10 replies to this topic

[whawes]

Model: Clubman GT
Year: 1972
Description of problem (please be as in depth as possible):

I've been trying to fit a standard horn to my car, as it was missing when I bought it. But I can't get it to work properly.

There are two wires just behind the grille, one purple and one purple and black. According to my Haynes manual, the purple one should go to the fuse box and the purple/black one to the horn push. I've used a multimeter to verify that the purple wire has 12v continually present at the horn end.

I've mounted the horn on the front panel behind the grille, but when I connect the purple wire on its own the horn sounds continuously. Thinking it might be the horn push I tried disconnecting the plug for the stalk, but this has no effect and the horn stays on.

What have I done wrong? I thought that if the horn is earthed directly to the body, perhaps it should be connected to whatever comes back from the switch rather than to a permanent live, but the purple/black wire doesn't show any current when the horn push is pressed and there are no other wires to connect. I'm further confused by the fact that the Haynes diagram doesn't show the horn as being earthed directly, even though it connects to the body of the car.

Edited by whawes, 17 August 2008 - 02:34 PM.

[whawes]

Anyone? Can't believe no-one knows the answer to this and could really do with some help

[paulrockliffe]

Evening!

There is one wire that takes the power from the battery to the stalk switches, when you open the switch 12v should flow down the wire you've identified fusebox and then to the horn to the horn, then the horn should be earthed.

If you're having problems in all honesty it might be easier to just put your own wiring in, but your description doesn't sound right. You've described the curcuit earthing through the fuse box, which isn't correct. It sounds like you've got it wired so that you've got a permanent 12v feed at the horn, or your switch is jammed shut, which I tihnk is unlikely.

Suggest you trace the horn wire from the back of the stalk switch and see that it goes to a fuse, then take your wire from that fuse to the horna nd it should work. Actually, sounds like you've got the horn on the wrong fuse perhaps?

Your post suggests that you know what you're doing, so if what I've suggested doesn't help, just wire it up how you know it shoudl be yourself.

From experience, don't trust components being earthed through their own body directly to the car as they always fail eventually!

[whawes]

Right, after a few weekends of dire weather I've now had a chance to get all the wiring apart and see what goes where.

My car has a single stalk like the one described here: (Mini Spares). There are two wires that go up inside the stalk to the horn push, one black and one purple and black. Inside the stalk are two prongs, which get touched together to complete the circuit when you push the end of the stalk down.

I've traced these wires down to the plug to see where they lead. The purple and black wire goes directly to the horn. The black wire has been cut just beyond the plug and goes into a bullet type connector. This has a wire in the other side going to the plug on the back of the clocks, but the connector seems to be coated in plastic inside and out so it looks like maybe this is just blanking both wires off for some reason - what the hell?

As I think I mentioned in an earlier post, the wiring diagram for the Clubman 1275GT doesn't show the horn earthing directly to the body of the car. Maybe these cars had a different type of horn fitted? Anyway, since I have a horn that does earth to the body, my plan was to run 12v up the black wire to the horn push and then down the purple/black wire to the horn when the horn push is depressed.

The problem is that having hooked it all up, I now get sparks and a puff of smoke as soon as I touch 12v to the black horn wire. I think, although I'm not sure, that this means the black wire must be earthed, so I'm effectively passing current straight from the battery to earth without it ever reaching the horn push. But I'm at a loss to explain how the black wire can be earthed - one end is attached to one of the prongs inside the horn push (which goes nowhere, as the end of the push is not fitted at present), the other end is where I'm touching a wire with 12v to apply some current. Unless the black wire is somehow earthed internally in the stalk.

It seems like this should be really trivial to fix and I thought I had the answer, but I've been completely thrown by this problem with the black wire.

Can anyone point out something obvious that I'vemissed?

[minimaff]

I had the same problem,

All i did was clean the terminals, worked a treat.

