There are four wires going to the rear. They are:

-Black/Red stripe for backup
-Brown for taillights and license plate
-Green for Right Turn&Brake
-Yellow/Blk stripe for Left Turn&Brake

I had similar issues with my brake/turn lights and replacing the cancelling cam in the turn signal switch did the trick. NAPA-$12. If you pull the steering wheel, you can access the switch wiring. That way you can see if you're getting juice through the turn signal. You can also do it from inside the dash, but I figure it's easier in the column--all the needed wires are right there.

I also had one rear light that wouldn't work no matter what, but turned out to be caused by a crappy, corroded ancient trailer harness that had been on there for decades.

Almost forgot- Check and clean the sockets too. They get corroded and don't make good contact after a long time.

 

Well, now I'm blowing the turn signal fuse so I can't check what I was going to check. Oh well. I'll get mine figured out sometime when I have time and feel like messing with it. I'd say that you're on the right track though. If all the switches are good then it's most likely the wiring. Bypassing the switches should tell you alot about what's going on. Good luck!!!!

 

On your lamp sockets front and rear, clean sockets and pin contacts as well the lamp base then apply KOPR-SHIELD a conductive anti-corrosion compound #CP8-TB, made by Thomas & Betts. Get it as a good electricial supply house and never have slow, loss of blinking or lamps out due corrosion problems.
As above stated those turn siginal switches can drive you crazy when they fail which they do fail in time.
.....=o&o>.....

 

In an earlier post you say your "rear headlights" work. I'm gonna guess that your front lights work and your rears don't, or do intermittently. Even if you think your rear taillights work I bet they do so sometimes and that you're just not there to look at them all the time. And when the fronts work and the rears do/dont on and off, it's always the turn signal switch and or the cancelling cam. Take the steering wheel off and press all the connections down together and try them again. I bet they intermittently come on and off. Or hell, just disconnect the turn signal switch harness and jumper the wires Vern listed. If your lights work then you'll know it's the switch.

 

I had the same problem on my 1970 and in my great search for the problem i learned that the blinkers and brake lights run through the hazard relay. Check the relay. Might be it.

 

I just joined and am having the same trouble with my '68 F-250 so I am taking notes. You might also check under articles / electrical for one on:
Turn and Stop Lamp Diagnosis for 70's Ford Pickup

Ken

 

Something to keep in mind- If you do mess with the turn signal switch and/or cancelling cam make sure that it seats properly when you re-assemble it. I went through literally hours of cussing that thing trying to figure out why my lights weren't working properly. That is, until I figured out my horn wire was doubled-over under the turn signal switch plate which was keeping everything from seating properly. Major PITA! The worst part is it was such an easy fix!

p.s. kmartin- Welcome to FTE! Good site to be checking into. Whatever you need to know about your old Ford, I guarantee there's someone here that's dealt with it at some point. They may not always be pleased with their dealings, but will be happy to let you know the truth either way...

 

Just got the wiring diagram from Blue Oval - what a mess. You could never debug this without a PhD. My turn signal switch is new, but it's been in and out a few times so I guess I better check it. All lights are not dead in the rear. Hopefully the turn signal will fix it.

 

Here's more to add to the confusion (or hopefully take some away).

-Your turn signals and taillights are two different filaments within the same lamp so you can have one without the other.

Do your hazard flashers work?

-Your turn signals can work while your brake lights and rear hazards don't. These are all firing the same filament, but the turn signals run through a different circuit in your fuseblock (14A) because they will never flash both sides at the same time (if you try it you'll likely blow the fuse--trust me). The brake and hazards run through another circuit (20A). This is because you have juice to the brake and hazards all the time while the turn signals are switched through the ignition.

-Your hazard flasher switch also controls two different circuits. One is for the front and dash indicator lights (14A) and the other goes to the rears (20A).

If you don't have rear turn signals, the problem is likely with the turn signal switch and/or cancelling cam. If you get turn signals, but no brake or hazard it's probably somewhere in the 20A circuit. There is a Red wire w/White stripe that goes from the cold side of your 20A fuse to the hazard flasher where it joins the Red wire coming from the brake light switch. You can run a continuity check from your brake light switch to the cold side of the 20A fuse and see whether this circuit is intact.

Also, since this mess just keeps getting bigger, you can have a problem with the hazard switch itself where one circuit is working and the other isn't. The hazard flasher can controls the 20A circuit while the turn signal flasher can controls the 14A. Somewhere in this mess (that I haven't quite figured out yet) there must be an 'ignore' signal to tell it not to energize the turn signals when it's getting juice through both sides from the hazard switch.

Crap, reading back through this I'm starting to confuse myself!! If this just made things worse instead of better, let me know and I'll try to clarify it better.

 

Vern,

Thanks for the great not.

I pulled apart the connectors at the bumper and in the engine compartment and confirmed that all my bulbs and the harness up to the dash are working, so I have definately have trouble in the dash. I also pulled the steering wheel and checked my new switch and it's fine as well.

However, after your note I checked the hazzard and it's not working. When I turn the hazzard on I get lights on my instrument panel but no clicking from the flash unit. Based on what you said the 20a circuit must have a problem so I'm going to take a look at that.

I've made lots of change to the dash lately - new instrument panel with all new gauges - so I could have caused a problem there.

Peter

 

OK, I'm making some progress. The dang PO had made a few "modifications" to the wiriing in the dash which I am still trying to figure out and apparently one was involved in this curcuit. When I replaced the turn signal switch I had two wires left over - yellow with brown stripe. I found the other end of the yellow with brown stripe under the dash along with a green. I connected the yellow and brown stripe wires and now have brake lights on one side but no flasher. So I have to figure out what connector the green wire came from, hook it back up and I should be good to go.

Peter

 

Got it. The instructions that came with the aftermarket turn signal switch weren't quite perfect (imagine that). Looking at the wiring diagram I was able to recreate the 2 wire plug that's supposed to be in the dash and move a couple of wires in the 6 wire plug and voila! it works.

Thanks for all the help. Anyone wants scans of the wiring manual let me know.

Peter

 

Yeah, between half-arsed directions and PO mods, troubleshooting wiring in these things can be no fun at all. Congrats on getting it hashed out!

 

I really appreciate all the help. The tips about the turn signal and flasher were invaluable. I could put my test light on each and confirm their operation.

Peter

 

Good job dog.