Horn not working - what to check

[tomwood]

Hi all,

car went in recently for its MOT and, to my annoyance, it was found that the horn had stopped working and needs sorting.

things I have already ruled out:

- the switch - I disconnected and completed the circuit with some improvised wiring - still nothing

- fuses - pulled them all out and they are all fine

my banner battery has been playing up recently and is clearly in need of replacement which I will be doing later, this might solve it but doubt it.

what else could I check or do to help diagnose the issue. What’s the quickest way of checking that the horn itself isn’t faulty?

any assistance much appreciated,

Tom

[SM25T]

Run a cable directly to the horn from the battery to test it. Then replace it with a louder horn. I recall the switch breaks the earth line from the horn, rather than the positive feed to it ?

[SM25T]

Have you got a multimeter to follow the 12V feed along the way to the horn and back to the switch ?

[DJ.]

Hopefully it is just the horn itself as that's an easy fix. I would take the horn off and just run two a short cables from the battery to the horn to test it. If you want a better horn the PIAA sports horns are a straight swap and very loud:

https://www.amazon.co.uk/PIAA-85110-115db-400HZ-Sports/dp/B00067BWBI/ref=sr_1_2?ie=UTF8&qid=1541333422&sr=8-2&keywords=piaa+horns

If it isn't the switch  or the horn, it's unfortunately a case of checking the power feed and earth with a multi meter.

[Molecular--Bob]

Relay fault? They can get sticky if they don't get used much?

[tomwood]

Thanks guys, great suggestions. Will investigate and report back,

Tom

[SM25T]

If the relay is in the block behind the fuse rail above the passenger knees .... you do need to push them a home every few months as they can work loose.

[tomwood]

 Ok, so have narrowed it down a bit.

have confirmed that the horns are not faulty. If I connect a positive feed from the battery directly to them then they both sound.

i have also checked the wiring that runs from the back of the switch to the horns for continuity.

In the image above, the top wire (purple / yellow) runs all the way to the horns. If I connect this direct to the battery then the horns also sound so that wire is good. The other wire (solid purple) is, I assume, the positive feed coming from the relays?

regarding the relays, I pulled all of the ones that sit behind the fuses in the passenger foot well and re-plugged them in again. I assume this is all of the relays? Any idea which one is used by the horn?

I defo have some battery problems as well. It would appear my second banner in 5 years needs to be replaced again as it was unable to start the car despite having been on a conditioner. That said I don’t think it’s that given I could get the horn to sound when connected directly unless... low voltage/current would stop the relays from working correctly, could that be a factor?

Tom

 

[tomwood]

Here’s the other end of that purple/yellow wire where it attaches to the horns. This is the only wire running to them so it looks as if the switch simply supplies a single 12v live to them both.

[CharlesElliott]

They earth to the chassis (but you have tested that earth by running the 12v from the battery?). So I assume you are not getting 12V at the connectors when you push the button, so trace it back with a meter to see why not....

[tomwood]

Hi Charles,

indeed, I’ve checked that either supplying a direct live feed from the battery or sending a live feed from the battery down the wiring from the switch results in the horn sounding so I just have no power coming to the switch.

i will aim to test that at the weekend and see if I can trace the issue.

Tom

[rj]

It certainly is a switched earth. 

If the relay is clicking then either the earth is not supplied to it or the relay is faulty.

You can pull the relay from the socket and short the terminals in the socket where the pins 30 and 87 goes. (The contact inside the relay).

If this makes no sound from the horns, then try to short the same terminals to earth. One of them should make the horn sound.

[tomwood]

Right, finally got to the bottom of this one and now I feel like a right numpty.

i checked all relays and all were fine. Thought I’d do another check of all the fuses, something I thought I fully checked first time round and would you believe it I found a blown one. I can only assume I pulled every fuse apart from the one that mattered!!

anyhow, fuse swapped and switch re-attached and it’s all good again.

interestingly, having had a good look over the wiring diagrams and fuse layout for my car in the handbook, I can confirm that the fuse layout is completely different on mine to what is shown in the handbook. I also have 6 relays when the handbook says I should have 5. A bit of a mystery!

thanks for the help,

Tom

[SM25T]

It's often something silly. Well done for seeing it through.

[John Vine]

...the fuse layout is completely different on mine to what is shown in the handbook.

Have you seen this thread?   It looks like your fuseboard and relays are the same as spiderlane's, except for the fuse (grey/clear(?) fuse "L"?) you have between the yellow 5A (M) and the brown 7.5A (K).

JV