Ground is applied at all times to the turn signal/multifunction switch from G300. The turn signal lamps may only be activated with the ignition switch in the ON position. The BCM controls the vehicle turn signals based on inputs from the turn signal/multifunction switch. With the turn signal/multifunction switch in either the left or right turn position, a ground is applied through the turn signal switch signal circuit to the BCM indicating a turn signal request. When the specific request is seen, the BCM then applies a pulsating voltage through the front and rear turn signal supply voltage circuits illuminating the turn signal lamps. At the same time, the BCM sends a GMLAN serial date message requesting the door module to illuminate the outside rearview mirror turn signal by applying a low reference to the ground side of the mirror turn signal lamp.

When a turn signal request is seen by the BCM, a serial data message is sent to the instrument panel cluster (IPC) requesting the respective turn signal indicator be pulsed.

Schematic Reference

Exterior Lights Schematics

Connector End View Reference

Lighting Systems Connector End Views

Electrical Information Reference

Scan Tool Reference

1. Ignition ON, observe the scan tool Left and Right Turn Signal Switch parameters while cycling the turn signal switch between the left and right positions. The readings should toggle between Active and Inactive.

⇒	If the readings do not change between the commanded states, refer to Turn Signal/Multifunction Switch Circuit Test.

2. Observe the left and right turn signal lamps while activating the turn signals. The front and rear turn signal lamps and the outside rear view mirror indicators should illuminate and flash when turned ON.

⇒	If the front or rear turn signal lamps are inoperative, refer to the Turn Signal Circuit Test.

⇒	If the front turn signals operate, but the outside rearview mirror turn signal is inoperative, refer to the Outside Rearview Mirror Turn Signal Circuit Test.

3. Ignition ON, observe the left turn signal lamps and turn signal indicator on the IPC while activating the left turn signals. The left turn signal lamps and indicators should illuminate and flash when turned ON.

⇒	If the turn signal indicator is inoperative, refer to Turn Signal Indicators Malfunction.

⇒	If the left turn signal lamps and indictors illuminate but flash in a rapid manner, refer to Tail Lamp Outage Detection Malfunction.

4. Ignition ON, observe the right turn signal lamps and turn signal indicator on the IPC while activating the right turn signals. The right turn signal lamps and indicator should illuminate and flash when turned ON.

⇒	If the turn signal indicator is inoperative, refer to Turn Signal Indicators Malfunction.

⇒	If the right turn signal lamps and indictor illuminate but flash in a rapid manner, refer to Tail Lamp Outage Detection Malfunction.

Caution: The high intensity discharge system produces high voltage and current. To reduce the risk of severe shocks and burns:

Turn Signal/Multifunction Switch Circuit Test.

1. Ignition OFF, disconnect the C3 harness connector at the turn signal/multifunction switch.
2. Test for less than 5.0 ohms of resistance between the ground circuit terminal G and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

3. Ignition ON, verify the scan tool Left Turn Signal Switch parameter is Inactive.

⇒	If not the specified value, test the signal circuit terminal F for a short to ground. If the circuit tests normal, replace the BCM.

4. Verify the scan tool Right Turn Signal Switch parameter is Inactive.

⇒	If not the specified value, test the signal circuit terminal H for a short to ground. If the circuit tests normal, replace the BCM.

5. Install a 3A fused jumper wire between the signal circuit terminal F and the ground circuit terminal G. Verify the scan tool Left Turn Signal Switch parameter is Active.

⇒	If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

6. Install a 3A fused jumper wire between the signal circuit terminal H and the ground circuit terminal G. Verify the scan tool Right Turn Signal Switch parameter is Active.

⇒	If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

7. If all circuits test normal, test or replace the turn signal/multifunction switch.

Turn Signal Circuit Test.

1. Ignition OFF, disconnect the C4 harness connector at the BCM.
2. Ignition ON, verify a test lamp illuminates between the following B+ terminals and ground.

