The park, tail, marker, license, are turned on when the headlamp switch is placed in the PARK or ON position or anytime the headlights are requested. The I/P BATT fuse in the engine wiring harness junction block supplies battery positive voltage to both the parklamp relay switch contacts and the parklamp coil circuit. The body control module (BCM) provides a ground or control circuit to the parklamp relay coil circuit. When the parklamps are turned on, the BCM energizes the parklamp relay. Current flow is from the parklamp relay to the individual parklamp circuit fuses and to their respective park, tail, marker, and license lamps. The front park and marker lamps are grounded at G107 and G103. The tail lamps are grounded at G401 and G402. The license lamps are grounded at G401. If the driver places the headlight switch in the ON position after the ignition switch has been turned OFF, the park, tail, marker, license, will remain on until turned off or the battery runs dead.

Schematic Reference

Exterior Lights Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Exterior Lighting Systems Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, observe the scan tool Parklamp Switch Parameter while cycling the headlamp switch between the PARK and OFF positions. The parameter should cycle between Active and Inactive.

⇒	If the parameter does not cycle between the specified values, refer to Parklamp Switch Malfunction.

2. Command the parking lamps ON and OFF with a scan tool. Verify the park, license, and tail lamps turn ON and OFF.

⇒	If any of the lamps do not cycle between the commanded states, refer to Exterior Lamps Malfunction.

Parklamp Switch Malfunction

1. Ignition OFF, disconnect the harness connector X1 at the headlamp switch.
2. Ignition OFF, test for less than 5.0 ohms between the ground circuit terminal C and ground.

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

3. Ignition ON, verify the scan tool BCM Parklamp Switch parameter is Inactive.

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

4. Install a 3A fused jumper wire between the signal circuit terminal E and ground. Verify the scan tool BCM Parklamp Switch Parameter is Active.

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

5. If all circuits test normal, test or replace the headlamp switch.

Exterior Lamps Malfunction

1. Ignition OFF, remove the fuse of the inoperative park/marker lamps and inspect.

⇒	If the fuse is OPEN, test the control circuit of the blown fuse for a short to ground.

2. Ignition OFF, disconnect the parklamp relay.
3. Verify that a test lamp does not illuminate when connected between the control circuit terminal 87 and ground.

⇒	If the test lamp illuminates, test the circuit for a short to voltage.

4. Verify that a test lamp illuminates between the parklamp relay B+ circuit terminal 85 and ground.

⇒	If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance.

5. Verify that a test lamp illuminates between the parklamp relay B+ circuit terminal 30 and ground.

⇒	If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance.

6. Ignition OFF, reconnect the parklamp relay.
7. Ignition OFF, disconnect the inoperative park, marker, or license lamp.
8. Verify that a test lamp illuminates when connected between the listed ground circuit terminal and B+.

•	Front Park/Turn Signal Lamp terminal A - Chevy with W49

•	Front Park/Turn Signal Lamp terminal G - GMC, Isuzu

•	Front Park Lamp terminal A

•	Marker or License lamp terminal B

•	Tail Lamp Circuit Board terminal E

⇒	If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

9. Connect the harness connector at the inoperative park/license/marker lamp.
10. Connect a 20A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the park/marker lamps are activated.

⇒	If the park/marker lamps do not activate, test the control circuit for an open/high resistance. If the circuit tests normal, replace the inoperative park/license/marker lamp.

11. Connect a test lamp between the B+ circuit terminal 85 and the control circuit terminal 86.
12. Command the Parking Lamps 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 OFF, test the relay control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

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

13. If all circuits test normal, replace the inoperative lamp.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.