Test Description

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

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

DTC U1500

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	Turn the ignition OFF. Disconnect the VCU connector. Turn ON the ignition, with the engine OFF. 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	Turn the ignition OFF. Reconnect the VCU connector. Disconnect the VIU connector C1 Remove the OnStar serial data return circuit from the connector at the Vehicle Interface Unit (VIU). Reconnect VIU connector C1. Connect a DMM between the OnStar serial data return circuit and a good ground. Turn the ignition ON. Set the DMM to the DC voltage scale and enable the MIN/MAX feature. Allow the meter to record for 45 seconds. 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 . 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 . 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 . 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 . 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 . 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 (VIU) 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 (VCU) Replacement .Did you complete the replacement?	-	Go to Step 14	-
14	Use the scan tool in order to clear the DTCs. 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