For 1990-2009 cars

### Circuit Description

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 U1064 sets when a controller does not receive specific information from the body control module (BCM) after it had already learned its source ID in that ignition cycle.

### Conditions for Setting the DTC

DTC U1064 will set if the powertrain control module (PCM) or sensing and diagnostic module (SDM) lost Class II communications with the BCM when:

• | The condition exists for longer than 3 seconds.

• | The PCM or SDM already learned the BCM source identifier in the ignition cycle. The BCM had initially communicated with both the PCM and SDM when the ignition was first turned On.

• | The ignition is On.

DTC U1064 diagnostic runs continuously with ignition On.

DTC U1064 is a type C DTC.

### Diagnostic Aids

To locate an intermittent problem, use a scan tool to monitor the BCM located under the DIAGNOSTIC CIRCUIT CHECK---CLASS II MESSAGE MONITOR with the ignition On and the engine Off. INACTIVE will be displayed if the BCM is not communicating on the Class II link.

If an intermittent loss of battery power to the BCM via CKT 2540 exists, scan tool communication with the BCM will be lost and the engine will not start.

If an intermittent loss of ignition power to the BCM via CKT 339 exists, scan tool communication with the BCM will NOT be lost; however, the engine will stall and not restart

If CKT 743 or 1640 is open, communications with the BCM will exist and the engine will start and run normally.

If an intermittent open in CKT 550 will cause a lack of BCM communication and the engine will not start.