Diagnostic Instructions
DTC Descriptors
DTC U1866: Battery Energy Control Module Lost Communication With Body Control Module
DTC U1886: Battery Energy Control Module Lost Communication With Engine Control Module/Powertrain Control Module
DTC U1897: Starter/Generator Control Module Lost Communication With Battery Energy Control Module
DTC U1899: Starter/Generator Control Module Lost Communication With Engine Control Module/Powertrain Control Module
Diagnostic Fault Information
Circuit
| Short to Ground
| Open/High Resistance
| Short to Voltage
|
Battery Positive Voltage Circuits
| U1866-U1899
| U1866-U1899
| --
|
Ignition Voltage Circuits
| U1866-U1899
| U1866-U1899
| --
|
Module Ground Circuits
| --
| U1866-U1899
| --
|
Circuit Description
Modules connected to the CAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The 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.
Conditions for Running the DTC
| • | Voltage supplied to the modules is in the normal operating voltage range. |
| • | The vehicle power mode 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 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 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. |
| • | Communication may be available between the BCM and the scan tool with either the low or high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the low and high speed GMLAN systems. |
| • | Use to determine if the module uses high or low speed GMLAN serial data communications. |
| • | 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. |
| • | This diagnostic can be used for any control module that is not communicating, regardless of the type of serial data circuit it is connected to, providing the vehicle is equipped with the control module. |
Reference Information
Schematic Reference
Data Communication Schematics
Connector End View Reference
Component Connector End Views
Electrical Information Reference
Circuit/System Testing
- Using the DTC Descriptor and Diagnostic Aids above, determine the module that is not communicating that should be tested.
- Ignition OFF, disconnect the harness connector of the module that is not communicating.
- Ignition ON, verify that a test lamp illuminates between all battery positive voltage circuits and ground.
| ⇒ | If the test lamp does not illuminate, repair the circuit for a short to ground or an open/high resistance. If the circuit fuse is open, also test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace
the faulty module. |
- Verify that a test lamp illuminates between all ignition voltage circuits and ground.
| ⇒ | If the test lamp does not illuminate, repair the circuit for a short to ground or an open/high resistance. If the circuit fuse is open, also test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace
the faulty module. |
- Test for less than 1 ohm between the module ground circuits and ground.
| ⇒ | If greater than 1 ohm, repair the ground circuit for open/high resistance. |
- Test for less than 1 ohm in the CAN serial data circuit(s) between the non-communicating module and the module that displays the DTC.
| ⇒ | If greater than 1 ohm, test the CAN serial data circuit(s) for open/high resistance. |
- If all circuits test normal, replace the module that is not communicating.
Repair Instructions
Perform the
Diagnostic Repair Verification after completing the diagnostic procedure.
