Circuit Description
Modules connected to the Class 2 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 data link about once every two seconds. When the Body Control Module (BCM) and radio receiver detects no messages on the Class 2 data link, U1255 is set.
Conditions for Setting the DTC
| • | Voltage supplied to the modules are in the normal operating voltage range (approximately 9-16 volts). |
| • | No Class 2 messages are 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 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
| • | This DTC cannot be retrieved with a current status. Diagnosis of a current U1255 is accomplished via the symptom, Scan Tool Will Not Communicate With The Class 2 Serial Data Line. |
| • | An intermittent condition is likely to be caused by a short to ground on the Class 2 serial data line. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This test checks whether the lack of communication is due to the scan tool rather than the vehicle.
-
This test checks for a good connection to the power supply ground and battery feed circuits of the DLC.
-
This test checks for battery voltage at the DTC.
-
This test checks for the power supply ground at the DLC.
-
This test checks for a partial malfunction of the Class 2 data link as opposed to total malfunction.
-
This test checks for additional information concerning the location of the malfunction.
-
This test checks for a good connection to the signal ground circuit of the DLC.
-
This test checks for the signal ground at the DLC.
-
This test checks for a good connection to the Class 2 data link circuit of the DLC.
-
This test checks for a short to voltage in the Class 2 data link circuit. The check is made with the key OFF and then with the key ON.
-
Before replacing a module the power and ground circuits need to be checked. This might be the reason the modules can not communicate.
-
This test checks for DTCs in the modules that communicate on the Class 2 data link. These DTCs need to be cleared to avoid future misdiagnosis.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Can the scan tool be powered up? | -- | |||
Is the connection good? | -- | |||
Measure the voltage from terminal 16 of the DLC to ground. Is voltage approximately 12 volts? | -- | |||
Measure the resistance from terminal 4 of the DLC to ground. Is the resistance within the specified range? | Less than 5 ohms | |||
5 | Repair the connection as needed. Is the repair complete? | -- | -- | |
6 | Scan tool may be malfunctioning. Refer to scan tool instruction manual. | -- | -- | -- |
7 | Repair an open in CKT A13 / A103. Is the repair complete? | -- | -- | |
8 | Repair an open in CKT F114. Is the repair complete? | -- | -- | |
Are there any modules on the Class 2 data link which are communicating? | -- | |||
Are DTCs U1000 and/or U1128 (Lost Communication DTCs) set in the modules which will communicate? | -- | |||
11 | Diagnose the malfunction using the appropriate diagnostic table. Is the procedure complete? | -- | System OK | -- |
12 | Diagnose the malfunction using the Diagnostic System Check for the BCM and RKE if they do not communicate or the diagnostic table for DTC U1128 if the radio does not communicate. Is the procedure complete? | -- | System OK | -- |
Is the connection good? | -- | |||
Measure the resistance from terminal 5 of the DLC to ground. Is the resistance within the specified range? | Less than 5 ohms | |||
Check for proper connection of the scan to the data link at terminal 2. Is the connection good? | -- | |||
16 | Repair open in CKT F115. Is the circuit repair complete? | -- | -- | |
17 | Measure the resistance on the DLC from terminal 2 to terminal 4. Is the resistance reading as specified? | OL | ||
Are both voltage readings within the specified value? | Less than 1 V | |||
19 | Repair the short to ground in the serial data circuit. Is the circuit repair complete? | -- | -- | |
Is the repair or replacement complete? | -- | System OK | -- | |
21 |
Is the repair complete? | -- | -- | |
Do the DTCs clear from memory? | -- | System OK | -- |
