped, vehicle in Service Mode, verify that a test lamp illuminates between each ignition circuit terminal and ground.

• If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the K114B hybrid powertrain control module 2 for a short to ground. If the circuits test normal, replace the K114B hybrid powertrain control module 2.

5. If equipped, vehicle in Service Mode, verify that a test lamp illuminates between the accessory wake up serial data circuit terminal 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 a short to ground. If the circuits test normal, replace the K9 BCM.

6. Vehicle OFF, disconnect the X1 harness connector at the A4 hybrid battery pack.
7. Vehicle OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 2 ohm in each high voltage energy serial data circuit between the K114B hybrid powertrain control module 2 and the A4 hybrid battery pack.

• If greater than the specified range, test the serial data circuit for an open/high resistance between the K114B hybrid powertrain control module 2 and the A4 hybrid battery pack.

8. Vehicle OFF, disable the high voltage system. Refer to High Voltage Disabling High Voltage Disabling.
9. Reconnect the 12 V battery.
10. Vehicle OFF, disconnect the X1 harness connector at the K16 battery energy control module (BECM).
11. Vehicle OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 2 ohm in each high voltage energy serial data circuit between the A4 hybrid battery pack and the K16 BECM.

• If greater than the specified range, replace the drive motor battery wire junction block relay.

12. If all circuits test normal, replace the K114B hybrid powertrain control module 2.

Repair Instructions

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

* GMLAN Wiring Repairs GMLAN Wiring Repairs
* Drive Motor Battery Wire Junction Block Relay Replacement Drive Motor Battery Wire Junction Block Relay Replacement
* Control Module References Programming and Relearning for control module replacement, programming and setup

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