•	B+ terminal 1

•	B+ terminal 2

•	B+ terminal 3

⇒	If the test lamp does not illuminate, test the B+ circuit for an open/high resistance. If the circuit fuse is open, test the B+ circuit and the respective turn signal circuits for a short to ground.

3. Ignition OFF, disconnect the harness connector at the inoperative turn signal lamp.
4. Test for less than 5.0 ohms of resistance between the appropriate ground circuit terminal listed below and ground.

•	Headlamp Assembly terminal A (and T72)

•	Headlamp Assembly terminal H (T84)

•	Repeater Lamp terminal B -- Export

•	Tail Lamp Assembly terminal C -- Domestic

•	Tail Lamp Assembly terminal H -- Export

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

5. Connect a test lamp between the appropriate supply voltage circuit terminal listed below and the ground.

•	Headlamp Assembly terminal C

•	Repeater Lamp terminal A -- Export

•	Tail Lamp Assembly terminal B -- Domestic

•	Tail Lamp Assembly terminal G -- Export

6. Ignition ON, command the inoperative turn signal ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the supply voltage circuit for a short to voltage. If the circuit tests normal, replace the BCM.

⇒	If the test lamp is always OFF, test the supply voltage circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

7. If all circuits test normal, test or replace the appropriate turn signal bulb.

Outside Rearview Mirror Turn Signal Circuit Test

1. Ignition OFF, disconnect the C1 harness connector at the inoperative outside rearview mirror.
2. Connect a test lamp between the low reference circuit terminal 2 and B+.
3. Ignition ON, command the appropriate LF or RF Turn Signal ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the low reference circuit for a short to ground. If the circuit tests normal, replace the door module.

⇒	If the test lamp is always OFF, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the door module.

4. Connect a test lamp between the supply voltage circuit terminal 1 and ground.
5. Ignition ON, command the appropriate LF or RF Turn Signal ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒	If the test lamp is always ON, test the supply voltage circuit for a short to voltage.

⇒	If the test lamp is always OFF, test the supply voltage circuit for an open/high resistance.

6. If all circuits test normal, test or replace the outside rearview mirror lamp bulb.

Tail Lamp Outage Detection Malfunction

1. Ignition OFF, disconnect the harness connector at the appropriate tail lamp assembly.
2. Ignition ON, test for B+ between the signal circuit terminal D and ground.

⇒	If less than B+, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

3. Ignition ON, connect a test lamp between the signal circuit terminal D and ground. The test lamp should not Illuminate.

⇒	If the test lamp illuminates, test the signal circuit for a short to voltage. If the circuit tests normal, replace the BCM.

4. If all circuits test normal, test or replace the tail lamp assembly.

Turn Signal Indicators Malfunction

Ignition ON, with a scan tool perform the display test for the instrument panel cluster (IPC) and observe the turn signal lamp indicators.

Turn Signal/Multifunction Switch

1. Ignition OFF, disconnect the C3 harness connector at the turn signal/multifunction switch.
2. Test for infinite resistance between the following signal circuit terminals and the ground circuit terminal E with the switch in the neutral position.

•	Signal terminal F

•	Signal terminal H

⇒	If not the specified value, replace the turn signal/multifunction switch.

3. Test for less than 2.0 ohms of resistance between the signal circuit terminal D and the ground circuit terminal E with the switch in the left turn position.

⇒	If greater than the specified range, replace the turn signal/multifunction switch.

4. Test for less than 2.0 ohms of resistance between the signal circuit terminal F and the ground circuit terminal E with the switch in the left turn position.

⇒	If greater than the specified range, replace the turn signal/multifunction switch.

Tail Lamp Assembly

1. Ignition OFF, disconnect the harness connector at the appropriate rear tail lamp assembly.
2. Test for infinite resistance between the appropriate outage detection signal terminal and the ground terminal listed below.

•	Domestic -- Signal terminal D and ground terminal C

•	Export -- Signal terminal G and ground terminal H

⇒	If not the specified value, replace the tail lamp assembly.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.