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Rear tail light problem on my 74 Eldorado

1537 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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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.
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Yeah, horn is key. So are brake lights. If you ultimately don't think you can pull these fixes off, you should bite the bullet and pay a pro.
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