Hi All,

I’ve finally got my ‘72 f250 in good running condition after being off the road for 24 years. I’ve moved on to addressing the things that I’ll need to pass inspection, mainly electrical.

My main issue right now is the taillights. Some things work on one side and not on others. On the driver side, the running light and side marker works, but the brake, flasher, turn signal and back up light don’t.

On the right side, the flasher turn signal and the brake light work but the running light side marker and backup do not.

I’ve already replaced all bulbs and the turn signal flasher (c-76 or c-79 in the diagram, not sure which)

I am still learning how to read wiring diagrams and wondering if someone could help me figure out the path of flow from the fuse to the driver side rear turn signal.

The attached is how I think it would go…

If I’m right I think it would most likely be the turn signal switch. I cut the yellow wire right before the light and checked it for voltage with the turn signal on and it read less than .1V so I don’t think it’s a ground issue.

Thanks!

 

Not 100% certain & should have already been n garage working on my truck so a quick guess is it is the turn signal switch has a broke piece. U will have to check all wire, connections , fuses, grounds, Many of the turn signal switches break a small arm in them and are replaced by a stronger made aftermarket switch........I goota go 4 now.........

 

Older Fords were notorious for ground issues that would cause the tail lights to act strange. I would double check any physical grounds under the light assembly. Most of the problems I used to have were with the bulb sockets themselves. Make sure there is clean metal on the inside of the sockets as well as under the two tips that you have to push past in order to twist the bulbs out.

 

As lah2420 talked about, poor grounds will drive you crazy on our old trucks. You can search for posts about adding ground wires throughout the truck. Sorry I'm not on my desktop or I could post some pics. You could add a tempory ground with a wire and a couple of clips on the wire ends. Add a SMALL amount of dieelectric grease to the bulb socket and the contacts. IF the bulbs are good and the grounds are clean and good, then check out the turn signal switch.

 

I would go back on the tail light harness to a factory plug and pull it apart to make sure it is making a good connection.
Before plugging it back together using a test light, meter will work also, check the truck side of the plug for power to each wire.
You should have
1 wire for tail lights
1 for left brake / turn light
1 for right brake / turn light
1 for back up light
I dont know if you have a ground that runs through it or not, my 81 F100 dose not but in 82 or 83 they do have a ground wire in the frame harness.

If you dont have power at this plug then you will not get anything out back so even if you added grounds (not a bad thing) you would still have no lights.
If you have power there you will need to move forward to next plug if there is one or a easy place to check the same wires that are out back for power.
Could have a broken wire in the frame harness, power up front but not out back.
For the brake / turn you may have to test at the plug on the column if you get power there. If not then I would say the switch was bad.
To cheat you can try contact cleaner sprayed into the turn switch and move the switch back and forth. The contacts can build out a film from sitting and stop working.

That is how I would go about fixing this
Dave ----

 

Update!

Okay so I got (almost) everything to work! Brake lights, turn signals all work. Turns out someone had put in some really bad trailer wiring and just cut wire’s galore. I wish I had looked for that before I did a lot of the other stuff but it’s all part of learning!
The only other issue is that the left turn signal is extremely slow like once every 3 seconds, and the indicator doesn’t light up. The right one is fine. After reading some other threads, I think it’s too much resistance? I’m not sure if the resistance is before or after the turn signal flasher but since the right one works, I’m thinking after.

also, I replaced the turn signal switch and took off the shifter collar on the steering column. If anyone has tips on getting the square head screws that hold it on, back in correctly, that would be so helpful!

 

You gotta love fixing up someone else's back yard repairs. I would say the problem is anywhere from the output of the switch to the bulbs. Are both front and back left bulbs coming on when you put the switch to the left? The flasher relies on all the bulbs working to make the signal work right. The flasher is really just a thermal switch that opens when the current heats up an element in it. Less current would make it open slowly. I would replace the indicator bulb too if it isn't coming on at all. Beyond that you will probably find a corroded splice somewhere in the left signal wiring circuit. Sounds like the previous owner blessed you with his wiring brilliance.