U1000 - Class II Data Link Malfunction (ID Not Learned)
Schematic:
CIRCUIT DESCRIPTION
Class II serial data is a 0 to 7 volt variable pulse width message scheme (logic 0's and logic 1's) used to communicate information between modules and for communication to an off-board Scan tool. Contained in each module memory is a list of serial data messages and default actions taken by the module if one of the important messages is not received. One of the most important messages is the state of health (SOH) message used to inform each module that other modules on the Class II link are functioning correctly.
Each module on the Class II link must first "learn" all modules connected to the Class II link every time the ignition is turned On. Each message that a module sends out on the link contains a source identifier used to identify where the information came from. Once a module receives information, it will store the attached controller ID. If this same information is not received later in the ignition cycle, lost communications with a specific controller DTC will set. DTC U1000 sets when a controller does not receive specific information when the source identifier was not learned.
DTC PARAMETERS
DTC U1000 will set if the BCM, PCM, SDM, auto climate control module (if equipped) or ONSTAR CIM (if equipped) did not received information from another Class II module when:
^ Condition exists for longer than 6 seconds for BCM, PCM or SDM and longer than 5 seconds for auto climate control module or ONSTAR CIM
^ One of the above modules was not able to "learn" the source identifier of another Class II module that it relies on for information.
^ Ignition is On
Possible combinations for U1000:
6-CYL (L81)
^ BCM cannot communicate with the I/P cluster, SDM or ONSTAR CIM
^ SDM cannot communicate with the BCM, ECM** or I/P cluster
^ Auto climate control module (if equipped) cannot communicate with the BCM or ECM
^ ONSTAR CIM (if equipped) cannot communicate with the BCM, ECM**, I/P cluster or SDM
DTC U1000 diagnostic runs continuously with ignition On.
DTC U1000 is a type C DTC.
DIAGNOSTIC AIDS
IMPORTANT:
^ If only one other U-type DTC is set, diagnose that DTC first. If two of more different U-type DTCs are set, perform the "Vehicle On-Board Diagnostic System Check" located.
^ ** L81 vehicles only: If the ECM is not communicating over the CAN link with the BCM as soon as the ignition is turned On, the SDM, auto climate control module (if equipped) and ONSTAR CIM (if equipped) will set U1000. If the ECM looses communication with the BCM after the ignition was turned On, these modules will set U1016 and the BCM will set U2105. Diagnose U2105 first.
This DTC indicates that a module did not communicate with another module(s) when the ignition was first turned On.
The Class II circuit is in a ring configuration. An individual open in circuit 1807 will NOT affect communications.
Check for module connector tightness and broken connector locks.
To locate an intermittent problem, use Scan tool to monitor each individual controller located under DIAGNOSTIC CIRCUIT CHECK---CLASS II MESSAGE MONITOR with ignition On, engine Off. If a controller is not communicating on the Class II link, either the controller name will not appear or INACTIVE will be displayed.
TEST
