Circuit Description
Modules connected to the class 2 serial data link monitor the link for
serial data communications during normal vehicle operation. Operating information
and commands are exchanged among the modules. In addition to this, Node Alive
(State of Health) messages are transmitted by each module on the class 2 serial
data link about once every 2 seconds. When the module does not detect
a message on the class 2 serial data link that the module expects to receive,
DTC U1255 is set.
Conditions for Setting the DTC
| • | Voltage supplied to the module is in the normal operating voltage
range (approximately 9-16 volts). |
| • | A class 2 message that the module expects to receive is not detected
on the data link. |
Conditions for Clearing the DTC
| • | A current U1255 will clear when a valid class 2 message is detected
on the class 2 serial data link or at the end of the current ignition cycle. |
| • | A history U1255 will clear upon receipt of a scan tool Clear DTCs
command. |
Diagnostic Aids
| • | Diagnostic of a current U1255 is accomplished via the symptom,
Scan Tool will not Communicate with a class 2 Device. |
| • | An intermittent condition is likely to be caused by a short to
ground on the class 2 serial data link. |
| • | When this DTC is set by the body control module (BCM), the symptom
observed by the customer is likely to be that the engine doesn't run. When
this DTC is set by the BCM and the engine runs, this indicates that an ignition
feed to either the BCM or the vehicle control module (VCM)/powertrain control
module (PCM) is open. |
