Horn stalling car...help!

Evening all, hopefully someone can point me in the right direction with getting this sorted!
Basically every time I use the car horn when the engine is running, it will stall the car very quickly. If I am driving on the road and use the horn the car will kangaroo until I stop pressing the horn-not great lol

The wiring at the horn seems fine and not earthing out but otherwise I’m not sure what to check, any help would be fantastic!
Thanks in advance

What is the age of the car? Is it a carburettor, k jet or digifant car? Instead of concentrating on the horn, look at what you are losing when you press the horn. Are you losing spark for example?  Once you find out what's missing we can help you track the problem down.

Yeah that would help!
Its a 1987 Cabby Gti, DX engine, K-Jet. Standard engine and electrics. I did find the low tone horn was broken so replaced it but haven’t changed anything else.
Dont know if it’s cutting the spark, it just loses all power until it stalls or I stop pressing the horn. If you just give a quick beep it seems ok but anything over a few seconds will stall it.

Was wondering if it was a short to earth somewhere but it won’t blow the fuse?
I hate electrics lol

With it being k jet, I would get someone to crank it while pressing the horn. Check for spark and fuel pressure. You must be losing something. Have a quick look at your grounds on the battery, chassis and engine.  

The horn is sounded by earthing it at the steering wheel so if its 'shorting' it would be sounding all the time.

It relies on a short wire at the bttom of the steering column/rack under the bonnet, check is this is intact.

It also uses a relay due to the current the two horns consume, it could be an issue with the relay/fuse board, esp if its been wet as with many MK1 leaking in water above the wiring/board.

As a test I'd remove the new horn and try it with that disconnected as thats the only recent change.

Tried it with a single horn and the same happens. The earth wire at the bottom of the steering column is intact and connected. The car is kept garaged and hasn’t seen rain in about 8 years! Does it matter which way the wires at the steering wheel are connected? Mine is the single middle horn button, didn’t know if it made a difference to which part the live and earth were wired?
Which relay is it- I might be lucky and have a spare!
Thanks for the help so far guys

Live goes direct to the horn and earthed at the steering wheel, the opposite way to the rest of the world.

See pic for diagram, loads more I think this may be from the US as it has A/C.

All Cabriolets are Euro Made in the Karmann Plant in Germany and Exported all over the place.
All have the same fuse panel, only the optional things are different, ie: A/C, Seat Heaters, Power hoods….

When replacing the horns you are near to the main frame to engine Ground.  If you pulled or pushed it by accident in replacing it, then you could of taken a IFFY wire and made it worse.

Horns have 12V full time on them, the Black/yellow wire.
The Horn relay switches Ground to the horn to engage it or make it honk.

Electrics are funny things a component that has a iffy power or ground wire, will use anything it can to make it back home.

Ground is always looking for a way to return to the Positive side of the battery.

I would do basic checks of your electrics.

Try this.

Take a pair of Battery Jumper Cables (Booster Cables)
Attach the red lead from the Battery negative post to the Strut tower nuts.  Take the Black lead attach it to the Battery negative post and run it to the Engine lifting eye.

Start your car and if it is running then honk your horn and see if it stumbles…. if it doesn't then you have a flaky ground wire from the Batter to the frame and or the frame to the engine.  If you haven't replaced your main battery to frame and frame to engine grounds then your major Earthing points are over 30 years old and since VW used an unshielded cable it is probably internally corroded so it's best to replace it. you can use 2 off the shelf cables and replace it as battery to frame and frame to engine.  

Other ground attachment points probably need a good cleaning as well. Engine to coil bracket, and all other earthing points.

If you have water leaks in to the Cabin, then you may have a corroded Fuse panel and a little corrosion can cause things to be bridged and pass current.

Test 2, is to measure the Voltage from the Battery Positive to Frame, then Battery to engine, if there is more than .5V difference then you have a earthing issue.

If you don't have 13.5-14V on a running engine, then you are looking at a iffy alternator, or battery.

It is really tight under the engine where the horns sit, and you may have crossed wires that have marginal cuts in the shielding and are leaking over to different things.

So you are using the power to the horns to power other things and when you honk the horn the direct path that you have for the horns takes away the leak from powering or earthing the other things.

A good place to start is
Electrical System  look for the major grounding points.

If you have ever seen the inside of a fuse panel, they look like this:


A little corrosion or water can cause a lot of things to go wonky.

You could also try the by-passing of your fuel pump relay to rule that being flaky.

Just my thoughts.

Just been looking at a diagram. The horn is powered from the ignition 15 circuit, the same as the coil and fuel pump relay. If there is voltage drop on this circuit from say a "bad ignition switch " then blowing the horn might drop the voltage enough to cut the engine off. As a test It's worth connecting a jumper wire from battery positive to terminal 15 of the coil, leaving the black wires connected. See if that makes any difference.

Just been checking the earth wires in the engine bay and the one that goes from the battery to the engine looks in a bad way-you can almost see through part of it  I will start by replacing this, any idea where I can get a new one?
Briano
Try this.Take a pair of Battery Jumper Cables (Booster Cables)Attach the red lead from the Battery negative post to the Strut tower nuts.  Take the Black lead attach it to the Battery negative post and run it to the Engine lifting eye.

I tried this assuming you meant putting the black lead on the positive terminal and all I got was a tonne of sparks lol
 

Try this.Take a pair of Battery Jumper Cables (Booster Cables)Attach the red lead from the Battery negative post to the Strut tower nuts.  Take the Black lead attach it to the Battery negative post and run it to the Engine lifting eye.

No I was specific.
You are substituting your pair of ground cables for the Earth to Battery connection (frame) and the battery to the engine FRAME.

Never did I say place the wire on the POSITIVE battery post.

Sorry Briano I see what you meant now!
Just been out to do it and the car stalls as soon as I press the horn

I have noticed that if I keep the revs high whilst pressing the horn it doesn’t stallProbably going to leave it tonight now, the neighbours might get a bit funny with me revving the bejesus out of the car whilst constantly beeping! Plus I have a 3 year old trying to sleep lol

Steve's ignition 15 circuit is the clue.

I would defo check that next.

So I connected a jumper lead to circuit 15 and on to the positive battery terminal Which made the fuel pump come on straight away. Car started but then died as soon as I pressed the hornBugger....

Try unplugging the horns at the connector. Not at the horns , lower down there is a 2 pin connector. Press the horn button. The horns will obviously not sound but does it stop the engine cutting out?

Ok, disconnected the horns at the pin and started the car. Pressed the horn and it didn't stall, just clicked away on the relay…

The you could power the horns off the Battery +12Vside with a 10 Amp in-line fuse to the +12V side of the horns, as the clicking relay is switching ground to make the horn honk.

Try this, with the engine running, put a jumper wire on battery negative. Touch the other end to the brown wire of the horn connector leave everything connected. You will have to go in the back of the connector. The horn should sound. Does it stall the engine?