GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2001 🢒 Century 🢒 DTC 🢒 DTC U1500

Circuit Description

The Vehicle Communication Unit (VCU) and the Vehicle Interface Unit (VIU) communicate using the OnStar® Serial Data (-) circuit, OnStar® Serial Data (+) circuit and the OnStar® Serial Data Return circuit. DTC U1500 is set when the VIU detects no communication between the VIU and the VCU.

Conditions for Running the DTC
    • The ignition must be in the RUN or ACC position.
    •  The system voltage is at least 9.5 volts and no more than 15.5 volts.
    • All the above conditions are present for greater than 300 ms.

Conditions for Setting the DTC
    • The VIU detects an open, short to ground or short to B+ on the serial data return circuit.
    • The VIU detects an open, short to ground or short to B+ on serial data (-) circuit.
    • The VIU detects an open, short to ground or short to B+ on serial data (+) circuit.
    • The VIU detects an open on the battery positive voltage circuit.
    • All the above conditions are present for greater than 300 ms.

Action Taken When the DTC Sets
    • The VIU does not communicate with the VCU.
    • No calls can be placed.
    • The OnStar® status LED turns RED.

Conditions for Clearing the DTC
    • The VIU does not detect an open, short to ground or short to B+ on the serial data return circuit.
    • The VIU does not detect an open, short to ground or short to B+ on serial data (-) circuit.
    • The VIU does not detect an open, short to ground or short or B+ on 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 ON the ignition, with the engine OFF.
  4. Measure the voltage from the OnStar® 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 OnStar® 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 OnStar serial data return circuit from the connector at the Vehicle Interface Unit (VIU).
  5. Reconnect VIU connector C1.
  6. Connect a DMM between the OnStar serial data return 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 OnStar® 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 OnStar® 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 OnStar serial data return 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:

    • OnStar® serial data return
    • OnStar® serial data (-)
    • OnStar® 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