A HORN THESIS help!

[Jazzman]

Dear all,

As a result of three days scratching my head, I have decided to give up and ask for advice.

Horns were a mysterious part (to me) of my spitfire mk3, but after losing them (important note: they started to fail randomly, now they don´t work at all) I decided to understand the circuit to mend once for all the problems.

To do so I have taken the following steps so far:

STEP 1: I checked the fuse. It seems OK, regardless, I changed it (FUSE IS OK)
STEP 2: New horn pencil: the previous one seemed OK, but it was old. (HORN PENCIL IS OK)
STEP 3: Dismantled the horn push. Cleaned the connexions (were a bit rusty). I checked for continuity between the end of the horn pencil and the metal plate of the horn push: it works, when pushed. (HORN PUSH IS OK)
STEP 4: Still doesn´t work....
STEP 5: I connected the horn itself with the battery with jumping cables. Not very H&S, but both work. (HORNS DO WORK)
STEP 6: I checked for continuity between the purple/yellow cable from relay to both horns as shown in the following diagram. The test light lightened up, so the cable is OK.
Although, the horn did not work --> I do not get that because (i) considering both horns work when connected to the battery directly; (ii) assuming the car is properly earthed (negative) to the engine and (iii) the circuit is closed since the test light works, how come the horn did not toot?


With my limited understanding of electricity I guess I still have to check:
1.     Relay – how do i check it is working?
2.     There is power at the ring behind the horn push, where the horn pencil makes connection – I guess that is as easy as checking for voltage (should read 12V?) between the hole in the boss (where the pencil fits) and the chassis (earth).
3.     Suggestions?


Many thanks in advance
A frustrated fellow enthusiast  :o

[GT6MK3]

Did you check the wire braid connectors:

(a) around the steering uni joint, and
(b) from the rack to the chassis?

C.

[bodders1]

1248 wrote:

Did you check the wire braid connectors:

(a) around the steering uni joint, and
(b) from the rack to the chassis?

C.

^^^^^
What he said!

It's the one part of your circuit that you do not say you have tested.
The horns on your car are "permanently live" - meaning that the switch to make them work is in the earth return part of the circuit.  When you press the horn button on the steering wheel, you are connecting the horns to earth via the metalwork of the steering column and steering rack.  For this to wrk, there must be continuity right down the steering column, including through the steering UJ, and across the rubber rack mountings.

[Richard B]

bodders1 wrote:

When you press the horn button on the steering wheel, you are connecting the horns to earth via the metalwork of the steering column and steering rack.  For this to wrk, there must be continuity right down the steering column, including through the steering UJ, and across the rubber rack mountings.

I must say, (if there is a relay in your system), When you press the horn button on the steering wheel, you are connecting the relay to earth via the metalwork of the steering column and steering rack not the horn.

Connect the tag on the relay that goes to the horn switch direct to earth. If the horns work then the fault lies in your switched part of the circuit. Most likely (as others have said), the earth braid across the steering U/J or the braid from the earth to the chassis.

[bob dunn]

fit air horns and a button  , use the relay for spot lights .

[heraldcoupe]

The grounding of the steering column is often compromised, I'd estimate it's responsible for 95% of horn issues.
Use your meter to check resistance between the top of the inner column and the body/chassis, you should get a minimal reading.

There are two critical links which cause a break in the ground path. Firstly, there is a link wire which usually runs from the steering rack's greasing point to the rack mounting. This is sometimes left disconnected after the rack is greased.
Secondly, and less visible, is the link across the bottom column joint. This was originally a wire which ran inside the joint, so not visible externally. The wire fatigues and eventually breaks, it can be supplemented by a short wire link externally across the upper and lower pinchbolts.

As Richard suggests, many Spitfires have relays to the horns, the column ground affects the switching of the relay, rather than the direct feed to the horns,

Cheers,
Bill.

[JensH]

On my mk 3 spit, the old original steering UJ had an internal earth lead only visible when you dismantle it from the steering column. New ones (the last one I bought didn't fit well) need an external lead.

You should be able to hear horn relay clicking (if you disconnect the wire at the horns)  8)

[mikeyb]

never seen on on a UJ - did you mean the the little rubber insulated coupling?

[JensH]

MikeyB wrote:

never seen on on a UJ - did you mean the the little rubber insulated coupling?

Yep - sorry for not being more clear  :o

[Nick Moore]

There are proper Universal Joints available to fit in place of the rubber-cupped steering coupling. These provide a better earth with no need for the wee wire. They also eliminate any lost movement through the rubber cups, which perish with age, heat and oil, firming up the steering response slightly.

Bill sells them, among other suppliers. I think Bill's version is slightly more compact than some (eg Ford Escort) meaning that they fit inside the awkward turret hole more easily.

[Jazzman]

Thank you all for your help.

I did some work yesterday night:
* the relay does work --> it sounded when I bypassed it
* the earth strap around the steering uni joint was not A1, I unbolted it and soldered the ends. I put it back in place and checked again, but the horns won´t work

I´ll check the earth strap that joins the rack to the chassis this weekend.

cheers

[Richard B]

Find the cable from the relay to the steering column. Try connecting that to earth and check if the horns sound.

[Jazzman]

Problem solved! thanks guys for all your help.

I was going nuts thinking about all the possible problems. In the end, guess what was the problem? the cable behind the dashboard giving power to the horn plate was disconnected  :o LOL

At least I am an expert in everything related to horns...

cheers!!

[JohnD]

Haven't investigated this yet, but perhaps the asembled gurus can help me here.

Last weekend at Snetterton, in gloriously unseasonal weather, every time I went into the new Murray's corner - maximal braking while still turning right  and then going sharp left - my horn sounded!

To save me time - any suggestions on where to look for a short?

John

[heraldcoupe]

JohnD wrote:

To save me time - any suggestions on where to look for a short?

It's usually the steering wheel making contact with the horn ring. It's often down to play in the column bushes, but some brands of aftermarket steering wheel, Mountney's in particular, aren't designed with enough clearance in the boss, and will short even when the bush clearance is quite tight. Grinding off a couple of mm from the boss increases clearance, a disc of plastic interleaved adds belt-and-braces to the fix.

Cheers,
Bill.