Class II serial data is a 0-7 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, EBTCM, PCM or SDM did not receive information from another Class II module when:
| • | Conditions exist for longer than 3 seconds for BCM, 5 seconds for EBTCM, 7 seconds for PCM and 5 seconds for SDM. |
| • | BCM, EBTCM, PCM or SDM 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:
| • | BCM cannot communicate with the PCM or SDM. |
| • | EBTCM cannot communicate with the PCM. |
| • | PCM cannot communicate with the EBTCM or BCM. |
| • | SDM cannot communicate with the BCM or I/P cluster. |
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 or more other U-type DTCs are set, perform the Vehicle On-Board Diagnostic (OBD) System Check (Chart 1 of 2) .
This DTC indicates that a module did not communicate with another modules 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 the scan tool to monitor each individual controller located under DIAGNOSTIC CIRCUIT CHECK- - - CLASS II MESSAGE MONITOR with ignition On, engine Off. INACTIVE will be displayed if a controller is not communicating on the Class II link.
