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Paging electrical gurus... courious problem

Centerline

Centerline

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OK, this one has me stumped.

Turn the "lights" (headlights) on and the left side tail lights light up with both filaments. Right side single "tail light" filament lit.

Checked voltage and the tail light line has 11 volts. Turn signal/brake line has basically 0 volts both right and left.

Remove left side socket from tail light housing and then it lights only the single (tail light) filament. Touch the socket to the housing and both filaments light. With the socket removed from the housing it's my understanding that it should not light at all... the housing is the ground. Right side single element says lit if you remove it as well but only one filament lit as before.

With the lights turned off the turn signals work perfectly as do the brake lights.

Here's what I suspect.... the sockets were aftermarket (Chinese) el-cheapos and I believe they're allowing the turn signal/brake light side to act as a ground. But.. I'm no expert so are there any knowledgeable gurus out there who might have an "educated" opinion on this??

Thanks in advance...
 
I have found that most of the time when working with tail/brakes lights not working or not working properly, the problem is in the ground. A bad brake light ground connection will result in the brake lights not working, or power cross feeding through the tail-light circuit, preventing both tail and brake lights from working at the same time or only working when the headlights are in the off position. . The situation you describe above could easily caused by a bad, loose, or corroded ground connection. Easiest way I have found to trouble shoot this is to get a ground direct from the battery (alligator clip etc) and hook that up to your tail light/brake light ground. You know you have a good ground now, so things should work. When the lights are off, the ground path(least resistance) for the turn signal and brakes is being sought thru the tail light filament (not correct but it will work), as soon as you turn the headlights on, this alternate ground is taken away and the turn signal and brake lights will not work. I don't think your sockets are the problem I think it is just a bad circuit ground.
 
Yes, as said above, but to be clear, I would run temporary ground from battery neg terminal directly to tail/ brake socket itself.
 
Old rule of thumb - when an electrical issue especially DC current, makes no sense, it's most likely a ground. Filament style bulbs present a direct high resistance path to ground and any insufficiently grounded circuit will find whatever path is available.
 
Thanks for the help.

Maybe I'm just not understanding what you guys are suggesting. The tail light housing is the ground for the rear light circuits. Each housing (which holds three bulbs) is grounded to the chassis by 6 mounting bolts. When the sockets are not touching the housing on the driver's side and the headlights are turned on only the tail light filament lights. However, when I touch the bulb socket to the housing both filaments come on (left side only).

Today I completely disconnected the tail/brake light circuits. There was no difference. Once again both filaments on the driver's side come on when the headlights are turned on and the sockets are mounted in the housing. Passenger's side works fine. I believe this tells me the brake/turn signal lines are not the problem. Somehow when the bulbs on the driver's side are mounted in the housing voltage is reaching the brake/turn signal side of the bulbs (drivers side only).

Am I just being stupid? I just don't understand how this could happen just on one side. It would seem to me if there was a bad ground it would effect both sides.
 
Last edited:
Now you have me a little confused. Just to be sure what year and model are we working on?
 
On the driver side, with the sockets mounted in the housing, remove the brake/tail light bulb from the socket and take a voltage reading (to a good ground) for each positive terminal with the head lights on.
 
Could also be a bad turn signal switch in your steering column.
 
Builderguy said:
On the driver side, with the sockets mounted in the housing, remove the brake/tail light bulb from the socket and take a voltage reading (to a good ground) for each positive terminal with the head lights on.
Click to expand...

I've done that. With the headlights on each tail light terminal of each light socket reads around 11.79 volts. Each turn/brake terminal reads just under 3 volts. Here's the kicker... when the sockets are removed from the housing they are on (single filament tail light) and they shouldn't be, because theoretically they're not grounded. However, when the brake/turn signal line is disconnected the lights all go out which tells me they're getting a ground through the brake/turn signal line. At the same time the reading when the turn/brake signal line is disconnected is 11.75 volts for each positive terminal on each socket....which makes no sense. It sounds to me that somehow there is voltage reaching the turn/brake side of each socket from the tail light signal line.....that may explain the 3 volt reading on the turn/brake signal line but doesn't explain the fact that when that line is disconnected they all go out (drivers side only). When disconnected the turn/brake signal line reads 0 volts so I don't think power is getting to the sockets from there.

Any ideas???
 
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Here is the wiring diagram, Lets see what is going on:

64PolaraA.jpg
 
Builderguy said:
Here is the wiring diagram, Lets see what is going on:

View attachment 998500
Click to expand...
I have the wiring diagram. I rewired the car with a new kit and fuse panel from "Keep it Clean". I'm also using relays for the headlights and electric fuel pump so I get a full 12 volts at all times. This isn't the first car I've wired so I'm not a novice when it comes to this, but I'm not an expert either.... especially when it comes to diagnosis.

The tail light wiring comes directly from the switch. The brake/turn signal lines are wired as per the diagram. Everything works fine EXCEPT the driver's side tail lights. Now, all that said..... I am using an aftermarket sequential flashing system on the tail lights. I swapped out the backup light lens for a tail light lens and wired it accordingly. With the headlights off the turn signals sequence properly on both sides. Also the brake lights work properly. It's just when the headlights are on when I have the problem and its on the driver's side only.

