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 ro B+ on the CMP serial data circuit. |
| • | The VIU does not detect a short to ground or short ro 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 numbers below refer to the step numbers on the diagnostic table.
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 OnStar System Check | ||||||
Does the voltage measure greater than the specified value? | 0.04 V | Go to Step 3 | Go to Step 6 | |||||||
Measure the voltage from the CMP serial data circuit to a good ground. Does the voltage measure greater than the specified value? | 5 V | Go to Step 4 | Go to Step 7 | |||||||
4 | Measure the voltage from the TRU serial data circuit to a good ground. Does the voltage measure greater than the specified value? | 5 V | Go to Step 5 | Go to Step 8 | ||||||
5 | 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 10 | Go to Step 9 | ||||||
6 | Test the RTN serial data circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 12 | ||||||
7 | Test the the CMP serial data circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 12 | ||||||
8 | Test the the TRU serial data circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 12 | ||||||
9 | Test the battery positive voltage circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 12 | ||||||
10 | Test the following circuits for a short to voltage:
Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 11 | ||||||
11 |
Important: Perform the OnStar® setup procedure. Replace the VCU. Refer to Vehicle Interface Unit and Vehicle Communication Unit Modules Replacement . Did you complete the replacement? | -- | Go to Step 13 | -- | ||||||
12 |
Important: Perform the OnStar® setup procedure. Replace the VIU. Refer to Vehicle Interface Unit and Vehicle Communication Unit Modules Replacement . Did you complete the replacement? | -- | Go to Step 13 | -- | ||||||
13 |
Does the DTC reset? | -- | Go to Step 2 | System OK | ||||||
