I gotta 77 F250 with after market tail lights on it. I didnt put them on so i dont know anything about the wiring for them. But right now it doesnt have any tail lights and so far havent figured out why. The brake lights work except there so dim you have to stare straight at the light to see them, and the blinkers only work when im not stepping on the brake. When i step on the brake all the marker lamps come on with the brake lights and the blinkers stop working if there on. The tail lights are brand new. So, any ideas?

 

Go over all the grounds stem to stern, start at the battery and clean the terminal then the ground that connects there near the batt. the ones on the motor all the tiny ones on the body, all the frame ones front to back. Actually pull them of sand paper them to shiny, get a dremel or drill with a wire brush and shine up the frame or body where it connects, Then apply some dielectric grease and reconnect. You could run a wire straight from the battery negative terminal to the back of the truck and connect it to the main ground for the tailights back there or T into the ground wire right into the harness if you want the quick and dirty way!!

 

Aftermarket trailer harness?? Check for melting in the harness or a short somewhere. disconnect the harness and see if that fixes your problem....

 

Also, if the taillights are the circuit board type, check the board itself. I've seen issues with chevy's that have the circuit board type rear lights and they act up in such a way that you are describing. pull the light assembly out, and with a screw driver pry up on the circuit board assembly lightly and see if the lights change. that would be bad contacts in the circuit board assembly.

 

What I would do is pull the taillights out, and figure out how they work. Get a small battery charger and hook the taillight wires to the charger so you know what wire does what, including which wire is the ground. Once you mark them with masking tape or something, then you can use the diagram in this link to hook them up.

http://www.autozone.com/images/cds/g...3d800ce985.gif

 

[QUOTE=Franklin2]What I would do is pull the taillights out, and figure out how they work. Get a small battery charger and hook the taillight wires to the charger so you know what wire does what, including which wire is the ground. Once you mark them with masking tape or something, then you can use the diagram in this link to hook them up.

http://www.autozone.com/images/cds/gif/large/0900823d800ce985.gif[/QUOTE]

Thanks I shall try that

 

If by all the marker lamps coming on when you step on the brake, You mean the front marker lights are coming on. That is a indication that the lights are searching for ground, meaning somewhere you got a bad ground in the circuit somewhere. I go along the thinking if one is bad the rest are probably not to healthy either. I would go over all of them. You could just do the ones in the back if it's too much work to do all of them starting at the battery.