Tomorrow I'll bypass the sequential turn signal system and see if that is causing the problem, although I don't see how.

Thanks for all your help. I'll report back tomorrow.
 
Quick question, is your car a 500?
This your backup light and brake light assemblies?

What I would do, with the sockets in their holder or out (doesn't matter) alligator clip a good ground wire to each of the sockets. Use a meter to make sure of the ground. Doesn't have to fancy just enough to have a good ground to each one. Spring clips and bare wire would be fine. Anyway, once that is done try the function test for each socket. If we know we have a good ground, and the lights still don't work, then the problem is not the socket getting a ground thru the assembly, the problem is somewhere else. Wiring, etc., but at least the most common problem, a bad ground is eliminated.
s-l1600.jpg


s-l1600.jpg
 
Per your description in Post #12, I have only seen the condition you describe (works fine with the head lights off, but messed up with head lights on) when there is a bad ground and the ground when the head lights are off, is the head light circuit. When you turn the head lights on, then that ground path is taken away. This is the only explanation I can think of, or have seen for this problem. I have not seen every thing and they may be some other way to cause your problem. but I have seen the same type of thing several times in the past. The was an article about this exact problem several years ago and the writer explained much better that I can. I will see if I can find it.
 
Builderguy said:
Quick question, is your car a 500?
This your backup light and brake light assemblies?

What I would do, with the sockets in their holder or out (doesn't matter) alligator clip a good ground wire to each of the sockets. Use a meter to make sure of the ground. Doesn't have to fancy just enough to have a good ground to each one. Spring clips and bare wire would be fine. Anyway, once that is done try the function test for each socket. If we know we have a good ground, and the lights still don't work, then the problem is not the socket getting a ground thru the assembly, the problem is somewhere else. Wiring, etc., but at least the most common problem, a bad ground is eliminated.
View attachment 998515

View attachment 998516
Click to expand...
It's a standard Polara. However, I removed the backup lens and replaced it with a red lens. I also replaced all six socket with new ones so I have three tail lights on each side. I'm using an aftermarket sequential flashing system on the turn signals. I've tested the grounds and they're good.... but there's more going on. The turn signals sequence properly and brake lights work properly as long as the headlights aren't on. With the head lights on the passenger's side sequences properly and the brake lights work fine, but the drivers side has both the tail light and brake lights on constantly. When the sockets are removed from the housings the bulbs stay on but only one filament is lit and this shouldn't happen because they're theoretically not grounded. When you ground them against the housing both filaments light. When the socket is grounded to the housing and the bulb removed the tail light positive terminal reads 11.75 volts. The turn/brake light terminal reads just under 3 volts. However, when the brake/turn signal signal wire is disconnected from the system both terminals on each bulb read 11.75 volts which isn't right either. The brake/turn signal side should read 0 volts.

This isn't the first car I've wired so I'm not a novice with electrical stuff, but I'm not an expert either, especially when it comes to diagnosis. Tomorrow I'll bypass the sequential flashing system just to see if that might be the problem, although I don't see how. Don't ya just love diagnosing electrical problems. ;-)
 
Last edited:
Builderguy said:
Per your description in Post #12, I have only seen the condition you describe (works fine with the head lights off, but messed up with head lights on) when there is a bad ground and the ground when the head lights are off, is the head light circuit. When you turn the head lights on, then that ground path is taken away. This is the only explanation I can think of, or have seen for this problem. I have not seen every thing and they may be some other way to cause your problem. but I have seen the same type of thing several times in the past. The was an article about this exact problem several years ago and the writer explained much better that I can. I will see if I can find it.
Click to expand...
I'd be interested to read it. I don't think the headlights are grounding the system though. I'm running the headlights through relays. The signal wire from the switch goes to the relay and isn't grounded till it receives power from the switch and flips the relay.
 
PROBLEM SOLVED:

First, many thanks to all who offered help. Although a 'bad ground" didn't turn out the be the problem it never hurts to confirm the grounds are good. What did cause the problem was ME. Which I suspected was the weak link in the system to begin with.

Today I disconnected all three circuits.... turn/brake circuit for each side and the tail light circuit. I then checked each circuit to make sure all voltages were correct. ie. 11+ volts when on and properly grounded. Then I bypassed the sequential turn signal electronics and hooked up the tail lights only. The result was the brake filament was working on drivers side and tail light filament was working on the passenger's side just like before. So... after a few swear words I switched the terminals on the driver's side bulbs, and that did the trick.

Apparently I had wired the driver's side brake light terminals to the tail light circuit and the tail light terminals were wired to the brake/turn signal circuit on the driver's side only. This is why when the tail lights were turned on, the lights were too bright on the drivers side and the turn signals failed to work properly. After this DUH moment I wired back in the sequential system and everything now functions correctly. See short video below.

Thanks again for everyone's help. It is greatly appreciated.

 
I have a 70 Roadrunner car was apart for years my harness was used my taillight issues were ground related I just run a wire from one of those tailight housing studs to the trunk latch ,it’s out of site and gives you a better ground ,my lights are nice an bright
 
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