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For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2002 🢒 DeVille 🢒 Body and Accessories 🢒 Cellular Communication 🢒 Scan Tool Does Not Communicate with Class 2 Device

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. Connecting a scan tool to the data link connector (DLC) allows communication with the modules for diagnostic purposes.

Diagnostic Aids

The engine will not start when there is a malfunction of the class 2 serial data circuit between the VTD system and the PCM while the engine is not running. The following conditions will cause a total loss of class 2 serial data communication:

    • A class 2 serial data circuit shorted to ground.
    • A class 2 serial data circuit shorted to voltage.
    • An open in the class 2 serial data circuit between the VTD system and the PCM.
    • An internal malfunction of a module on the class 2 serial data circuit that causes a short to voltage or ground.

On some vehicles, if the PCM is unable to communicate with the VTD system after the vehicle has started, the PCM will consider the VTD system to be malfunctioning. The PCM will enter a fail enable state and will command the security indicator to illuminate. When the PCM is in a fail enable state the vehicle will NOT stall or stop running. If the PCM is in a fail enable state when the ignition is switched OFF, the PCM will remain fail enable until communications with the VTD system has been restored. When the PCM is in a fail enable state the VTD system is NOT active and the vehicle will start. This feature is NOT available on all GM vehicle lines.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. A partial malfunction in the class 2 serial data circuit uses a different procedure from a total malfunction of the class 2 serial data circuit.

    2.

  2. The following DTCs may be retrieved with a history status. These DTCs are not the cause of the present condition.

    3. • U1300
    • U1301
    • U1305

  3. A State of Health DTC with a history status may be present along with a U1000 or U1255 with a current status. This indicates that the malfunction occurred when the ignition was on.

    4.

  4. An open between the DLC and the class 2 serial data circuit will prevent the scan tool from communicating with any module. This condition will not set a DTC.

    5.

  5. Splitting the class 2 serial data circuit will help isolate the location of the malfunction. If communication can be established, the malfunction is located in the wiring or one of the modules that is no longer connected to the DLC. If communication cannot be established, the malfunction is located in the wiring or one of the modules that is still connected to the DLC.

    6.

  6. If there are no current DTCs that begin with the letter "U", the communication malfunction has been repaired.

    7.

  7. The communication malfunction may have prevented diagnosis of the customer complaint.

Step

Action

Yes

No

Schematic Reference: Data Link Connector Schematics

1

Does the scan tool power up?

Go to Step 2

Go to Scan Tool Does Not Power Up

2

  1. Turn ON the ignition, with the engine OFF.
  2. Attempt to communicate with each module on the class 2 serial data circuit. If using a Tech 2, obtain this information using the class 2 Message Monitor feature.

Does the scan tool communicate with any module on the class 2 serial data circuit?

Go to Step 3

Go to Step 8

3

  1. Select the Display DTCs function for each module. If using a Tech 2, use the Class 2 DTC Check feature in order to determine which modules do have DTCs set.
  2. Record all of the displayed DTCs, the DTC status and the module which set the DTC.

Did you record any DTCs in the range of U1000 to U1305?

Go to Step 4

Go to Step 7

4

Are the DTCs U1300, U1301 or U1305 retrieved from any module?

Go to Step 5

Go to Step 6

5

Important:: Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Use the DMM MIN/MAX function to capture intermittent conditions.

Test the class 2 serial data circuit for an intermittent short to ground or an intermittent short to voltage. Refer to the following in Wiring Systems:

    •  Circuit Testing
    •  Connector Repairs
    •  Wiring Repairs

Did you find and correct the condition?

Go to Step 18

Go to Step 6

6

Are U1000 or U1255 the only DTCs displayed in the previously specified range?

Go to DTC U1000 and U1255

Go to DTC U1001-U1254

7

Diagnose the non communicating module using the DTC U1001--U1254 Lost Communications with XXX procedure for the module which is not communicating. Refer to DTC U1001-U1254 .

Did you complete the action?

Go to Control Module References for the applicable Diagnostic System Check

--

8

  1. Turn OFF the ignition.
  2. Disconnect the scan tool from the data link connector (DLC).
  3. Inspect for poor connections and terminal tension at the DLC. Refer to the following:
    4. •  Connector Repairs in Wiring Systems

Did you find and correct the condition?

