Automotive electrics fault finding sanity check

[harryagain]

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.

[David]

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

[Lee]

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.

[Brian Gaff]

Does the horn button twist with the steering? My late Father had a problem of intermittent horn that turned out to be a break inside the so called super flexible wire used for this, OK so it only operated a relay for the horn, but trying to get this wire out was a real pig of a job. I don't recall what he actually did, but I suspect it was a bit of a bodge! It was a Ford.

Brian

[DerbyBorn]

"Brian Gaff" wrote in news:mfh1bt$6ha$ [e-mail]:

Most horns are permanant live. The button earths it. I had a problem eons ago with a car that had a broken earth connection over a flexible coupling on the steering column (Old Beetle)

[David]

Well, diagnostics up to my usual standard :-)

The fuse carrier comes out easily and, looking at the back, each fuse has two in and two out.

The 'In' for this particular fuse is a meaty red power lead not switched by the ignition which is also looped with other fuse carriers.

I discovered this when I pulled the lead to investigate what was on the pins hidden below, and the radio/CD buzzed at me.

So the fuse is on the incoming side and the horn does NOT go through the ignition switch.

So I have (in wiring colours):

Red -> 25A fuse -> red/green -> horn -> black/white -> ???? -> red/green -> horn switch

Another reply suggests a relay in there somewhere; I will investigate.

Cheers

Dave R

[David]

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

[Drumtochty]

Agreed you need a wiring diagram but as it was made by fiat then messed about with by another vendor it mmay be difficult to get the correct wiring diagram.

The change in wire colour confuses me between leaveing the horn and getting to the horn push.

[Dave Plowman (News)]

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.

[Peter Parry]

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.

[David]

As reported back in another response my first analysis was wrong.

It looks now as though there is a relay involved, and the wire goes from the horn to the relay and then to the switch.

Cheers

Dave R

[David]

Oh, and thanks for the pointer to the Fiat forum - I keep forgetting to look there and they are the best place to ask about the base vehicle.

I now have two known common faults to chase - the relay and the contacts below the steering wheel.

Cheers

Dave R

[David]

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

[GB]

My experience with Fiat is that they don't bother with such niceties. As long as it lasts one day past the guarantee period....

[Peter Parry]

In't days of real motoring when the new fangled electric horn came along to replace the squeezy bulb some had an interesting failure mode. The contact on the steering wheel was pushed against the metal earthed plate by the horn push but power actually went through the spring and contact which the horn pushed. This was usually hopelessly under-rated and heated up, thus losing its springyness, every time the horn was used. The horn stopped working eventually but when disassembled everything appeared to test OK.

[Nick]

All horns used to be permanent live before the advent of tech advances (spit). There was a good reason for this that I have forgotten since it was explained to me about 45 years ago. At the same time an explanation was given for small holes in the roof lining. Your fault will undoubtedly be in any of (a) horn push (b) cabling or (c) fuse/fuse holder. My strong suspicion is that this will be the horn push. Find the multi-connector, pull it apart and check for continuity. Work onward from there. IIRC changing a column stalk on a Ducato is a bitch. The chaps at uk.rec.cars.maintenance may well know better that me. HTH Nick.

[nemo]

IME FIAT stands for Fix It Again Tomorrow.

[The Natural Philosopher]

Mo. I would expect ignition->fuse->horn->horn push->earth

[The Natural Philosopher]

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'

[Dave Liquorice]

This would be my guess as to the point of failure if the horn button is on the steering wheel. The unit has a name but I can't remeber what it is...