Weird Bulb Behavior

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Detroit Iron

Detroit Iron

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I'm pretty good a troubleshooting and only come to the forum when I'm totally out of ideas. Well.... I'm totally out of ideas.

'67 Dart dual filament tail/brake lights.... 1157 incandescents. If I remove the bulb/socket assembly from the housing and hold it, and then turn the lights on. One filament works as it should. With the turn the signal on, the second filament also works as it should...blinking. But then when I plug the bulb/socket assembly into the grounded housing.... both filaments come on and stay on. Unaffected by the turn signal, brake pedal or flasher.

Checking voltage in the socket, the tail light terminal reads 12v, and with the turn signal on, the signal terminal flashes 12v as expected. Somehow the grounding of the socket sends 12v power to both filaments.

Would appreciate any input anyone has. Thanks!
 
Is the socket contact plate twisting in the socket when you twist the bulb in and shorting between both?
 
Daves69 said:
Is the socket contact plate twisting in the socket when you twist the bulb in and shorting between both?
Click to expand...
I don't believe so. I pull the wires back as I insert the bulb and I can feel the bulb rotate into place. It feels solid. Good point though.
 
When you hold and test the socket it is no longer grounded but it works........why?
 
You may have a shorted bulb, and I believe it could be any one of the 4 corners. Otherwise it just about has to be a ground and or a short across something else in the harness. I would separate the tail harness at the left kick panel connector, make a jumper with a male flag terminal, so you can jumper power to the pins in the connector, and test operation of the tlamps at will

But on the "removed socket" thing, the tail lamp is getting a ground through the filaments of the turn signal, and vice versa. If you study up, they "reverse" so to speak. With the tail lights off, the turn seems to work, but with the tail lights on, the turn filament on the rear will light when the front one is out, because the two are in series, and with both filaments getting power--and no ground, they cannot light
 
Here is what I found on pre 70 cars with dash ignition. Turn on the flashers. Turn on the turn signal. Turn on the radio. The radio will play but go off and on with the flasher. Hit the brakes and the radio will play constant. When the flasher and the turn signal are on they back feed the Accessory circuit. The brakes bypass the flasher. Look for a short in the turn signal switch since its one side only.
 
Today I will take @67Dart273 advice and unplug the rear harness and poke around with a jump wire. Who knows what that will reveal. Yesterday I ordered a full turn signal harness so I can get that replaced if need be like @Oldmanmopar suggests.

Will update the thread. Thanks for all the input so far.
 
Oh and I forgot to mention this. I have an M&H (from Classic) restoration under dash wiring harness in this car. It has provisions for three flashers.... yeah THREE. Locations for these are one is on the left side of the steering column. One is on the right of the steering column just above the ignition switch, and one is mounted on the ash tray housing. WTF? I figured there would be two.... one for the flashers, one for the turn signals. And yes, in my troubleshooting I experimented with populating all of them, just one, and just two. But yeah... weird.
 
The one by the ashtray is turn flasher, left side of steering column is the 4-way? Not sure what the third one would be for on a '67. Wrong dash harness? I had a non working problem with my RR. The contacts in the bulb housing were were turned, bulb was out of place when installed. That does not sound like your issue though.
 
Detroit Iron said:
Oh and I forgot to mention this. I have an M&H (from Classic) restoration under dash wiring harness in this car. It has provisions for three flashers.... yeah THREE. Locations for these are one is on the left side of the steering column. One is on the right of the steering column just above the ignition switch, and one is mounted on the ash tray housing. WTF? I figured there would be two.... one for the flashers, one for the turn signals. And yes, in my troubleshooting I experimented with populating all of them, just one, and just two. But yeah... weird.
Click to expand...
The third one you have is for key light above the ignition. One wire is yellow if not mistaken by memory? If you have a flasher plugged in there take it out. That plug is for accessory light package . Key light, Map light, Uses the door switch circuit for the dome light. Which is always powered and grounds to aluminate.
 
I think that is for the timer relay. Smaller than a flasher, but don't they have 3x terminals? Photo or description and wire color is what we need
 
Ok, in an effort to not be that guy who comes here for help, then never updates all the thread contributors...... I had all kinds of weird stuff going on and typically these types of problems are in fact ground issues, but I didn't see anywhere this could be the case.

I installed a new turn signal harness as a craps shot. If that fixed it, then good, if not.... well it should have been replaced anyway. Haha.... But no, it did not fix the problem.

So, I turned my attention to the rear harness. It had been cut into by a previous owner for a third brake light. I removed the third brake light and repaired the wires. This is the only place I could see the harness has been cut in to. At any rate, I just didn't trust the harness. So I bypassed the wires to the tail lights and brake lights with test leads that I made. Everything worked perfectly. So, it's something in the harness. I decided not to spend the money on a reproduction harness, so I ordered the correct color and gauge of primary wire to run new leads.

Thank you all for the input. See ya'll here on the forum.
 
Hacked and or aftermarket wiring and mistakes can drive you nuts, but yet another "monkey wrench" is "in the harness" either pinched or melted wires from some short. This might be, say, connecting the turn wire(s) to the dome light circuit.
 
This is why I am in the replace the harness camp, with M&H or Evans. I am not a make it up wiring guy.
 
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