I have a F100 1978 302, i replaced the brake light socket and now my brake light will not work, signal works hazards work but brake light will not anyone else have this happen

 

Since the hazards and the turn signal circuit use the same wire to the rear as the brake lights the issue might be further upstream. Simplest to check first would be your brake light switch (assuming you already checked the fuse, which I think is also t/s fuse so should be good), it is a simple on/off switch located on the brake pedal arm. With a multi-meter you can test continuity of that switch.

With the plunger fully compressed on the switch you should have infinite resistance across those terminals. Release the plunger and you should see zero resistance. If the switch tests okay, find a good ground and test the wires going to the switch, one of them should be 12v+

If you have zero or low voltage on that line, there are only two more items upstream, which are the t/s switch in the steering column and the fuse itself.

 

Test the ground on the actual side of bulb Body.

 

i changed the brake light switch and it worked for that night and its out again. I will check the grounding and see thank-you

 

not much of a mechanic so how would i test the ground?

 

If the t/s and flasher are working, as well as tail lights, the ground is probably okay back by the bulb, but it doesn't hurt to clean it up and eliminate it as a potential cause.

If it worked for a little bit then stopped either the fuse blew and needs replaced or the issue is intermittent and I am guessing there is a loose/corroded connection somewhere under the dash or one of the switches has failed.

I dug out a schematic to refresh my memory. I forgot the headlight switch is also in the mix.

Constant 12v+ (so hazards work with key off) from fuse panel into hazard flasher, then to the 4 way switch located in column (part of t/s switch assembly). This circuit is working from what you have said.

Keyed 12v+ from fuse panel to t/s flasher > t/s switch > front & rear t/s bulbs. This circuit is working from what you have said.

Constant 12v+ from ignition switch > headlight switch > brake light switch > t/s switch > rear t/s bulbs. If t/s is activated that overrides the brake light on the side that needs to flash. This is the circuit that is not working.

If you have power to headlight switch, but not to brake light switch the headlight switch may be bad or has bad connections.

If you have power at brake light switch, but that signal is not being passed through to the t/s switch, it could be a bad connection between the brake light switch and t/s switch or a failed brake light switch.

If you have power out of the brake light switch into t/s switch, but you're not getting it back out of the t/s switch it could be a bad connection between or failed t/s switch.

I am thinking your culprit is either the headlight switch or t/s switch at this point. Leaning more towards the headlight switch since that is where you get power for the brake light circuit. The t/s and hazards are working, so the t/s switch is likely fine, it is just not getting the brake light signal to pass through to the rear. You will need to test with a multi-meter to confirm though.

Wires going to rear should be left turn/brake, right turn/brake (pulling the 4-way switch activates hazards by sending power to both left and right), activate left t/s only sends power to left t/s bulb, activate right t/s and only the right t/s bulbs. The other two wires heading to rear tail lights would be the running/tail light signal and reverse light signal.

If t/s and hazards are working in the rear, the wires along frame rail and grounds at the rear are probably okay. If brake light switch tests okay, but you do not have 12v+ to one side of the brake light switch, or you are getting an intermittent signal to the brake light switch when you wiggle the wires, you will have narrowed it down to a bad connection or what I am leaning towards which is your headlight switch. There is a lot of testing you can do though before spending more money on parts that may not fix the issue.

I hope all that makes sense, electrical is can be very tedious to diagnose, but if you break it into smaller parts and test each leg of the circuit individually you can usually determine the source of the problem. And on the note of grounds, there are a few under the dash too. It wouldn't be a bad idea to clean the ring terminal and contact area on everyone you find while you're inside the dash.

 

There is a 2-piece connection in the ground wire at the socket. You need to check from bulb contact to ground.

 

I'm guessing that the other side brake light is working since you only complain of the one you replaced a socket on. Signals and hazards use exactly the same bulb filament as the brake lights. If a brake light is not working but the hazards and signals do, then the issue is up inside the cab at the turn signal switch.
Normally if you step on the brake, juice goes to the filament, but if you are also signaling left or right the turn signal switch interrupts the juice to that same brake light so people can see it flashes.
Hazards flash those same brake and signal filaments all as one including the front signals. If you step on the brake pedal with the flashers on, all the flashers will stay lit continuously as long as the pedal is depressed, it gives priority to brake light over flashers.

My bet is your issue is in that turn signal switch, likely is the cancel cam and those contacts just under the steering wheel. Turn signal switch is some like the train master at the rail yard.

Now having posted that, I'll add that the lights in a same socket all use the same ground, but if a ground does go bad then that brake light sharing a ground with a tail light will find a ground through that tail light filament to it's hot wire and then through marker lights that are part of tail lights across those filaments to their ground, but it's a current limited ground as it depends on tail / marker light filaments and will not allow enough juice to allow a bright brake light, only a dim one. If that replaced socket is not also well grounded, then how would a good signal or flasher be seen?

Did all these lights work before the swap?

 

Yes they worked before swap now breaklights are good but when i turn on lights the light wont work but brakelights do. I was mistaken

 

Then you might indeed have a weak / defective ground issue there.

Did you solder spliced wires or use simple crimp connections when you replaced that socket. I strip, slide shrink tube on, wrap splice, and solder using 60/40 and my old Weller Junior gun, wrap a round of 3M Super electrical tape, then I slide that shrink tube into place over the tape and heat it with a match or heat gun, but some say I use 4 nails where 2 will work.

The sockets in '78 have three wires, and one is ground, one is brake / signal, and one is tail. The brake / signal and tail bulb filaments both share that one ground and it may be that your ground is not capable of handling the current of both. The brake / signal filament is the fatter one in the bulb, it lights up the brightest using about 27 watts, but the tail filament only uses about 7 watts. The ground has to handle all the 34 watts if they are both expected to light up. If it can't do it, both suffer brightness, but you may not notice a slightly dimmer brake or signal, but that tail is really gonna be weak .... maybe even so weak you can't even see it in daylight.

I am just using watt numbers to illustrate.

 

yes i just used the simple crimp connections, i will try what u said. Thank you

 

Fixed it, and im not saying how lol its embarrassing

 

A come on now. Ya can't leave us hanging like that...

 

Bet he only crimped two of the three, third was just in there. Easy to do.

 

Mistakes happen, that's how you learn. I too have made embarrassing mistakes and hope that I learned from them so I won't repeat them.

Instead, I like to make new and more interesting mistakes to be laughed about later...