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.
This diagnostic procedure supports the following DTC:
DTC U1500 Inter-device Dedicated Bus Malfunction
• | 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. |
• | 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. |
• | The VIU does not communicate with the VCU. |
• | No calls can be placed. |
• | The OnStar® status LED turns RED. |
• | 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. |
The number below refers to the step number on the diagnostic table.
Step | Action | Value(s) | Yes | No | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: OnStar Schematics Connector End View Reference: Cellular Communication Connector End Views | ||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | ||||||
Does the voltage measure greater than the specified value? | 4 V | Go to Step 3 | Go to Step 7 | |||||||
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 vehicle interface unit (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 |
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? | 0.10 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:
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 | Replace the VIU. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 14 | -- | ||||||
13 | Replace the VCU. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 14 | -- | ||||||
14 |
Does the DTC reset? | -- | Go to Step 2 | System OK |