'78 Coupe de'Ville Headlights Won't Dim
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  • 1 Post By 1980coupe
  • 1 Post By jayoldschool
  • 2 Post By bigm57ict
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RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, '78 Coupe de'Ville Headlights Won't Dim in Past Cadillac Vehicle Discussion; Hello, New to the forum. Please forgive my ignorance. I have searched this issue here but I am still stumped ...
  1. #1
    Mercedesman is offline Cadillac Owners Member
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    '78 Coupe de'Ville Headlights Won't Dim

    Hello, New to the forum. Please forgive my ignorance. I have searched this issue here but I am still stumped on the issue. The lights operate but they will not dim with the floor switch or via the automatic headlight dimmer option. I suspected the floor switch so i removed it from the car. Should the headlights still work if the floor switch is removed? I figured the lights would go out without the floor switch connected. I am also finding out that the floor switch is obsolete and the only one I have been able to find is $75!!! I would appreciate any sound advice that could be given. The car is not a daily driver but I really would like to be able to drive it at night and not blind oncoming traffic.

    1978 Coupe de'Ville 61,250 miles! I love this car!!!

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  3. #2
    Mercedesman is offline Cadillac Owners Member
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    Re: '78 Coupe de'Ville Headlights Won't Dim


  4. #3
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: '78 Coupe de'Ville Headlights Won't Dim

    The switches that I find are all "without Guidematic" automatic dimming. So, my guess is that there is a different dimmer for your auto dimming setup.

    First up, your FSM will likely have a trouble-shooting tree to figure out exactly what part is the problem. If you don't have an FSM, I can think of three options:
    1. Spend the big money, and change the switch (and hope that's the problem).
    2. Rewire the system, use the cheap switch, and bypass the Guidematic.
    3. Unplug the high beams.

  5. #4
    Mercedesman is offline Cadillac Owners Member
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    Re: '78 Coupe de'Ville Headlights Won't Dim

    I just did option #1 so while I wait on option #1 to arrive how do I go about disabling the high beams, option #3? If this were a Mercedes w123 chasis then I would know what to do. Oh the joy of the learning curve! LOL!

    Unfortunately at the moment I do not own a FSM.

  6. #5
    1980coupe's Avatar
    1980coupe is offline Cadillac Owners Connoisseur
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    Re: '78 Coupe de'Ville Headlights Won't Dim

    Quote Originally Posted by Mercedesman View Post
    Hello, New to the forum. Please forgive my ignorance. I have searched this issue here but I am still stumped on the issue. The lights operate but they will not dim with the floor switch or via the automatic headlight dimmer option. I suspected the floor switch so i removed it from the car. Should the headlights still work if the floor switch is removed? I figured the lights would go out without the floor switch connected. I am also finding out that the floor switch is obsolete and the only one I have been able to find is $75!!! I would appreciate any sound advice that could be given. The car is not a daily driver but I really would like to be able to drive it at night and not blind oncoming traffic.

    1978 Coupe de'Ville 61,250 miles! I love this car!!!
    If you remove the floor mounted dimmer switch all front headlights would be inoperative. I can check my service manual to see what it says related this your issue, but that won't be until tomorrow morning. I'm at work right now..more than likely, it's the dimmer switch that is the culprit. Or it can be the guidematic control, which has sensors up front behind the grille. But definitely tomorrow I'll post..
    Mercedesman likes this.

  7. #6
    Mercedesman is offline Cadillac Owners Member
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    Re: '78 Coupe de'Ville Headlights Won't Dim

    Thanks!!!!!!!!! I have the dimmer switch completely removed from the car and the headlights STILL WORK but of course they are on high beam. I thought that when I removed the dimmer switch that it would cause the lights to go out BUT they did not. My car must be special. Haha!

  8. #7
    1980coupe's Avatar
    1980coupe is offline Cadillac Owners Connoisseur
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    Re: '78 Coupe de'Ville Headlights Won't Dim

    That or someone in the past rigged up a direct wiring. I don't see how they'll still work without it. Like I said, first thing in the morning I'll see what it says and I'll scan the images...

  9. #8
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: '78 Coupe de'Ville Headlights Won't Dim

    Remove the high beams, unplug them. Just for now.
    bigm57ict likes this.

  10. #9
    The Ape Man's Avatar
    The Ape Man is offline Cadillac Owners Connoisseur
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    Re: '78 Coupe de'Ville Headlights Won't Dim

    I'd check the circus diagram to see of your auto gizmo is in parallel with the dimwit switch. Could be a stuck relay inside.

    Ex 124, 126, current W-140 X2 owner : ) That's so you know I'm a confirmed nutcase.

  11. #10
    bigm57ict's Avatar
    bigm57ict is offline Cadillac Owners Connoisseur
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    Re: '78 Coupe de'Ville Headlights Won't Dim

    "circus diagram" I like that. Very appropriate
    cadillac_al and talismandave like this.

  12. #11
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: '78 Coupe de'Ville Headlights Won't Dim

    Also, pick up an FSM on ebay. It's a must have for any classic car owner. It's what the dealership tech used to fix your car, you should use it, too.

  13. #12
    Mercedesman is offline Cadillac Owners Member
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    Re: '78 Coupe de'Ville Headlights Won't Dim

    Fixed it!!! At some point someone had been rooting around at the fuse panel and managed to plug the
    system up wrong. Plugged everything up correctly and now the headlights dim properly. Guidematic isn't
    working but at least the floor dimmer works!
    The Ape Man likes this.

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