GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2000 🢒 Eldorado 🢒 DTC 🢒 DTC U1500 Inter-Device Dedicated Bus Malfunction

Circuit Description

The DTC U1500 is set when the VIU detects no communication between the VIU and the vehicle communication unit (VCU) .

Conditions for Running the DTC
    • The ignition must be in the RUN or ACC position.
    • The VIU will run this test every 300 ms.
    • Battery voltage must be greater than 9.5 volts.
    • Battery voltage must be less than 15.5 volts.

Action Taken When the DTC Sets

When DTC U1500 is current the VIU does not communicate with the VCU. No calls can be placed.

Conditions for Clearing the MIL/DTC
    • The VIU does not detect a short to ground or short to B+ on the RTN serial data circuit.
    • The VIU does not detect a short to ground or short to B+ on the CMP serial data circuit.
    • The VIU does not detect a short to ground or short to B+ on the TRU serial data circuit.
    • The VIU does not detect an open on the battery positive voltage circuit.
    • A history DTC clears after 50 malfunction free ignition cycles.
    • The VIU receives the clear DTC command from the scan tool.

Test Description

The number below refers to the step number on the diagnostic table.

  1. By disconnecting the VCU you can check the 3-wire bus supply and carrier voltages supplied by the VIU.

Step

Action

Value(s)

Yes

No

Schematic Reference: OnStar Schematics

1

Did you perform the Cellular Communications Diagnostic System Check?

--

Go to Step 2

Go to Diagnostic System Check - Cellular Communication

2

  1. Turn the ignition OFF.
  2. Disconnect the VCU connector.
  3. Turn the ignition ON, with the engine OFF.
  4. Measure the voltage from the TRU serial data circuit to a good ground.

Does the voltage measure greater than the specified value?

4 V

Go to Step 3

Go to Step 7

3

Measure the voltage from the CMP serial data circuit to a good ground.

Does the voltage measure greater than the specified value?

4 V

Go to Step 4

Go to Step 8

4

Measure the voltage from the battery positive voltage circuit from the VIU to the VCU to a good ground.

Does the voltage measure greater than the specified value?

10 V

Go to Step 5

Go to Step 9

5
  1. Turn the ignition OFF.
  2. Reconnect the VCU connector.
  3. Disconnect the VIU connector C1
  4. Remove the RTN serial data circuit from the connector at the Vehicle Interface Unit (VIU).
  5. Reconnect VIU connector C1.
  6. Connect a DMM between the RTN serial data circuit and a good ground.
  7. Turn the ignition ON.
  8. Set the DMM to the DC voltage scale and enable the MIN/MAX feature.
  9. Allow the meter to record for 45 seconds.
  10. Record the minimum and maximum values displayed on the DMM.

Does the maximum voltage measure less than the specified value?

4 V

Go to Step 10

Go to Step 6

6

Does the minimum voltage measure greater than the specified value?

.10 V

Go to Step 11

Go to Step 13

7

Test the TRU serial data circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 12

8

Test the CMP serial data circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 12

9

Test the battery positive circuit from the VCU to the VIU for an open or a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 12

10

Test the RTN serial data circuit for an open or a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 13

11

Test the following circuits for a short to voltage:

    • RTN serial data
    • CMP serial data
    • TRU serial data

Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 12

12

Important: Perform the OnStar® setup procedure.

Replace the VIU. Refer to OnStar Vehicle Interface Unit Replacement .

Did you complete the replacement?

--

Go to Step 14

--

13

Important: Perform the OnStar® setup procedure.

Replace the VCU. Refer to OnStar Vehicle Communication Unit Replacement .

Did you complete the replacement?

--

Go to Step 14

--

14
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK