For this problem the only thing in the ECU that would effect it is the internal connection from D29 to D25 being open. However that seems unlikely.

If you can get the steering wheel off and access the connectors, do some continuity checking with a multimeter (set to resistance, ohms) referenced to chassis ground per the diagram. Disconnect the Z9 connector from the horn switch. Connect a test lead to the output of the horn switch. Other test lead to chassis. Push the horn pad and see if you get a 0 ohm reading at the output of the horn switch. If instead you read open or a high resistance value: tens or hundreds of ohms, that’s your culprit.

If you do get 0 ohms when the horn pad is pushed , the switch is OK. Next reconnect Z9, disconnect Z11 from the spiral coil. Test Z11 pin 1 in a like manner. Reconnect Z11. Disconnect connector F23 from the spiral coil and test the coil’s output pin the same way. If you do not read near 0 ohms the coil is bad.

Lets clear up some incorrect assumptions and information.

There are 3 horns on the truck. The S-horn is located on the right fender behind the air filter box. It is associated with the security system, has it's own "s-horn" relay, and is powered through the security fuse. The road horns (2) are behind the front grill, are powered through the "horn" fuse and the "horn" relay. Two different circuits.



The sounding of the security horn on lock/unlock (if enabled) or panic button on the fob is NOT an indication of the condition of the "horn" circuit. That event is all on the "s-horn" circuit.

The road horns fuse and relay are located on/in the "integrated relay" also referred to as 'Unit K' in the manuals. This module is in a slot at the rear of the fuse/relay box.



Top view:


Inner map:


Looks like this:


--------------------------
--------------------------

Since smoisan says that the relay can be heard clicking when the steering pad is pressed then the button, spiral coil, and all ecu connections up to the relay are good and the circuit is complete up to the integrated relay.

I would start at the horns with really basic checks. Does the wire connected at the horn look good as far back as you can visually trace it? Is there +12v at the connection with the horn when the steering wheel pad is pressed? If the wires look good and there is no power then it's either hidden wire damage or the integrated relay is suspect.

The integrated relay can be bench tested and tested in place. I can give you the procedure for that if needed. But first do the really basic checks.

Might be as simple as a critter damaged wire behind the grill.

That's what we get from referring to incomplete diagrams and not verifying our assumptions.

You are correct, sir. Nice research.

Thank you very much for the detailed write-up.

The first thing I did when the horn stopped working was to remove the wire leading up to it and connect the old one. I have Flamm horns but the old horns are still behind the grill. The old ones did not work either.

I will try to see how far I can follow the wire.

Sylvain

The main horns do go off with panic or the OEM alarm is triggered. The S-horn is for confirmation chirps with lock/unlock.

Good additional info. Will be useful in determining which side of the horn relay the problem is on.
I've edited my original post to reflect this new knowledge.

Sounds like someone forgot to change their horn fluid. Didn't you even read the manual?

HI @Deep_palancar

I have had the time to test the circuit today. First I wanted to confirm there was a third horn for the alarm and indeed there is. I disconnected the street horns and press the panic button. The alarm horn was loud and clear.

Next I removed the street horns and cleaned the contacts and tried again. Nothing.
I have tried the original horn without any more success.

The cable is not damaged as far as I can follow it. There is not much to see though as it soon enters a cable protector that runs hidden within a narrow channel of the frame above the radiator and then it's visible again until the fuse box.

I removed the extended wire the previous owner added to connect the Flamm horns and got some help to verify if there was any voltage at the end of the original cable. My gf pressed the horn and my multimeter showed +12.66V which is the same voltage I have measured at the battery poles.

I guess I have to test the integrated relay?

How can the horns not work when the circuit is closed since I am reading +12V?

Thanks,
Sylvain

12V is not necessarily a true test of a “closed” switch. When you measure the voltage of a circuit, you are actually measuring the amount of voltage dropped by resistive components in the circuit. If you are measuring across one component, Ohm’s Law will apply. If you are measuring ALL the components of a circuit together, you should end up with source voltage.

My recommendation would be to see if you have current in the circuit of your closed switch. That will provide some valuable insight to the problem at hand.

An easy troubleshoot I used for wire diagnostics:

Current/No Voltage = Closed circuit, no resistance (Potential short to ground)
No Current/Source Voltage = Line Break or Open Switch
Current/Voltage = Normal circuit. Check Ohm’s Law values against schematics.

Street horns? Like the normal horn that is OEM? Flamm horns?
It's a pretty simple circuit. At the OEM horn, green/red should have power when the relay energizes by hitting the horn on the wheel. You also have ground, which the OEM horn should be securely bolted down. 12v and ground is all that is needed to make the OEM horn go off.

If you have +12v at the wire end then the only thing needed is a ground and a good horn.
Some horns have two posts, one for + and one for -(ground), some have only one post for +12v and are grounded via the mounting post.
Test the horn with direct connections + and ground to the battery.
If they work then clean the ground connection; either wire or post to ensure good metal to metal contact.

Hi,
I will check the direct connections later this week. The original horn has only one male post while the Fiamm horns have two. But it was connected with only one cable while being attached to the frame and this was working. They look like this:
New Fiamm Horn Install

And that's basically the mod that the previous owner did.

Thank you again for the help.
Sylvain

I have checked the OEM horn today. It was removed from the truck so not grounded at first.
I ran two wires, (+) and ground, from the battery terminals directly to the horn. It worked.
Then I re-attached the horn to the truck in its original spot so it was grounded.
I ran a wire from the positive terminal of the battery to the horn only connector and it worked.
I then connected the original wire with female connector to the horn and pressed on the horn pad. Nothing.
So the issue appears to be between the fuse box where the relay is located and the horn itself.
What would be the next step?
Thanks,
Sylvain

...did you say that the Horn relay is clicking when the horn pad is pressed? That does NOT mean that the relay is good; could have bad load feeding contacts. But if it clicks, that just means that the primary circuit from the horn-pad is working, which activates the relay coil... first-things-first!

rig, cheers

It's looking more and more like a bad integration relay. One final check that I would do before purchasing a $130ish part is to check the whole wire from the relay to the horn.
I pulled my integration relay and found that the connectors and wire bundles come out of the box far enough to easily probe the back of the connector. (I was expecting the connectors to be affixed to bottom of the fuse box and difficult to access.)
Pull the integration relay up and out of the fuse box.


Stick the back of connector 1C pin 8 (Grn/Red) with a wire or your multimeter probe. touch the other end to the battery +. If the horn does not sound, run a new wire. If the horn sounds, replace the integration relay.


This part is plasti-welded closed so opening it up to try to replace just the actual component would be risky. There is circuitry in there that controls the fuel injection and ignition so a screw-up would leave you parked until you could get a new one.

FWIW, you stated earlier that you did get +12v at the horn wire; It's possible to get a good voltage read when there is no load on the circuit (it only takes a little current for the meter to register) but there is not enough current to activate the device. This would be a classic symptom of bad contacts in the relay.

Curiosity questions.. What size fuse is the horn slot? Were all four horns originally connected?