Hi all,

I'm trying to get to the bottom of some electrical issues with my 77 f150 with 351m and 4x4 manual transmission.

As the title states, the horn is always on, whenever plugged in, and the fuse in the lower left for emergency flashers and brakes blows instantly. I can verify that there is a short with a test light hooked up to battery. When I pull each fuse i don't find the short, that is the test light stays on each time.

I would love to get any input as to things to troubleshoot next or how to get to the bottom of this issue.

Thanks in advance!

 

I'm guessing some damage to the harness going up inside the steering column...

 

There are 9 wires that run up the steering column. That single "Blue with Yellow tracer" wire is from the horn relay, it serves as the ground for the coil in the relay, so when it touches ground the activation circuit activates as it gets the plus side otherwise. In normal operation, that wire is grounded when you press the horn buttons on the steering wheel. If that "Blue with Yellow tracer" wire develops a bare spot to ground from wear or a cut, the horn will sound off.

 

Thanks for the input guys. I did also change out the stock wheel for a grant signature that was new old stock. I believe I cut the coil as specified in the instructions. I'm doing a bit of restoration and upgrade at the same time, so I can't recall specifically if the steering wheel could be causing the horn issue. I don't think the horn ever worked. I'll pull it tomorrow and check again. Pretty sure the emergency fuse was blowing previously to the steering wheel upgrade. I also upgraded the radio but it's been working fine and I think the wiring all seemed pretty cleanly done.

I was just surprised to see that a fuse was blowing and I had a short without even after pulling all fuses individually and checking. The emergency fuse that blew was also pulled for all of the testing.

Not sure how I can get in and test the wire in the column.
Also, I tested the wires at the relay. The yellow with green stripe was constant 12v and the yellow wire was showing no voltage. This isn't what I expected.

​​​​​​I Just wanted to throw some more info out there in case it helps. Clearly I'm still learning a lot but this site has been a great resource and I'm thankful for it.

 

The wires going into the steering column look good and clean. I pulled off the grant steering wheel and can confirm the short is still present after removing wheel and horn hub. Not sure if I'm dealing with 1 issue or two. Emergency fuse blowing and horn ground shorting, it seems. Any other ideas are most welcomed. I'm still trying to trace wires and see where there may be a problem.

Thanks!

 

The horn relay should have three wires. Only one should have 12 V on it.
One of the others goes straight out to the horn itself, and should not have any measurable voltage on it when you’re just testing the connector.
The other goes up to the horn button in the steering wheel and may also not show voltage.
Then again, now that I think about it, it might.

Someone else will have to answer that. I don’t have a factory three wire relay on any of my Fords. All of them are standard Bosch type cube relays that I installed myself. So I don’t have a Ford circuit to verify that you can measure the voltage on the ground wire.

is your truck rusty at all around the horn relay? This might have nothing to do with anything, but I would still clean the mounting surface and make sure it’s got good contact with the body and screw for the relay ground.

 

Thanks for the reply. I've been digging quite a bit into this yesterday. Let me first say that the relay is new. I'm getting 12v at the horn constantly. It would appear that PO may have installed a new turn signal switch and I'm guessing they did so incorrectly. The blue/yellow horn relay wire does not connect to the blue and yellow wire on the switch.

Blue/yellow on switch is going to blue/red which appears to be something in the cluster... Still looking. And the horn relay wire which is blue/yellow is connected to a green wire that goes to the top of the turn signal cancel switch. So I'm not getting 12v at horn button. If there is a wiring diagram showing the connections of the turn cancel switch to factory wires on that gray horseshoe looking plug, if love to see that as I can't seem to find that. Fordification doesn't seem to have 77 diagrams.

Even more helpful, if someone could let me know what the connections are like on their 77 that would be ideal. Do the harness wires match up color wise at the plug and if not which color goes to which on that plug and what are they connected to on the harness?

Blowing the 20A fuse seems to be a separate issue that I'm trying to track down in parallel. Dies anyone know how to hook a test light to the fuse box so I don't keep blowing these hard to find fuses?

Thanks so much for all the help.

 

For anyone who may be interested, the horn issue has been resolved. Thanks to all for the help. The turn signal switch wiring was hooked up wrong. So it was definitely an issue with the 8 wires going up into the column.

I still have a dead short to ground blowing the emergency hazards/brake fuse that I could use some insight on if anyone has any clues. I looked at wiring back to brakes/lights and it seemed okay. I'm about to try a cable tracer. It would be cool if there was a way to hookup a light bulb to the fuse panel and see what makes the light go out.

 

does the fuse still blow with the turn signal switch unplugged?

 

Sounds like the mishmash wires were a dead giveaway. And yes, the blue with red stripe wire is for cluster illumination. But that’s also for the steering column drive indicator illumination. Hence its presence in the column. At least on automatics.

 

Thanks again. Yeah, I have a 4spd. Also, the fuse was blowing with steering column wiring unplugged.

Both issues have been sorted out. I can't thank folks enough for posting resolutions to their threads as I've found them invaluable when troubleshooting issues.

Long story short. The horn wiring was wrong from the column to wiring harness. Hooking up the green wire to the blue/red fixed and hooking blue/yellow up to blue/yellow fixed the horn.

​​​​​​The fuse was blowing due to a dead short in the brake switch which was spliced into cigarette lighter. The haynes schematic helped find this issue.

Thanks again!!! Onto the next ones. 😁

 

Onto the next one, indeed! There’s always the next one! :-) Congratulations on sorting it out so quickly.

 

Nice! Well done. I learned a difficult horn wiring lesson a few weeks ago. I rewired the whole truck and learned the hard way why the electrical engineers never put two identical connectors right next to each other when they design a harness. I had two 1x6 weatherpack connectors in the same area. After I disassembled the front clip to take the cab to the body shop, put it back on, and plugged everything in again the horns would blare when I turned the headlights on but the headlights wouldn’t work. I nearly pulled my hair out and annoyed the s*** out of the neighbors chasing the problem. Hours went by before I realized that I simply had the weatherpacks reversed. Still feel stupid about that one.

 

I swapped a ‘79 cab into my ‘77-1/2 back in the early 90s. I was surprised that not all the connectors, or even the colors were a match. I spent a lot of time ringing out all the wiring. I used wire nuts until it was all sorted out. I figured I would find a connector later, or at least solder and heat shrink them. The wire nuts were still there thirty years later. You know, the cobblers kids and all. In my defense, the truck hasn’t been on the road in 25 years. I’m going to treat it to one of those expensive wiring harnesses during the rebuild.

I’m glad you got the horn wiring sorted out. It’s probably easier troubleshooting in a sewer plant than working on these.

 

Not that you’ll ever do that again, or perhaps never even need to take the two connectors apart, but a good thing to do, perhaps for future generations, might be to give a quick spritz of paint (color of your choice) to either one of the two connectors. Leave the two halves connected, and just give it a quick shot.
That way no way to mistake them if anyone ever has to do electrical work.
Well, at least make less likely! 😉