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MG MGA - horn fuse keeps blowing

The fuse the the horn uses keeps blowing. What else is on this fuse, and how do I start to find the fault. I've looked at the wiring and it seems OK!
Thanks
Nigel Munford

Nigel,
I am away at present so no wiring diagram to hand, but as I recall there is nothing else on that fuse (unless of course the PO has wired something else up). No doubt someone will soon be along and confirm or advice otherwise.
Does it only blow when you hoot the horn, or all the time?
If the former, I would recommend you disconnect the power to the horn and then press the button to see what happens. Obviously if the fuse doesn't blow, you will know where the fault is.
Good luck
Graham M V

If you don't find anything wrong with the wiring, you may have to refurb and tune the horn(s). If the center rod in the coil is frozen then the horn can draw more current than normal and blow the fuse. X2 if you have 2 horns trying to run off the same fuse, as is the normal case in the MGA. It could take some PB blaster (or your personal favorite go-juice to free up the rod. Even then, you may need to adjust the contact points for a clean sounding note.
Chuck Schaefer

Nigel, if your car is wired normally, that fuse supplies the horns and, as Graham says, any accessories the owner has connected. If you only have the horn connected (a single brown and green wire) then you have a faulty horn as Chuck says, or the wire is shorting out somewhere. Does the fuse blow when the horn button is pushed? If so, it is a faulty horn. If it blows without the horn button being pressed, you have an earthing wire. If you have a second wire on that fuse terminal, then whatevever is connected to that wire should be investigated.
Lindsay Sampford

Simple circuit for the horns. See here:
http://mgaguru.com/mgtech/electric/circ_fd.htm
Before anything else, check to see if there may be some extra wires in the circuit connecting to something else that should not be there.

Run separate hot and ground jumper wires to the horn(s) for testing. If the horns sound normal, they are likely not drawing enough current to blow the fuse (unless you have the wrong fuse). The fuse should be a Lucas 25/50 slow blow fuse. By USA standards, a 25 amp fuse. If you use a Lucas 17/35 fuse, dual horns may draw enough current to blow that one.
Barney Gaylord

Thanks all. The fuse blows when the button is pressed, sounds once and then nothing. So the horn seems to be working. I am using 35amp fuses.
Nigel Munford

I suppose the wiring is okay. If there was any short in the wire before the horn it would blow the fuse immediately. If there was any short in the wire between the horn and the button the horn would sound continuously. This does sound like the horn is drawing too much current.

Is there one horn or two? If two, then you might disconnect one at a time to see if one particular horn is responsible for too much current. Otherwise run a hot wire to the horn to bypass the fuse and harness, and use an ammeter to measure actual current draw.

There may be an outside chance you got some bad fuses that won't carry the specified current.
Barney Gaylord

It isn't the fuses. I used one I thought had gone in the other fuse holder and it works. Therefore it is either the horn or the switch. I will try Barney's test next.
Nigel Munford

I doubt if it could be the switch. I merely grounds the circuit to activate the horn(s). The only fault I could see it having would be either to not close the contacts when you push the button, in which case the horn would not work, or short across the terminals, in which case the horn would stay on, but not blow a fuse.
Unless there is another circuit and wire connected to the switch. There should only be 2 wires connected to it. It is likely the horn(s). Maybe one of the coils is shorting to ground? Barney's test is perfect. I would follow that.
That is what we do at work. We have fairly complex metal forming machines, and if a certain fuse starts blowing, we disconnect everything on that circuit, and reconnect the devices one at a time until the fuse blows, then we have our culprit. It saves a lot of fuses. LOL

Good Luck,
Ralph
L Poupard

This thread was discussed between 14/10/2011 and 16/10/2011

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