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MG TD TF 1500 - Horn works...ALL the time!

Hello Experts:

I'm having horn problems with '52 TD MKII. The horns and button are all new (reproductions). If only the right side horn (High note) is hooked up, everything works fine. If both sides are hooked up, they both blow constantly!

I cleaned the fuse block, and checked the wires for bare spots and found nothing. I'm not sure what to do next. Suggestions?

Thanks,
Matt Cavicke
MCC Cavicke

Who was the supplier Matt?
Cheers
Peter TD 5801
P Hehir

Well I got them through Abingdon Spares, but I neglected to mention that they were blowing all the time when hooked up before I started replacing things.

To be clear, the original horns and button did the same thing.

I know I probably didn't go about this in the most economical order. I also know that I could live with just the right side horn working, but the problem baffles me and I'd really like to know what is going on.
MCC Cavicke

It's a safety feature :)

Seriously, there must be a short somewhere in the switch circuit, I would think. I'd start with the horn itself. The fact that the high note horn works fine indicates it's probably in the low note horn or the wire connecting it.
Geoffrey M Baker

Hi Mark,
Well, if it was doing the same before you bought the new parts, there seems to be a problem in the wiring. I suspect it's not original. If you look at the wiring diagram, if either one of the wires on either horn is removed, then the other should still blow. They should be hooked up in parallel. The only proper ground is thru the horn "push", and 12v power is always on one terminal of each horn, each supplied by it's own wire, from the fuse box. I can't visualize how, but when you hook up the left horn, somehow it grounds both of them. Maybe both the new and the old left ones have an internal short to ground (not likely), and you've been disconnecting the ground wire from it to shut it off?
Ain't these things fun? That's why we work on them, isn't it?
good luck,
Al
54TF "Emma"
A W Parker

I did a little more testing. Both horns when hooked up have power at the fuse box.

When the highnote (properly functioning one) is hooked up it has power to at the push and when you push it, the test light goes out.

The low note doesn't light the test light at the horn push. The wire condition is fine. Just to be sure, I replaced it with a new wire. Same problem. What are the possible causes of power at the fuse block, but no power at the horn push?

MCC Cavicke

Matt, if the wiring of the horns on a TD is similar to the TF then I would suspect the non voltage wire on the low horn is earthed somewhere before it returns to the horn switch. When you put the 12v connection on the low horn disconnect the earth and see what happens.
If it stops blowing connect the earth of the high to the low externally, that is not through the loom.
Regards, Don TF 4887
D J Walker

I'm afraid I'm too electrically clueless to understand. I need some vocab help and nube friendly instructions.

Earth I know is ground, but which wire is the ground? The one to the fuse block or the one to the horn button?
MCC Cavicke

To pinpoint the problem to the wiring, re-wire the horns with new wires as they are routed in the factory schematic. At first, just run them loose in the engine compartment. If the parts work as expected, you'll know where the problem is.
MAndrus

Look at the horns on this schematic

http://www.ttalk.info/LateTDWiring.htm

At the horn button, the short black line on the bottom left side with the inverted Christmas tree (for lack of a better description) is the symbol for ground. It doesn't actually look like this on the car. It's accomplished with a group of black wires ( if the loom is correct) that have eyelets that are fastened to various points on the back of the dash to complete the circuits.

If you follow the chart for the horns, you will notice at some point that the two wires join together with a double connector and then continue to the horn button with a single wire.

Since you said that if you "remove one wire from the right horn" that the left one functions as designed when you push the horn button, but when you connect the wire back to the right horn, both horns are trumpeting without pushing the button, that means that the circuit is being compleated before it gets to the horn button. Compleated meaning it is shorting out to ground.

The first test I would do is remove both wires from the right horn and connect them together with a test light.

Does the test light glow?


If it does then you have a short somewhere in the wire that runs from the right horn to the switch.

If it doesn't , then the short is in the horn itself.
MG LaVerne

On a TF, which I believe the TD is wired the same on a positive ground car, the horn button is wired to ground/positive earth. Make sure you have the horns wired to the correct terminals, if their not, the scenario you are encountering will happen. Make sure their wired as in the diagram. Hope this helps. PJ



Paul161

I re read your first post... so correct my suggested test on your horn to the left horn. Sorry.
MG LaVerne

We know the switch works because one horn responds correctly when the switch is pressed. You say you have replaced the wire. This suggests the next place to look at is the horn itself. You may want to open it up and check the internal connections.
Geoffrey M Baker

Matt, to answer your question, the one that goes to the fuse block is positive, the one that goes to the horn button if earth or ground, this is the one I suspect is earthed before it gets to the button.
Regards, Don TF 4887
D J Walker

I seem to recall someone in the archive mentioning that an overlong cover screw on the horn could earth the horn. Could this possibly be the cause of your problem?
Cheers
Peter TD 5801
P Hehir

Thanks for all the great input, but i need a little more info on the Test LaVerne is suggesting.

