Automotive electrics fault finding sanity check

The horn isn't working on the camper, so I have started fault finding (which lead to a previous question about test leads).
Long post - can someone please sanity check process and conclusions :-)
Base vehicle is a Fiat Ducato, 2001 vintage.
Much to my surprise, the horn has a permanent live connection.
(1) Removed horn (spark from live lead alerted me to the permanent live) and bodged the leads from a battery charger onto the terminals. The horn squawked.
(1a, later) Made up test lead with spade clip and bare end. Connected usual live to the horn, test lead to the negative side, touched the bare wire to earth. Horn sounded.
First conclusions: the horn works and has a working power supply. Therefore the problem must lie between the horn and the final earthing.
(2) Prised off the horn switch from the centre of the steering wheel. Established that it looked very simple. Incoming lead to horn push, earthed when pushed down on contacts.
(3) Turned ignition on and checked for 12V present at horn push, by putting meter between incoming lead and brass strip on wheel. No voltage.
(4) Turned ignition off, tested for voltage between fuse carrier for horn, and earth. Nothing.
(5) Turned ignition on, tested for voltage between fuse carrier for horn, and earth. Voltage indicated on meter.
Second conclusion - the circuit goes
+ -> Horn -> Ignition -> Fuse -> Horn push.
Fault is somewhere between fuse carrier and horn push.
Except that the fuse carrier also (allegedly) serves the electric window so power could be coming to the fuse via the electric window circuitry with a break between the horn, ignition switch, and fuse.
However I can't see how this would work - if two switched circuits share the same fuse then closing either switch would allow current to flow. and the horn would sound when you worked the window!
[Go out to van Ignition on; test window; Ioff; remove fuse; Ion; test window; still works.
Search for another 25 amp fuse allegedly serving something which isn't fitted. Ah! - heated window/mirrors. Test/remove/test and this one does the windows.]
So I am trusting that the fuse carrier I am testing does in fact serve the horn.
It does look like to me like a wiring fault between the fuse carrier and the horn push. If so, the next step is presumably to gain access to the back of the fuse carrier to bypass the wire leading to the horn push and use a convenient earth to prove the horn works. Then find a way to replace the wire.
Does this sound a reasonable conclusion?
Thanks for reading this far!!
Cheers
Dave R
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Permanent positive/live to the horn is normal. (That way you only need one wire up the steering wheel.) The button on the steering wheel just earths the wire coming from the horn. If the horn is live, next check is the wire to the horn button, that should be "permanently live" too. If you short it to earth, the horn should sound. If not, the wire runnung from the horn to the button is open circuit or the horn itself is duff. If the horn does sound, there is a problem with the button or the slip ring device on the steering wheel column the earths the horn wire.

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On Wed, 01 Apr 2015 14:07:04 +0100, harryagain wrote:

Yes - slip ring coming up again as the main suspect.
Once I get a 24mm socket I can take the steering wheel off and check.
Cheers
Dave R
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On 02/04/2015 12:19, David wrote:

Bear in mind that the horn push switch may be "grounded" through the relay coil *not* straight to ground - unless you can see that it does otherwise. If the relay coil is open circuit (uncommon, but happens), then it will look like a wiring fault...

Remember to be careful if it has an airbag :)
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On 02/04/2015 13:49, Lee wrote:

Doh!, looking at the wrong wiring diagram* - thought it was somewhat unusual to do it that way :)
*much earlier Fiat, shows the horn being powered from the stop light switch supply and the relay in the earth return from the horn push...
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On 02/04/2015 12:19, David wrote:

Ok, if you have access to the back of the switch already, shouldn't grounding the wire (to a proper separate ground of course) sound the horn? If it still doesn't, and the 12v is present, then it's not (just) the slip rings...
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On Thu, 02 Apr 2015 14:08:43 +0100, Lee wrote:

Slip ring is underneath the base of the steering wheel - you need to remove the steering wheel to get at it.
12V is not present at the wire directly behind the horn push.
Cheers
Dave R
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On 02/04/2015 19:03, David wrote:

Ok, I misread that it was.
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On 01/04/2015 15:22, David wrote:

Most stuff is ground side switched on modern vehicles...

Likely to be a relay in there as well...
A post on Fiatforum suggests relay T03 - means nothing to me, lol.
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On Wed, 01 Apr 2015 15:45:53 +0100, Lee wrote:

Thanks.
I am wondering what good a relay would do - because as I understand it a relay is there to carry a heavy current between power and a device (such as an air horn) and the relay is activated by the switch circuit.
Therefore the switch circuit is low amperage and thin wire, and the power circuit via the relay is high amperage and thick wire.
In this case the wire to the horn is thin and there is a permanent live to the horn itself.
There could be a relay on the negative side to complete the circuit - the wiring thickness just doesn't seem to match the usual setup.
However the fuse is 25 amp which is fairly meaty.
Now looking for a wiring diagram.
Cheers
Dave R
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Correct.

Cars tend to have a minimum gauge wire. Just so it has adequate mechanical strength, even if not carrying much current.

Fairly normal.

Normally referred to as the ground side on a car. I'd ignore the thickness.

The reason to use a relay may be more down to the push itself.
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The electromechanical horn generates some awful reverse voltages and if simply connected to the push switch in the steering wheel will soon cause sparking and damage to the moving contact. Putting a relay in the circuit minimises this problem and when the relay finally fails its easier to replace than the switch.
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On Wed, 01 Apr 2015 17:58:40 +0100, Peter Parry wrote:

Thanks to you and Dave P for this explanation. Makes sense now. At the moment I am trying to locate which relay is the one for the horn.
Cheers
Dave R
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On 01/04/2015 17:58, Peter Parry wrote:

My experience with Fiat is that they don't bother with such niceties. As long as it lasts one day past the guarantee period....
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On 01/04/2015 18:11, GB wrote:

IME FIAT stands for Fix It Again Tomorrow.
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FORD: Found On Road, Dead.
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On Thu, 02 Apr 2015 04:11:02 +0100, Mike Tomlinson wrote:

LOTUS: Lots Of Trouble Usually Serious
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scribeth thus

Fix or Repair Daily

Lots Of Time Underneath Servicing;!..
Or as Colin Chapman the lotus factory owner once said;
"If it hangs together for more than the one race, we've built it too well"
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Tony Sayer

Former Lotus owner
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On 01/04/15 16:17, David wrote:

Probably because horn pushes normally opearate via a slip ring on the steering and high currents through that lead to arcing and corrosion. A relay is a crude 'amplifier'

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On Thu, 02 Apr 2015 00:08:24 +0100, The Natural Philosopher wrote:

understand it

device

circuit.

And it enables the use of a less heavy duty (aka cheaper) switch.
Slip rings are probably what are used but I'm sure there is a name for the unit that houses them on the steering column.
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Cheers
Dave.
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