GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 Savana 🢒 DTC 🢒 DTC U1000 and DTC U1255 Class 2 Communication Malfunction

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 identification number of the module which sent the message for State of Health monitoring. A critical 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 identification number with at least one critical parameter within about five seconds of beginning serial data communication, DTC U1000 or U1255 is set. When more than one critical parameter does not have an identification number associated with it, the DTC will only be reported once.

Conditions for Running the DTC
    • Voltage supplied to the module is in the normal operating voltage range (approximately 9 to 16 volts).
    • Diagnostic trouble codes U1300, U1301 and U1305 do not have a current status.
    • The vehicle power mode (ignition switch position) requires serial data communication to occur.

Conditions for Setting the DTC

At least one critical operating parameter has not been associated with an identification 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 DTC U1000 or U1255 will clear when all critical operating parameters for the module have been associated with an identification number or at the end of the current ignition cycle.
    • A history DTC U1000 or U1255 will clear upon receipt of a scan tool Clear DTCs command.

Diagnostic Aids

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

Test Description

The number(s) below refer to the step number(s) 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. The following modules communicate on the class 2 serial data circuit:

    2. • EBCM
    • Passlock™ Module (Domestic)
    • PCM (Diesel)
    • VCM (Gas)
    • VIM (Export)

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

    3. • U1300
    • U1301

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

    4.

  4. Data link connector terminals 2 and 5 provide the connection to the class 2 serial data circuit and the signal ground circuit respectively.

    5.

  5. This tests for a poor ground circuit or connection.

    6.

  6. An open in the class 2 serial data circuit between the DLC and splice S207 will prevent the scan tool from communicating with any module. This condition will not set a DTC.

    7.

  7. This test isolates the class 2 serial data circuit. When attempting to isolate a particular module, it is necessary to disconnect the other module due to the redundant connection between the two.

    8.

  8. If there are no current DTCs that begin with a U, the communication malfunction has been repaired.

    9.

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

Step

Action

Value(s)

Yes

No

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 leaving 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 19

Go to Step 6

6

Is U1000 or U1255 the only DTC displayed in the previously specified range?

--

Go to Step 7

Go to DTC U1001-U1254 Lost Communication with XXX

7

Diagnose the malfunction using DTC U1001-U1254 Lost Communication with XXX .

Did you complete the action?

--

Go to Control Module References or 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 at the DLC terminals 2 and 5. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Control Module References or the applicable Diagnostic System Check

Go to Step 9

9

Test the DLC ground CKT 451 (BLK/WHT) CKT 551 (TAN/WHT) for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Control Module References or the applicable Diagnostic System Check

Go to Step 10

10

  1. Disconnect the electronic brake control module (EBCM) connector C1.
  2. Test the class 2 serial data CKT 1807 (PPL) for an open between the DLC terminal 2 and the EBCM connector C1 terminal G. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Control Module References or the applicable Diagnostic System Check

Go to Step 11

11

  1. Leave the EBCM disconnected.
  2. Turn ON the ignition leaving the engine OFF.
  3. Attempt to establish communications with the vehicle control module (VCM - gas) or the powertrain control module (PCM - diesel).

Does the scan tool communicate with the VCM/PCM?

--

Go to Step 13

Go to Step 12

12
  1. Turn OFF the ignition.
  2. Disconnect the vehicle control module (VCM - gasoline engines) or the powertrain control module (PCM - diesel engines).
  3. Turn ON the ignition leaving the engine OFF.
  4. Attempt to establish communications with the Passlock™ module (domestic) or the vehicle immobilizer module (VIM - export).

Does the scan tool communicate with the Passlock™ module (domestic) or the vehicle immobilizer module (VIM - export)?

--

Go to Step 14

Go to Step 15

13

Inspect for poor connections at the EBCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 19

Go to Step 16

14

Inspect for poor connections at the VCM/PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 19

Go to Step 17

15

Inspect for poor connections at the Passlock™ module or VIM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 19

Go to Step 18

16

Replace the EBCM. Refer to Control Module References for the appropriate repair instructions.

Did you complete the replacement?

--

Go to Step 19

--

17

Replace the VCM/PCM. Refer to Control Module References for the appropriate repair instructions.

Did you complete the replacement?

--

Go to Step 19

--

18

Replace the Passlock™ module or VIM. Refer to Control Module References .

Did you complete the replacement?

--

Go to Step 19

--

19

  1. Reconnect all of the disconnected modules.
  2. Reconnect any other disconnected connectors.
  3. Install a scan tool.
  4. Turn ON the ignition leaving the engine OFF.
  5. Wait for 10 seconds.

    6. 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 prior to performing this test.

  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 you record any DTCs which begin with a U and with a current status?

--

Go to Step 21

Go to Step 20

20

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

--

Go to Step 21

Go to Step 23

21

Diagnose the DTCs as directed by the diagnostic procedures for the particular module or malfunction. Refer to Control Module References for the applicable Diagnostic System Check.

Did you complete the action?

--

Go to Step 22

--

22

Did you diagnose all of the DTCs?

--

Go to Step 23

Go to Step 21

23

Use the scan tool in order to clear the DTCs.

Did you complete the action?

--

System OK

--