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Pete, GMC Mechanic
Category: GMC
Satisfied Customers: 29037
Experience:  GM training and dealer tech for 14+ years. Diagnostics
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GMC Yukon Denali: The headlights in 1999 Denali are not switching

Customer Question

The headlights in 1999 Denali are not switching on automatically. I can manually switch on the lights. Once I manually turn on the lights, the relay sounds like it is trying to switch over...I hear a constant, rapid clicking from under the dash. Also, while the apparent relay is trying to engage, there is a slight rapid pulsing of the headlights. The clicking of the relay will randomly stop and start with no particular reason for it to start and stop. When I take this issue to the dealer, as with may electrical issues, it never shows up so it goes unrepaired (The GMC dealer also doesn't seem to understand that it is supposed to switch over on this model year). I also cannot determine where the clicking is coming from under the dash. Suggestions
Submitted: 2 years ago.
Category: GMC
Expert:  Pete replied 2 years ago.
My name is ***** ***** I am a professional here at Just Answer. I noticed that your question was not getting a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.
This model should have a DRL (day time running lamp) control module under the dash that is common to fail and can cause this here.I have had to replace this module before for this issue here.
Thanks Pete
Customer: replied 2 years ago.
Thank you Pete. I am going to look into replacing the DRL module. Is there a way to locate the module under the dash. I am not sure I have a 'Map" of its location as I am sure there are several modules under the dash. How do I determine which one it is?
Expert:  Pete replied 2 years ago.
Is the dash indicator coming on near the gauges to let you know the day time lamps are on?It usually looks like a light/picture of a light being on.
Let me know,thanks Pete.
Customer: replied 2 years ago.
Thanks for the response Pete. I am out of town for a couple of days so will get back to you shortly after I return.
Customer: replied 2 years ago.
The answer to your question about the dash indicator is no, it is not coming on. I replaced the diode today but that has not solved the problem. I have read that there is a circuit board under the dash but don't know where to find it...could that be the problem? or the relay?
Expert:  Pete replied 2 years ago.
I apologize for the delay.I was on vacation for a bit there.
Was the diode the component under the dash that you had replaced?
Customer: replied 2 years ago.
Yes, I replaced the diode under the dash. Still no change and I have ordered the circuit board that is also under the dash. As I previously indicated, the green light on the dash does not come on.
Expert:  Pete replied 2 years ago.
O.k.,the drl module is located under the dash to the far left/drivers side of the dash as seen in the links below.Since the diode did not correct this issue I suspect the module is the cause here.The diode is usually most common to fail that's why I suggested it first.
Thanks Pete