I've been chasing a tail light problem on a '77 LTD for a while...it's your run-of-the mill 70's full size Ford, and the wiring has never been altered. The problem is that on the passenger side tail light, the bottom bulb doesn't work, even with a new bulb. As for the top bulb, it functions correctly, except when the headlights are on, and you signal a right turn, the bulb won't flash. However, if the headlights are off, the bulb flashes fine.

Anyway, I thought maybe there was a ground problem, so I tended to the tail light ground...and it was in perfect condition. For kicks, I disconnected it to observe the effect on the lights...and to my surprise, they act just the same. That is to say, without a ground, the working lights still light up. The driver's side tail light works as normal, and the passenger side tail light acts as described above...ground or no ground.

What is happening here? The only possibility I could think of is maybe the fuel sending unit grounds to the tail light ground, and maybe I'm grounding through the fuel tank? I did notice there were two wires running up from the fuel sending unit under the car; however only one of these wires continues under the rear seat, up to the dash. I'd assume the second wire is spliced into the tail light ground, inside all the electrical tape?

Why do I still have current with the ground gone? Is it somehow grounding through a faulty turn signal switch, or was my guess about the fuel tank right? I'm stumped. I still don't know why the passenger side tail light is not functioning properly. The sockets seem fine as well.

 

Your ground is bad. You have proven that by taking it off, and it makes no difference in operation.

But when people say "bad ground" when talking about the tailight circuit, they automatically think of the wire bolted to the body. The "ground" starts at the brass part of the bulb. It then has to travel to the contact on the side of the socket, and then run through the socket to the ground wire. There are many things that can go wrong here, including the socket going bad.

If you want to do another test, take a scrap piece of wire, bolt it to a good ground, and then jamb the other end beside the bulb in the socket. while holding it in there, see if the lights work normal.

 

Thanks for your response. I will definitely try that tomorrow afternoon.

Quick question - when I took the tail light ground away, and the lights stayed on, how did they become grounded? Where did the current go? Would there be a ground wire coming from the fuel sending unit? I noticed two wires emerging from the trunk that came from the sending unit. These wires went to a winding of tape behind the rear seat. This section of harness continues under the carpet to the dash...the brown lighting wire, orange and green signal wires, backup light wire, the dome light wire, a green constant power wire for a trunk light, and a yellow wire...but no ground wires. All wires were accounted for, except for a yellow one; I'm assuming this was from the fuel tank sender? Is it safe to assume that the second wire coming from the fuel tank sending unit was a ground, since only one wire from the sending unit went up to the dash? If this wire is indeed a ground wire, then I assume this is how the remaining tail lights operated with the tail light ground removed?

 

What's happening is the current is going down the turn/brake wire, running through the turn filament(making it flash on and off) and then returning on the running/taillight power wire. It is actually finding a ground by running backwards up through the running light circuit.

When the headlights are on, there is power being sent down the running/taillight wire. So this circuit runs to the taillight filament, and then it finds it's ground by running backwards up through the turnsignal wire.

So when you have power on both circuits, the light goes out or won't blink because there is 12 volts on both sides of the bulb filament. 12 volts on both sides means there is zero across the filament, so it goes out.

It is very hard to predict what will happen when you lose a ground. The current runs backwards through the circuits, trying to find the path of least resistance. Of course as you use the car and switch buttons on and off, the path changes. So all kinds of crazy things can happen.

I had a truck once that when I hit the brakes, the dash lights lite up. It turned out I lost the ground between the bed and the frame. I had to run a jumper wire and bolt it to the bed sheetmetal, and then the frame, and the problem went away.

 

I have an 1992 F 250 5.8 liter gas engine w/ automatic overdrive transmission. My tail light's does not work when I turn on the headlight's. My turn signal's and brake light's work fine. I have changed the bulb's and the fuse's, still know tail light's. If anyone possibly know's the problem, could you let me know?

 

There is nothing I could tell you that I haven't said in the above posts. You have a bad bulb to socket connection, bad socket, bad ground.

 

The problem for me was the passenger-side sockets. They were old and corroded after 30 years. I ordered quality replacements from Napa and have had no problems since. Franklin2 was right on the money.