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Discussion Starter #1
The daytime running lights go on, but when we pull the knob to turn on the headlights, they shut off. We have changed the bulbs, checked the fuses, replaced the relays, and bought a new switch and still have the same issue. It was suggested that it could be the turn signal switch that controls the brights. That theory seems to make sense because the cruise will not work (fuse good) so it's plausible that there could be more wrong in there. Any ideas from you all? It looks to be a job and a half to get in there to change that turn signal switch, so I really would like to rule everything else out before tearing into it. My son bought this car on Monday and the headlight issue was not disclosed which ticks me off a bit. But it won't pass inspection without working headlights and he is leaving for college next week. So I'm trying to get it fixed as quickly as I can.
 

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2011 DTS Premium w/1SD | 1999 Deville Base (RIP @179K Miles)
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The car has two main relays for the headlights. One main headlight relay that supplies power to the lights and one that directs the power between low beam and high beam.

If the DRL works, that means the fuses and wiring is good to the bulbs. A relay wires them in series to make them dim.

Check the main headlight relay again. The label for the headlight relay in the owners manual was wrong for my 1999 Deville.

If you remove the plastic cover from the underhood fuse box, that has a sticker that shows the correct location of the headlamp relay. For me, it was the relay on the other side. My main headlight relay had corrosion on one pin. I cleaned the corrosion and changed the relay and the lights worked again.
 

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2011 DTS Premium w/1SD | 1999 Deville Base (RIP @179K Miles)
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Here's the wiring diagrams for the headlights. They're fairly simple. Do any of the headlight relays click when you turn on the lights? Focus on the main power relay going to the lights for now. The Low/High beam relay defaults to low beam when its not powered.


Either the Twilight Sentinel or the Headlamp switch activates the headlamp relay to supply power to the headlights.

Do you have power going to the main headlight relay? If not You can try running a jumper wire from the positive battery terminal to the pin supplying power to the headlights.
 

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Discussion Starter #5
Thank you all! It ended up being the hi/lo beam relay. It's supposed to have a 5 post relay and someone had a 4 post in it.
 
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