Correct the headlamp relay controls all the headlights. Fuse - one feeds both high beams other feeds both low beams. Does the lamp work when the daytime running lights come on? If not and no high beam then it is one of two things, either the DRL relay in the under hood junction box has failed and it isn't switching or the wiring has a problem. Testing the wiring will need a jumper wire or a test light. If you look at the wiring diagram the pink wire to the left headlight is the first to test. It is the ground control from the switch. With a test light connected to the battery positive and touched to the pin connected to the pink wire, with the headlight switch and key on the light should come on if the wire from the other side is good. If the light comes on you will want to check the DRL relay in the box under the hood as it may be faulty. On some of the GM's the pins are marked on the relays and on the fuse box. Others are only on the relay. To test the relay socket you can put the test light onto battery positive, then use the probe with the key on and headlights on. First touch the relay box terminal that pin 87 connects to, the light should come on, that verifies that the ground for that pin is good, next test pins 85 and 86. One will light the light as it's the ground to the control module. Now connect the test light to ground. Now touch 85 and 86 again. One will light the light if there is power to the relay. Next touch pins 87A and pin 30. The light should be bright on both if the wiring is good. If they all test out then replace the relay, or if you don't want to do the testing you could just replace the relay as the test. However the testing would show if there was a wiring issue.
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Thursday, December 3rd, 2020 AT 8:35 AM