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Rear tail light problem on my 74 Eldorado

1543 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.
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?
That plug socket looks rather corroded from here. If so, replace it.
I cleaned up the one on the right, where the bulb was out, but doesn't it seem odd that everything else is working? Why would the brake lights only NOT WORK when the headlights are on, but otherwise work properly when they are off? For all I know it was never working when I bought the car a year and a half ago. I rarely drive it at night, so I rarely have the headlights on.
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 :)

Thanks for the education. I found that same link after I posted :) There are monitor lights on the front left and right fenders and they both work. I think it's just 2 lights on each side though, I forget. I'm not sure I care enough to do the work on removing the seat! I'll just let a police officer pull me over and tell me it's out lol ..... I do have to figure out my rear tail light/braking problem though! Or I can just only drive during the day .... ha! Anyways, this car has other issues too so I just take them one dollar at a time :)
Bernt beat me to it... he's right. It's a very low-tech, non-electrical fiber optic backup system to let you know your bulbs are working. I'll add that the system is very likely explained in the owner's manual; definitely in the service manual.

Bulb-wise... Sounds like you have a short somewhere. I've learned that shorts or burnouts anywhere in that circuit can cause weird gremlin behavior. Recently my turn signal stopped blinking, turn bulb just lit solid. Found and replaced an unrelated burned out bulb, I think it was a running light bulb... then turn bulb blinked again.
So step one with any such lights issues, for me anyway, is checking all the turn/brake/running light bulbs everywhere. All of them. Replace any burned out bulbs.

If they all work: those bulb sockets get dirty/corroded and also very hot. The sockets often melt and warp. Either of those could be your problem. First, remove the bulbs and check for dirt/corrosion or melting. If not melted, then clean the contacts inside with Electrical Contact Cleaner and Q Tips. In a pinch, use alcohol. If really dirty, use an old toothbrush. (To go the extra mile, put Dielectric Grease on the socket and bulb contacts.) Then test.

If melted/warped, replace the sockets. If you still have the problem, personally I'd start looking at the electrical diagram and try to figure it out. You might have a bad headlight switch or brake light switch, or loose wires somewhere in the circuits.
Definitely "gremlin behavior". I replaced the bulb and couldn't figure out why it still wasn't working! Including the turn signal. I was doing the operation at night so I had my headlights on to see the lights better. Then I turned off the lights and was giving up for the night after some aggravation and suddenly they worked. That's when I discovered that everything worked as long as the headlights were off! LOL! Ok I gotta dig deeper. I'm just going to replace all the bulbs even though they all are functioning. I'll do the cleaning as suggested and see if anything gets it going. The light bulbs are a tight fit and on a bad angle to install. It was super tough pushing them in and then turning the bulb. Originally it wasn't hitting the contacts because it was so difficult to get in and turn.

I don't know enough about this car but I'm certainly learning! My horn is going off randomly so I had to unplug it. Horn pad must be stuck. I'm going to attempt that DIY fix next. You can't live in L.A. without having a horn !!!! You don't realize how much you use it until it's gone! It's also quite dangerous actually. I've been nearly hit several times because I couldn't warn them with my horn :)
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