Good luck

[stormintrooper]

from my understanding, the purple/black wire does indeed go to the horn, and the wire on the other side of the stalk should have the live feed, so that when u push the horn it completes the circuit and sends the power to the horn and the earthing is the bodywork, sounds to me like you think it works the other way around

[whawes]

from my understanding, the purple/black wire does indeed go to the horn, and the wire on the other side of the stalk should have the live feed, so that when u push the horn it completes the circuit and sends the power to the horn and the earthing is the bodywork, sounds to me like you think it works the other way around

No you've described exactly what I'm attempting to get working. The problem is that the black wire sparks if I try to apply 12v to it, in the same way as taking a wire from the battery +ve terminal and touching it to bare metal on the car body does.

As far as I understand it this sparking is because there's nothing in the circuit to take the load, so the implication is that the black horn wire in my earlier post is somehow earthed. This is just guesswork really though...

[dklawson]

I don't have a wiring diagram in front of me so please excuse this information if it is not correct for your car.

On the British cars with Lucas wiring ( and NO HORN RELAY ) the horn gets continuous power (12V) on the purple wire. The purple/black wire goes off the other horn terminal and up to the horn push button. The depressed horn pushbutton provides an earth connection for the circuit through a wiper on the steering column. It is therefore necessary (on early cars) to use a horn that has two terminals, both of which are isolated from the horn's metal housing (because the horn gets its earth connection through the steering column push button). AGAIN, this is for cars without a horn relay. If your 1972 has a horn relay please disregard this post.

[whawes]

I don't have a wiring diagram in front of me so please excuse this information if it is not correct for your car.

On the British cars with Lucas wiring ( and NO HORN RELAY ) the horn gets continuous power (12V) on the purple wire. The purple/black wire goes off the other horn terminal and up to the horn push button. The depressed horn pushbutton provides an earth connection for the circuit through a wiper on the steering column. It is therefore necessary (on early cars) to use a horn that has two terminals, both of which are isolated from the horn's metal housing (because the horn gets its earth connection through the steering column push button). AGAIN, this is for cars without a horn relay. If your 1972 has a horn relay please disregard this post.

Thanks for your reply.

My car does not have a horn relay fitted.

The horn I am currently using does have two terminals, but if I attach a wire with continuous 12V to one of them the horn sounds continuously, presumably because it is earthing to the body. Does this suggest that the horn terminals are not isolated from the horn body as they should be? I have another horn handy, but it only has one terminal, would I still be able to verify using that?

[dklawson]

If you earth the body of the horn and the horn sounds as soon as the purple wire is connected, that does indeed suggest that the horn has its windings attached to its housing.

Adding a Bosch/ISO relay to your horn circuit will allow you to use your horn. See this picture:
http://www.67chevyii...bosch_relay.gif

Connect the relay terminals 85 and 86 to the existing horn wiring (purple to 85, purple/black to 86). Thus, any time you push the horn button you will energize the coil in the relay causing it to close. To use this to power your horn will take a couple more wires.

You know your horn will sound if it is bolted to the car and you connect the purple wire. Therefore, you know that you only need to run one power wire to the horn.

Relay terminals 87 and 30 are normally open contacts that close when the relay coil operates. Connect a jumper wire between terminals 85 and 87 (the purple wire will be connected to BOTH terminals 85 and 87). Run another length of wire from relay terminal 30 to the horn terminal that you KNOW will make it sound when the horn is earthed.

Here's what will happen. You press the horn button. The relay will close because you have applied power across relay terminals 85 and 86. Relay terminal 87 is always "hot" because of the jumper wire connecting it to 85. When the relay closes, power is passed from terminal 87 to terminal 30... which in turn will make the horn sound.

Bosch/ISO relays are the single most common horn and headlight relay there is. You will find them in almost any parts store. I've posted the schematic picture link above, the relay itself will look like this:
Isometric View: http://www.alanhorva...s/relay-070.jpg
Bottom View of terminals: http://www.67chevyii...ts/relaybot.jpg

[whawes]

Fantastic, thanks so much for this info. I went out and bought the relay you mention for £2.70, wired it up as instructed and it worked perfectly!