U1128

DTC U1128 Loss Of Serial Communications With IRC:

CIRCUIT DESCRIPTION
Modules connected to the Class 2 data link monitor the link for serial data communications during normal vehicle operation. Operating information (Report messages) and commands (Load messages) are exchanged among the modules. When a module receives a Report or Load message for a critical operating parameter, the module records the Source ID of the module which sent the message for State of Health monitoring (Node Alive messages). A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. Once a Source ID is learned by a module, it will monitor for that Source ID's NODE ALIVE message. Each module on the Class 2 data link which is powered and performing functions that require detection of communications malfunction is required to send a NODE ALIVE message every 2 seconds. When no message is detected from a learned Source ID for 5 seconds, a DTC U1228 is set.

CONDITIONS FOR SETTING THE DTC
^ The voltage supplied to the module is in the normal operating voltage range (approximately 916 volts).
^ DTC U1300 and DTC U1301 are not current
^ A message from a learned Source ID has not been detected for the past 5 seconds.

CONDITIONS FOR CLEARING THE DTC
^ A current DTC U1228 will clear when a NODE ALIVE message from the failed Source ID is detected on the data link or at the end of the current ignition cycle.
^ A history U1228 will clear upon receipt of a scan tool CLEAR DTCs command.

TEST DESCRIPTION
The number(s) below refer to the step number(s) on the diagnostic table.
1. This step verifies that the radio receiver which does not communicate is powered and will not communicate via the serial data link.
2. This test checks for an improper connection to the radio receiver preventing serial communication.
3. This test determines whether the malfunction is due to the serial data circuit or due to a malfunctioning radio receiver.
7. Since the radio receiver, which did not communicate due to a serial data circuit malfunction will have set LOST COMMUNICATION DTCs and the BCM which can communicate indicates that the radio receiver which cannot communicate, it is necessary to clear the DTCs from these modules to avoid future misdiagnosis.