My 68 350 coupe has an interesting issue. For no good reason,starting a couple of months ago, when i open the door, the horn goes off and stops when I shut the door, it stops.
I disconnected the two horns so I could drive without the horn attracting attention. Quite by accident, when driving at night, I went to hit the horn (forgetting I disconnected them) and the courtesy lights and dome light when on and do any time i hit the horn on the wheel, otherwise the courtesy/dome lights work fine and the car has no other electrical issues.
Any thoughts

Maybe a horn relay issue? The horn relay doubles as a key reminder buzzer when the door is opened. Both the dome/courtesy lights circuit and the horn circuit both meet up in that relay.

What happens when you put the turn signal on? Windows go down?
Sorry, kinda funny situation you have. In a lot of cases it can be a poor ground for one or many devices. Electricity will try to ground it self to complete the circuit, sometimes affecting other devices (lights go on/off, horn sounds). It would be a good idea to make sure everything is grounded properly on your car and then do further troubleshooting.

Someone on this site may have a diagram for all the grounding locations. Make sure your ground straps are clean and installed.
They exist in your engine compartment wiring harness, engine to frame, interior (cab and trunk) wiring harnesses and the instrument panel.

Hope that helps ... beep beep

Never heard of that before.
Was key buzzer an option. Don't remember mine have'n one.
But may of disabled it back in 77...like on all newer cars i've owned.
Would think they'd be on separate circuits...

Yes, the key buzzer is standard in '68. I'm not sure mine works now either. I think they are annoying.

Well, there you go.
Thanks.

Yeah my key buzzer still works. It sounds like a dying locust caught behind the left headlight...one of my favorite parts on the whole car!

It's a conversation piece for certain.

What's that awful noise under your hood?
Just the buzzer for the key in the ignition.
Why did you install it under the hood?
I didn't. That's where Pontiac put it.
That's factory?!
Yes.
Wouldn't it make more sense for them to have put the buzzer inside the car?
Yes. But I assume this was the beginning of a long standing tradition of putting the key buzzer near the hood release.
(silence)
The hood release isn't inside the car?
No.
Why not?
Different topic. Move on.
OK. Why does the buzzer sound like a dying locust?
Because it's 45 years old. That calculates to like 293 in car electronics years.
Why don't you disconnect it or drive a wooden stake through its heart?
Because it starts conversations like this one. I wouldn't want to miss out.
Why did you leave your key in the ignition?
So I could make sure the buzzer still works.

And so on...

X2 That's funny.

Didn't realize I had a buzzer...something else to fix.

LOL.
So their under the hood? Don't remember see'n one. Is it actually the horn relay?
You would think i'd know having owned a 67 and two 68's since 75...

Indeed. The dying/screaming locust is trapped inside the horn relay itself. Jim's diagram in post #2 shows it as the contacts at the top of the picture. Those contacts vibrate like mad when voltage passes through. Terminal 4 carries the voltage, feeding it toward the ignition switch whenever key is in the ignition and doors are open. The ground for that circuit which actually triggers the noise is the drivers door jamb switch...that's why there's an extra terminal on the drivers door jamb switch with an insulator between the 2 terminals (if memory serves it's a black wire for buzzer, and white for the light? Maybe.), and there is only one terminal for lights on the passenger side jamb switch.

Should be easy to test your buzzer by applying 12V direct to terminal 1 (which is already done assuming your horn is functional), and then you simply jump from terminal 4 to ground. The moment 4 touches ground it should buzz if it's working. If it does, you're just missing continuity somewhere between terminal 4 and the door jamb switch...and most likely you'd lose that continuity at the ignition switch itself...OR you have the wrong door jamb switch installed/or 2nd wire is disconnected.

I discovered this whole stupid thing when I replaced my door jamb switch. I never knew the buzzer was there either. All of a sudden I replace the switch and eureka I have interior lights! Yay! And then I put the key in the ignition and I get this terrifying sound from under the hood which I never heard before. WTH?!!? I learned all this about 10 months ago, and it took me some exploring in my service manual to solve the mysterious noise and come to the conclusion it was entirely normal LOL, after finding that text and diagram that Jim posted in my service manual. Initially I was worried something really bad was happening electronically and that my horn was somehow getting partial voltage thru my ignition switch, but soon learned it was working exactly as designed!

Mine has the buzzer as well. Very cool. Also has the single horn.

Henry, I got us all sidetracked here and it seems we're ignoring you sorta. Very sorry...

Here's what I think MIGHT be your problem. Wires on the horn relay could simply be swapped. Check wire colors to a wiring diagram. It just seems to me that if wire #4 was hooked to terminal 1 or 2, and wire 1 or 2 was hooked to terminal 4, and maybe in conjunction with a bad insulator at the drivers door jamb switch...or incorrect wiring at the switch...

I'm thinking some combination of this might do what you describe...

Failing that, I can easily see corrosion or some sort of failure within the horn relay itself sending voltage to places where it shouldn't be sent...like that coil broke and a wire from the coil is now shorting to terminal 2 and/or 4 internally? You might need to replace your horn relay.

Mine sounds like a sick tree frog.

Thanks for the reply. My service manual does not have this diagram and the buzzer never has worked so the horn relay makes sense, since there is no other electrical issues with the car. If anyone does have a diagram with the grounding points on the car that would be very helpful. Thanks
Henry

How could the wires be swapped if it started "for no good reason"...unless he changed or messed with it, and didn't make the connection...LOL

If someone has a diagram for all the ground locations that would be helpful. Like any old ride, chasing down electrical issues can be futile, but it seems the best place to start is to make sure there is good contact to ground

Full wiring diagrams are in the service manual. Free download from this site, along with some color re-makes of a PORTION of the 68 wire harnesses. Beware those color links. As pretty as they are, they simply don't get them all, and frankly I don't find them as easy to read as the original diagrams in the service manual, but everyone is different:
http://thefirstgensite.com/html/librarydex.htm

Thanks. I have a service manual but didn't know if there was a separate diagram that shows where the body grounds are such as the one on top of the gas tank.
My horn relay has three terminals and and two screw connections along with a capacitor at the screw that holds the relay to metal. The diagrams I have seen show four terminals and no screw connectors.

I am unaware of such a diagram. And remember there are FAR more grounds than just those completed by wires, if that's all you're looking for. In fact I'd say the vast majority of the grounds on these cars are not completed by wires at all...they're switch bodies, bulb sockets, the tail lamps themselves, the horn relay case, the engine block, the alternator case, radiator support, instrument panel...etc etc etc...

Any place that one rusted (or freshly painted) part is bolted directly to another could be a compromised ground path, if it is an electrical item.