So no brake lights, seems like a simple problem.
I don't know when they went out, I tested my lights a monthish ago and I'm pretty sure everything worked. Now no brake lights.
I figured it was the brake light sending unit, but before I just went and replaced it. I tested it for continuity. It has it when the pedal is pressed and none when it's up.
Ok so that tests ok right?
So I test for 12v on the one wire, check, we have 12v.
Hmm well I figure lets crawl under and follow the wire.
That appears to check out as well but of course I can't see every bit of it.
So lets see if I have continuity to the rear.
Yes I do, and I have 12v to the rear as well.
Ok this is weird, well lets clean the ground up even though I really don't think that is the issue because every other light works.
Turn signal, running lights, hazards, tail lights all work. clean ground and still no brake lights.
I figured also if it was a ground, I wouldn't have the issue on both sides and my tail lights wouldn't be working either since it appears they share a common ground, right? or am I looking at it wrong?
The bulbs appear to be good and clean contacts and I even swapped one out with the same results just to rule out both bulbs didn't fail at once.

It seems pretty dang simple to me but at this point I'm
jumping the 2 wires together my lights should come on, right?
Had no luck with that either
Usually when the switches go bad, you have brake lights all the time; I thought.
So again I really don't think its in the switch, but where can it be?
Disclaimer:
I had to replace my master cylinder/vacuum booster in the last month.
I did catch on some wires which caused some ignition grief but I attended to that and didn't think anything else was wrong. Related? I don't know but I will include it.

 

Dang, it pretty much has me stumped. The only things I can think of are the sockets, do they plunge down when you put the light in? If they do not, then that's probably it, but it would be weird for them to both go out at once. I can't remember if there is a fuse that goes exclusively to tail lights, but you might want to check that

 

Oh I forgot to include I pulled every fuse and looked at each one, they all appeared to be intact.
The sockets appear to be ok, the tail light turns on nice and bright. as do all the other lights.
Don't know if this is right, but I just went and jumpered the 2 wires again. and put my voltmeter to them. I have 12 volts until they are jumpered and it drops to almost zero than.
That doesn't seem right. but at this point I'm upside down.

 

The brake lights work through the turn signal switch. Do your turn signals work?

 

yes they do, all 4 corners.
I have an "engineered" turn signal. but everything worked. until now with the brake lights.
PO definitely did some rewiring on this truck.

 

Have you read this https://www.ford-trucks.com/forums/1...ml#post1812190 and this?

Turn and Stop Lamp Diagnosis for 70's Ford Pickup - Ford-Trucks.com

 

"engineered" makes me nervous.
If your turn signals work, engineered or otherwise, then the wiring, bulbs, sockets and grounds are good. If you have power going from the brake light switch to the turn signal, and the rear turn signals work, that's narrowing it down to something with the turn signal switch....or whatever "engineering" the PO did to it.

 

Three suggestions:
1) Ground issues at the front can affect the rear - I had a bad socket in the front which had a great ground and great power, but it had intermittent contact between the ground inside the socket and the ground wire - when I replaced it my rear lights started working right.

2) Don't just check your voltage and ground, check continuity through the sockets too (see above). The wire from the socket may ground great but if it has intermittent contact inside the socket, you'll play hell tracking it down externally.

3) Try replacing your bulbs anyway - I have occasionally had odd problems which went away when I put in new bulbs.

Maybe make a long alligator clipped jumper wire, disconnect your 12V or ground wires at critical points, and see if things work OK when it's jumpered? Might help narrow down where in the whole 12+ to bulb to ground process your problem lies. After that...I got nothin'.

 

Yes I read that article before asking here. Have it bookmarked it to help me figure out this mess.

I'll start checking the switch, in my case it's actually a toggle switch they wired up to replace the turn signal switch.
I've left it alone because of the saying if it ain't broke don't fsck with it.
But something is broke now...

 

Do your hazards work?

Both the brake lights and hazards are powered by fuse 8 in a '79. In my case, the back side of the fuse holder corroded away and no longer supplied any power to the circuit. In your case, it may have corroded enough to show voltage with a 10 meg ohm input digital meter but drop to zero when you put a real load on it.

I was able to figure out which wire it was in the bundle, cut it and reroute to to the front side of the fuse block so I could get home.

I also had the opposite failure, the brake lights staying on full time. A tye-wrap fixed that.

Michael

 

Also, check the fuse for continuity. I have seen some that have blown but at the very end where the cap hides it. Pretty rare, but it can happen. I always check with a multi-meter to be sure.

 

I don't know what re-engineering the PO did to the turn signal switch but the wire that comes from the brake light switch to the t/s switch should be red with a black trace. There should also be two wires for the brake lights, one green and the other yellow with a black trace.
I would find the red/black wire and follow it to the t/s switch. Apply the brakes and check for voltage as close to the t/s switch as I can get. If you show 12 volts, I would then find the green and yellow/black wire and again, as close to the switch as I can get, measure for voltage. With the t/s in the "off" position you should get 12 volts at both wires when you depress the brake pedal. If you don't, then the switch needs to be replaced.

 

I think I'm going to try my hand at a video, it would be easier to show.
My red/black wire does have 12V, I just need to get into the column wiring so I can trace back.
In troubleshooting yesterday, I discovered, when signal(s) are on,
they work normal and than over time (minuteish plus) they start slowing down and eventually stop. or something is causing it to overheat/
Just guessing; but the relay is probably going bad as it feels slightly warm by the time the blinkers stop working.
wyrm73, fuses have 12v on both sides,
lasermike hazards do work

 

The slowing blinker is more than likely a flasher on the way out. That shouldn't have any effect on your brake light problem though. When the turn signals are off, the flasher is removed from the equation.

 

Here is my first video ever.