Go to Control Module References for the applicable Diagnostic System Check

Go to Step 9

9

Test the ground circuits of the DLC for an open or high resistance. Refer to the following:

    •  Circuit Testing in Wiring Systems
    •  Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Control Module References for the applicable Diagnostic System Check

Go to Step 10

10

Test the class 2 serial data circuit between the DLC and first 2 modules on the class 2 serial data circuit for the following conditions:

    • High resistance
    • Open
    • Short to ground
    • Short to voltage

Refer to the following:

    •  Circuit Testing in Wiring Systems
    •  Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Control Module References for the applicable Diagnostic System Check

Go to Step 11

11

  1. Turn OFF the ignition.
  2. Split the class 2 serial data circuit in half by disconnecting 2 modules in order to help isolate the location of the fault within the class 2 serial data circuit. The DLC must remain connected to at least 1 module.
  3. Turn ON the ignition, with the engine OFF.
  4. Attempt to communicate with modules still connected to the DLC through the class 2 serial data circuit.

Does the scan tool communicate with any module still connected to the class 2 serial data circuit?

Go to Step 13

Go to Step 12

12

The fault is located within the portion of the class 2 serial data circuit still connected to the DLC. Perform the following, one module at a time starting at the module furthest from the DLC, for each module on the class 2 serial data circuit now attached to the DLC.

  1. Turn OFF the ignition.
  2. Disconnect the furthest module from the DLC still connected to the class 2 serial data circuit.
  3. Turn ON the ignition, with the engine OFF.
  4. Attempt to communicate with any module still connected to the DLC through the class 2 serial data circuit.
  5. Repeat steps 1 through 4 until communication is restored.

Does the scan tool communicate with modules on the class 2 serial data circuit?

Go to Step 15

Go to Step 13

13

The fault is located within the portion of the class 2 serial data circuit that is not connected to the DLC.

  1. Connect one of the modules that was disconnected to split the serial data circuit.
  2. Perform the following, one module at a time starting at the module furthest from the DLC, for each module on the class 2 serial data circuit now attached to the DLC.
    3. 2.1. Turn OFF the ignition.
    2.2. Disconnect the furthest module from the DLC still connected to the class 2 serial data circuit.
    2.3. Turn ON the ignition, with the engine OFF.
    2.4. Attempt to communicate with any module still connected to the DLC through the class 2 serial data circuit.
    2.5. Repeat steps 2.1 through 2.4 until communication is restored.

Does the scan tool communicate with any module on the class 2 serial data circuit?

Go to Step 18

Go to Step 14

14

Repair the class 2 serial data circuits and in-line connectors to the last module connected in the branch being diagnosed.

Did you complete the repair?

Go to Step 18

--

15

Test the class 2 serial data circuit after the last module disconnected for the following conditions:

    • Short to ground
    • Short to voltage

Turn ON the ignition with the engine OFF when testing for a short to voltage.

Refer to the following:

    •  Circuit Testing in Wiring Systems
    •  Wiring Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 18

Go to Step 16

16

Inspect for poor connections and terminal tension at the harness connector of the last module disconnected. Refer to the following:

    •  Connector Repairs in Wiring Systems

Did you find and correct the condition?

Go to Step 18

Go to Step 17

17

Important: Perform the module setup procedure if required.

Replace the last module disconnected. Refer to Control Module References for the appropriate Repair Instructions for module replacement.

Did you complete the replacement?

Go to Step 18

--

18

  1. Connect all of the modules.
  2. Connect all the connectors.
  3. Install a scan tool.
  4. Turn ON the ignition leaving the engine OFF.

    5. Important: The scan tool may require a power up reset before communication will occur due to a short on the class 2 serial data circuit. Turn off or disconnect the scan tool before you display DTCs.

  5. Wait for 10 seconds.
  6. Select the display DTCs function for each module. If using a Tech 2, use the Class 2 DTC Check feature in order to determine which modules do have DTCs set.
  7. Record all of the displayed DTCs and the DTC status.

Did your record any DTCs which begin with a letter U and with a current status?

Go to Step 20

Go to Step 19

19

Did you record any DTCs which do not begin with a letter "U"?

Go to Step 20

Go to Step 21

20

Diagnose the DTCs as directed by the diagnostic procedures for the particular module or malfunction.

Did you complete the action?

Go to Step 21

--

21

Did you diagnose all of the DTCs?

Go to Step 22

Go to Step 20

22

Clear the DTCs using the scan tool.

Did you complete the action?

System OK

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