For 1990-2009 cars
Data Link Communications Components
Data Link Connector Schematics
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the ID number of the module which sent the message for State of Health monitoring (Node Alive messages). A critcal operating parameter is one which, when not received, requires that the module use a default value for that parameter. When a module does not associate an ID number with at least one critical parameter within about five seconds of beginning serial data communication, DTC U1000 or DTC U1255 is set. When more than one critical parameter does not have an ID number associated with it, the DTC will only be reported once.

### Conditions for Setting the DTC

At least one critical operating parameter has not been associated with an ID number within about five seconds after beginning serial data communication.

### Action Taken When the DTC Sets

The module uses a default value for the missing parameter.

### Conditions for Clearing the DTC

• | A current U1000 or U1255 will clear when all critical operating parameters for the module have been associated with an ID number or at the end of the current ignition cycle.

• | A history U1000 or U1255 will clear upon receipt of a scan tool CLEAR DTCs command.

### Diagnostic Aids

When a malfunction (such as a blown fuse to a module) occurs while modules are communicating, a Loss of Communication DTC is set as current. When the modules stop communicating (ignition is turned off) the current Loss of Communication DTC is cleared but the history DTC remains. When the modules begin to communicate again, the module with the blown fuse will not be learned by the other modules so U1000 or U1255 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only U1000 or U1255 is set.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Important: A short to ground will cause an open fuse(s). Before performing this diagnostic procedure, inspect the fuse(s) for an open.
1 | Were any DTCs in the range of U1001-U1199 recorded with a history status? | -- | Go to appropriate Loss of Communications DTC | Go to Step 2
2 |

1. Turn ON the ignition leaving the ignition OFF
2. Use the scan tool to communicate with each module on the class 2 serial data circuit.
3. Record all the modules communicating on the class 2 serial data circuit.
4. Compare the list of modules which are communicating to the schematic showing which modules are connected to the class 2 serial data circuit.

Does any module(s) on the class 2 serial data circuit not communicate?

--

Go to Step 3

Go to Step 11

3 | Test the battery positive voltage circuit of the module that is not communicating for an open or a short to ground. Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 4
4 |

1. Turn OFF the ignition.
2. Test the ground circuit of the module that is not communicating for an open.

Did you find and correct the condition?

--

Go to Step 9

Go to Step 5

5 | Inspect for poor connections at the battery positive voltage circuit, the ground circuit and the class 2 serial data circuit of the module that is not communicating. Did you find and correct the condition? | -- | Go to Step 7 | Go to Step 6
6 | Replace the module that is not communicating. Refer to appropriate repair instructions for module replacement. Is the repair complete? | -- | Go to Step 9 | --
7 |

1. Connect the scan tool to the data link connector.
2. Turn ON the ignition leaving the engine OFF.
3. Select the DISPLAY DTCs function for the module that is not communicating.

Does the scan tool display any DTCs that do not begin with "U"?

--

Go to the applicable DTC table

Go to Step 8

8 | Use the scan tool in order to clear the DTCs. Is the action complete? | -- | Go to Step 9 | --
9 | Select the DISPLAY DTCs function for the module(s) which had U1000 or U1255 set as a current DTC. Does the scan tool display any DTCs that do not begin with "U"? | -- | Go to the applicable DTC table | Go to Step 10
10 | Use the scan tool in order to clear the DTCs. Is the action complete? | -- | Go to Diagnostic System Check - Data Link Communications | --
11 | Did you record any other DTCs for the module(s) which had U1000 or U1255 set as a current DTC? | -- | Go to the applicable DTC table | Go to Step 12
12 |

1. Install the scan tool
2. Turn ON the ignition leaving the engine OFF.
3. Use the scan tool to clear the DTCs.
4. Turn OFF the ignition for at least 5 seconds.
5. Turn ON the ignition leaving the engine OFF.
6. Select the DISPLAY DTCs function.

Does the scan tool display U1000 or U1255 set as a current DTC?

--

Go to Step 13

Go to Diagnostic System Check - Data Link Communications

13 | Replace the module which had U1000 or U1255 set as a current DTC. Refer to appropriate repair instructions for module replacement. Is the repair complete? | -- | Go to Diagnostic System Check - Data Link Communications | --