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Rear tail light problem on my 74 Eldorado

1540 Views 9 Replies 4 Participants Last post by  scottelderado
So during the day over the weekend someone told me I had a tail light out. I checked it and sure enough it was out. The culprit bulb crumbled as I took it out so I replaced it. Now everything is working when the headlights are off, brake lights, turn signal etc, but when the headlights are ON, the tailights work, the turn signal works, but as soon as the brakes are pushed both right lights turn off. This is only happening with the headlights on and only on the right tail lights. Left is working fine.

When I first looked I saw this wire was dangling and I thought that might be the problem (though I don't even know what it does!), but even after I plugged this wire back-in I get the same result. Does anyone have any ideas?


Here is a birds eye view and then a shot of the bulb area where the loose black cable plugs into


Automotive lighting Metal
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The "wire" you plugged back in, is in fact not a wire, but the fiber optics to your lamp monitor system, located over your rear window (in the garnish moulding) if it's a coupé or the "snake head" attached to the rear seat if it's a convertible. In the latter case it would not be unusual that it has gone missing behind the seat back over the years.
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It's a convertible. I'll look for it when I get home. I had the seats reupholstered last year so maybe it's just fallen somewhere. So it does what exactly? Tells the dummy light on the dash to turn on when a tail light is out or something? Pardon my ignorance, I'm new to the classic car business! :) I assume that's not the reason my tail lights are shutting off when I hit the brakes?
You are absolutely right about the light monitor has no influence on your defective tail light. :) The system is not powered at all. The monitor system is simply a set of fibre optics transmitting the light from the taillights to the "snake head" shaped light monitor on top of the rear seat (should be fastened to the back of rear seat with two small screws) allowing you to check that your tail lights are actually functional just by checking you rear view mirror. There would even be a set of light monitors on each of your front fenders for the headlights. Those will have three indicators on each fender. One indicator for the park/indicator lamp and one for each headlamp. The brilliant part about this system is that what you see is the light from the actual light bulb transmitted to the monitor via the fibre optics. Hence it is not dependent of the electric system, like on modern cars.

Here is an article about light monitors.

If your upholstery shop has forgotten to reattach your lamp monitor on the rear seat, you need to remove the seat cushion (push it backward and upward. A no-tool operation). Then the rear seat back can be removed by unscrewing the seat belt retractors. They are secured with a Tx 45 or Tx 50 bolts. Once they are out, the seat back can be pushed upward releasing it from three hooks securing the upper part. Once you have done it once, its very easy. Your cab being a convertible will give you plenty of space for the operation too :) If you don't find the lamp monitor behind the seat, look for the fibre optic cables. If they are not cut, you can buy a replacement monitor with one of my friends who 3D print them.

Enjoy :)
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