I can connect the two horn wires to each other easily, but how do I connect the test light and battery?

Thanks,
Matt
MCC Cavicke

Matt

Plenty of good advice being offered however if your not electrical literate it all becomes very confusing.

Dont know if you possess a multimeter however you appear to have a test light. I would be totally isolating the horns from the vehicle wiring so you can determine whether the fault is in the horns or the wiring.

Run an external temporary live wire from your battery negative terminal to the front of the car, touch the live wire on each of the horn terminals individually. The horns should not operate unless they have a short to ground(earth).

Wire a temporary link between the two common terminals on the horns,touch the temporary live wire onto one of the common terminals, the horns should not blow.

If there is not a short in the horns you then need to prove the vehicle wiring. Connect your test light across the horn wiring leads of one horn the light should remain unlit.
Have an assistant press the horn button the light should light whilst the button is depressed and go out immediately the button is released.

Repeat this test with your test light connected across the horn wiring leads of the other horn.

If these results are not as stated you have a short in the vehicle wiring or alternatively the wiring crossed somewhere in the loom.

Graeme
G Evans

So I did the test best as I understand. I am a noob. And have almost no understanding of auto electrics.

I clipped the Red (horn push) and Blue (fuse bock) wires from the horn together with the gater clip of my test light and then put the pointy end of my test light on the neg battery terminal. See photo.

Not sure I did the test right though, cause the light shined with red wire only, blue wire only, and both wires together....

Did I get it right? Does this mean both wires are shorting?

Thanks for past and future auto electric coaching.

MCC Cavicke

Matt, I'm assuming you still have positive earth on your car. The light should only shine on the red wire if the horn button is pushed. I think you have identified the problem, that is, the red wire is earthing somewhere other than the horn button.
Regards, Don TF 4887
D J Walker

In general if a harness (loom) fault, look for any repairs, blobs of tape, splices, chafing where it goes through the bulkhead, or anything else that a DPO may have done that looks out of place. You have to look on both sides of the bulkhead where the individual horn wires come out of the harness as well as where the main harness passes through the metal. Look under the dash as well for problems. That will often be the source of the problem. George
George Butz

Matt, Take the horn that is causing the problem and remove the two wires that go into the horn. I no longer have a TD so I don't remember if they are connected to the harness by bullet connectors or if the wire just screw directly into the horn from the harness. In either case...once you have those two wires removed from the horn causing issue...clip the gator of the test light to one of the wires and touch the pointed end against the other wire. One of those two wires will be hot ...coming from the fuse block...the other goes to the horn switch which when you push it creates the ground and completes the circuit. If the test light lights up when you touch the other wire, it means that there is a short to ground before it gets to the horn switch. You may find that if you have to remove the horn cover to disconnect the wires that one of the wires may be shorted out inside the horn.
MG LaVerne

Did the test you suggest LaVerne. And the test light didn't light. So I reconnected the wires to the horn but left the cover off. When I turned on the power I saw sparks! So the new horn comes with a short! See photo (spark just left of the center of the photo). I will probably just exchange for a good one, unless there's an easy fix.

Is there a simple fix for this short?

Thanks,
Matt
MCC Cavicke

photo

MCC Cavicke

I'd like to see a close up picture..like your last one...where the two incoming wires connect to the horn.
MG LaVerne

Not like the original horn that connected with bullets. These are soldered connections.

MCC Cavicke

Take the cover off the good horn and see if you can see any differences between the two.
MG LaVerne

This is the good horn (high note). Only difference I can see, is it's inverted.


MCC Cavicke

I'd have a close look at the red wire connection on the bad horn. My guess is that is is shorted out to the chassis of the horn
MG LaVerne

Because the horn button operates to complete the earth (ground) side of the circuit, it is important that the horn mechanism be electrically isolated from the body/frame of the car. It might be worthwhile to see if there is something grounding the horn mechanism and thus bypassing the horn switch. Jud
J. K. Chapin

I find this thread to be very interesting, because.......I have the same issue with the horns on my 54TF. For now, I'm using only the right horn, as there is an issue when the left horn is connected. One of these coming winter days, I'll get back into that.
Cheers,
FB
Frank Bice

I imagine you and I aren't the only single horn people out there. Best of luck Frank.
MCC Cavicke

From you image appears as though there has been an over indulgence in solder by the assembler. Maybe it is only a matter of spreading the contacts to rectify the fault.

Suggest you find someone who owns a multimeter and is proficient in its use.
G Evans

This thread was discussed between 21/10/2015 and 25/10/2015

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