This is a seemingly daft question but ...

We know the horn push works OK, we know the horn works OK, but there is a relay between the horn push and the horn.

If we disconnect the horn from the relay and press the horn push we get 12v on the relevant terminal on the relay measured using a multimeter.

If we connect the horn to that terminal then we get 0v

We tried connection a 21w bulb on that terminal and got 0v measured

The relay clicks correctly and remember with no load on the output we get 12v.

We took a wire from the output from the relay direct to the horn to eliminate a wiring problem, when running a wire from the battery to the horn it hoots so the earth the horn end is fine.

How can this happen please? I can't understand how the relay can be faulty to cause this.

Mike

 

drmike, gotta love the classic cars. I have a pretty good idea of what is happening for you. What I want you to try is taking a small gauge wire and wrapping it around the post on the relay that has the terminal that looses the voltage. Plug the relay in and take the other end of this wire and take it to the positive of the battery. Now, try the horn. It should sound.

What is happening is you have a piece of wire that has broken and it broke in such a way that the wire inside the insulation is just barely touching. This allows voltage with no current to flow through this point, but you put a load on it, the break drops all the voltage and gives you 0 VDC on the output. Unfortunately, the fix is to simply run a new piece of wire from the terminal that the relay plugs into (on that one post) back to where it gets power. Not a hard job, but not something that is a piece of cake either. You should be able to get all the connectors at your local auto parts store (one that is more electrically savvy is going to be your better choice) and if you can, find the same gauge and color wire. I mention the color wire (the insulation color) as it really sucks when you are trying to troubleshoot a problem and you are looking at a sea of red wire and trying to find that one wire that goes from A to Z.

If you are lucky, what you can do is hand over hand this wire and if you give it a gentle bend over your finger, most of the way, the wire is going to conform to your finger. But, you will find a spot that it is going to make a sharp bend. This sharp bend is where the wire broke. If you decide to simply cut the wire at that point, strip back say a centimeter of insulation on each side and then solder the wires together (or use a bare metal butt splice) cover the exposed wiring with an 8 cm piece of raychem. This is heat shrink on steroids. The main benefit raychem has over general heat shrink is that it has a temp sensitive glue on the inside. So, you slip it over the bare copper and then you heat it up, it will glue itself to the current insulation and make a water tight seal (aka, it will be like the wire never broke). If you are worried about it surviving under hood conditions, DON'T!!!!!! I use it at my work to survive a nuclear reactor melting down, literally. It will laugh at under hood conditions. Yes ,the raychem is a bit expensive (a 100 cm piece will probably run you $10 USD), but once you use it, it is worth every penny. In your case, you are going to want to find what is called "WCSF-070 raychem". That will be the correct size to fit on to the wiring and be able to make a good seal. You will find that the raychem comes in many, many sizes. The number at the end is the final size if you let it shrink to as small as it will go in thousandths of an inch. So, WCSF-070 will shrink to as small as 0.070" (the diameter of the inside hole that is left). The hole is going to start out at around 0.160" (before shrinking). So, you can do some measuring and figure out if you may need to get the next size bigger "WCSF-125". Most raychem has a shrink ratio of somewhere between 2 and 3. So, a tube may start out at say 0.750", but will shrink down to somewhere between 0.250" and 0.375" at its smallest. Granted, you have something that is between 0.750" and 0.375", then then the raychem will stop shrinking when it comes in contact with the wiring.

 

Thank you for that advice and help. I have few fears for electrics under the bonnet of such cars and we will probably run a new wire if we don't find the weak point quickly.

The point about colours is well taken as a previous owner had only one spool of wire - black. So anything he touched, and boy did he touch a lot, uses black wire. It makes it quite hard to trace stuff!

Thanks again - we will prevail.

mike

 

And in the end it was roughly what you said inasmuch that there was a weak connection - the glass fuse had a good central wire but the ends were loose causing a weak contact. Changed the fuse and bingo we had a strong horn. That fuse looked good and would seat well but was 40+ years old.

 

Good to hear that you found the issue. Yes, fuses can do some tricky things.