I'm continuing to slowly fix my 1994 4Runner. The next problem is that my horn is stuck ON.

1. Initially, neither the horn nor the hazard lights worked because the previous owner removed the HAZ-HORN fuse.
2. Installing the fuse made the horn constantly sound.
3. I pulled the horn relay and then installed the fuse, so now my hazard lights work but my horn still doesn't. Whenever I plug in the relay, the horn sounds.
4. Both the existing relay and one I just pulled from a junkyard have the same behavior, so either the problem isn't the relay or they've both failed the same way. (I also tried putting voltage across pin 1 and 2 of one of the relays and the contact closes, so I guess they work.)
5. I removed the horn button from the steering wheel and found that I'm getting 0V between the green wire and ground.
6. I checked the voltage at the horn relay socket and found that pin 1 is at 0V, pin 2 is at 12V, and pin 3 is at 0V.
7. I realized that I'm being stupid and that #5 and #6 probably don't make any sense with the relay disconnected, so I tried jumpering between horn relay socket pins 1 and 2 at the relay, expecting it to cause the green wire to be 12V at the steering wheel if the connection were working correctly. Instead, the whole dash started lighting up and something started emitting a high-pitched whine intermittently. It was at this point that remembered that there's some mystery wiring installed by the previous owner and started wondering if that has something to do with it. (Maybe it's some kind of alarm system?)
8. I also tried doing the smart(ish) thing and checking for continuity between pin 1 on the relay socket and the green wire at the horn switch on the steering wheel, and yes, there is continuity.

I'm a bit stumped about what to do next. I suspect I have to figure out the aftermarket mystery wiring, but I'm not confident in my ability to do that. Help?

(By the way, I've been mostly using the FSM I downloaded and this thread as resources, but let me know if there are any better ones I should look at.)

 

If I remember correctly the horn grounds out thru the button on the steering wheel. So, maybe there's a grounded ground grounding your ground....

 

so you are saying the ground is grounded like an F-14 HOrnet?

Pull your steering wheel, look at the brass ring under there, also under the horn button there is a brass rod with a spring that likes to wear out about 25+ years or so.

 

When I removed the steering wheel pad from the steering wheel, I did a continuity test and the horn button seemed to work fine (continuity between the spade connector the green wire plugs into and the metal frame that touches the rest of the wheel when pressed, no continuity when not pressed). Is that what you're talking about something inside the steering wheel pad itself, or are you talking about something that's part of the rest of the steering wheel or steering column (before the brass ring)? If the brass rod with a spring is inside the steering wheel pad, I think it's fine.

I'll see about doing that next.

 

Correct.

Measuring voltage to ground won't tell you whether the point is connected to ground, or not connected at all (you'll get ~0v either way). Pin 3 of the horn relay socket is connected to ground (through the horns), and pin 1 is only connected to ground when the horn button is pushed. Connecting pins 1 and 2 is shorting (what would be) the relay coil, so it would put 12v on the green-red wire at the steering wheel, and blow the fuse if you pressed the horn button.

So to find whether pin 1 is grounded (it almost certainly is) v. disconnected, you can measure the voltage from battery+ to that pin (~12v means its grounded). You already know that pin 2 is at battery+, so just measure the voltage pin 2 to pin 1. You'll probably get ~12v, which is closing the relay, sounding the horn. That would mean the green-red wire at the relay socket is grounded. But you tested the green-red wire at the steering column, and it was only grounded when the horn was pressed.

What this suggests to me is that your alarm system has disconnected the horn button somewhere along the green-red wire, and grounded the green-red wire on the horn relay side. That's where I would look.

 

What Scope says above makes sense where pin 1 is directly connected to ground.
If you removed the steering wheel plate/ring that contacts the pin that looks like a 22-cal shell, with haz-horn fuse in, does the horn still sound?
To eliminate doubt above, Remove steering wheel so noting is contacting that pin. Does horn still sound?
That pin is connected to G-R (Pin 1 of relay).
On a normal system with steering wheel removed, grounding that pin to the steering column should make horn sound.