Seeking a direction for resolving an electrical glitch on my '73 F100. I can't get my right (passenger) brake light to work. The left one does work & as well as all other lights (head lights, side markers & turn signals) . The right tail light & turn signal both function but can't get
the blasted brake light to come on when brake pedal is depressed. All of the wiring going to rear of bed looks solid. I cleaned up grounds to bed & the tail light sockets look to be in great condition. Any ideas on where to begin ?
Thanks !

 

The right tail light & turn signal both function
If the turn signal works on the right side then the brake light has to work as the same wire to the bulb dose both.
Well maybe not
I forgot when the brake is pushed and the right turn is on the turn signal switch cancels the brake light to that side.

Has the truck been sitting for a bit?
You can try spraying electrical contact cleaner in where the switch is and move the lever back & forth to see if the contacts will claen up and start working.
Other wise replace the switch.
Dave ----

 

Dave, thanks for the feedback. I suspected it might be the brake light switch. Because it looks like it's original I will plan on replacing it with an after market switch. I just noticed that when emergency flashers are on I get nothing from that right tail light also. So, it is just the right brake light & emergency flasher that don't function on that side. All other lights function as they should. The truck has sat for a few months & the last thing I did (in Jan) was a full rebuild of the auto steering column. Because of that I wondered if it could have something to do with the turn signal switch (socket issue ?). In any case will start with the brake light switch replacement. Cheers

 

It's not the brake light switch. There is only one wire that comes out and goes to both sides in the back. If one side works, the switch is fine. I would double check your ground, check the light bulb, and light connections. Then check the wire coming from the driver's side to the passenger's side rear.

 

Yeah, FuzzFace was referring to the turn signal switch.
The TS switch is literally the only other thing that all those gremlin clues have in common. The hazard switch is wired into the group as well, but it's usually the turn signal switch that fails first.
The fact that the bright filaments in the lights work (turn and flasher) means that it's likely not a bad ground. However it never hurts to check them, so you can refresh them while you're at it. If the difference between running lights and turn/hazard is not that notable, a bad ground is more likely.
Connections do deteriorate over time.

One test might be to put your foot on the brake so you can see that the left side is lit, then flip the turn signal lever to both sides and see what happens. The working side should go out as soon as you turn the lever to that side.
Hopefully you'll at least see a flicker on the side that does not work. But if not, it's very likely you have a turn-signal switch replacement in your future.

I forget, does the '73 have the hazard/flasher switch on the dash, or is it integrated into the turn signal switch and sticking out the side of the column?

Paul

 

The hazards are part of the T/S switch.
My 2c on the question.
The brake lights are a single wire up to the turn signal switch. The circuit then gets routed through the switch where it gets split into two separate wires, one going to the left turn, the other going to the right.
The fact that everything else works except the one side brake light would lead me to concentrate on the turn signal switch.

 

Double post!

 

Guys, thanks for all of the great insight. As stated in my earlier post it's only the right brake light & the right tail light emergency flasher that I can't get to work. The tail light & turn signal both work on that side. Paul, with brake pedal depressed I can get both turn signals to flash (rt & lf). And, both flash brightly. Would that still speak to having a faulty turn signal switch ? I am going to try & find the wiring circuit for my T/S switch so I can ID which wire controls the right brake light.

 

Here is the relevant factory diagram.
The t/s wiring is in the upper right quadrant.

 

I missed where you said hazard in your second post. Only saw the brake light aspect. Glad you reiterated the details. Still the same results I think.

I'm going to say yes.
The fact that the turn signals are bright and the tail lamps dim, says to me that the bulbs and contacts in the housing are good, and that nothing is hunting for a good ground through the wrong path.
Still sounds like it's shouting out "turn signal switch" in a loud voice!

Thanks to Mike, there ya go. One wiring diagram, comin' right up!
The right hand brake lamp (stop/turn/flasher) according to that diagram is the White w/blue wire. Again, same single wire handles the turn signal, hazard flasher AND stop lamp duties.
So from at least the firewall back to the rear of the truck, there is only one wire working that corner's bright filament.
Nothing wrong with the wire or the lamp housing itself because you have turn signals.

The Brown wire handles the running lamp duties on the second filament of the tail light socket AND for the side-marker lamps.
In theory your '73 should still have the early style lamps in the rear, where you can remove the lens and there is a metal reflector housing with integrated sockets. Nothing wrong with checking them out at least, but the problem is not back at the lights themselves, but somewhere upstream.
Hence the focus on the turn signal switch.

Regarding the tail lamps though, trust the diagram, but verify! Because some things just don't work for the craziest reasons!

Paul