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testing a horn coil
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Ray White



Joined: 02 Dec 2014
Posts: 6286
Location: Derby

PostPosted: Sat Sep 26, 2020 11:27 am    Post subject: Reply with quote

...anyway...back to business. The beep function on my multimeter is working and using THAT same setting the coil is reading 1.4

I presume that is 1,4 Ohms?

If it is 1.4 it is higher than the 0.75 Ohms that it should read (according to my info.)

Perhaps the points could do with more cleaning. They were leaving white deposits on the fine abrasive paper.
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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Sat Sep 26, 2020 11:46 am    Post subject: Reply with quote

And when you just connect the probe tips together on the beep function the meter reads 0 ohms ??? I'd be very surprised if the horn couldn't be made to work with the coil reading 1.4 ohms.

What reading do you get with the meter probes on either side of the closed points?

Peter
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1939 SS Jaguar 2 1/2 litre saloon
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Peter_L



Joined: 10 Apr 2008
Posts: 2680
Location: New Brunswick. Canada.

PostPosted: Sat Sep 26, 2020 1:35 pm    Post subject: Reply with quote

Peter_L wrote:




With 12v applied direct to A & B there should be a magnetic field created, do you have a compass, or a needle floating on water or hanging from a thread ? There should also be a spark, and/or put your meter to DC Amps and in series.

Note: The resistance/resistor is there to reduce arcing at the contacts.
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Ray White



Joined: 02 Dec 2014
Posts: 6286
Location: Derby

PostPosted: Sat Sep 26, 2020 3:10 pm    Post subject: Reply with quote

peter scott wrote:
And when you just connect the probe tips together on the beep function the meter reads 0 ohms ??? I'd be very surprised if the horn couldn't be made to work with the coil reading 1.4 ohms.

What reading do you get with the meter probes on either side of the closed points?

Peter


The good news is that I am now getting the correct reading 0.8 Ohms from the terminals. I am using the 200 setting with the beep function. If I touch the probes together I get 0.2

I should now be able to reassemble and adjust the horn so it can work. Having cleaned up the diaphragm I think it should be O.K. even though it is a bit rusty. All I need are a few small items and we are in business!
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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Sat Sep 26, 2020 3:24 pm    Post subject: Reply with quote

That sounds much more hopeful.

Peter
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1939 SS Jaguar 2 1/2 litre saloon
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Ray White



Joined: 02 Dec 2014
Posts: 6286
Location: Derby

PostPosted: Fri Oct 09, 2020 11:20 am    Post subject: Reply with quote

http://www.mg-tabc.org/library/Altette_Horn.pdf

I know it is a long shot...but does anyone have a push rod adjustment tool for a horn like mine.?

Page 2 of the above.

(Hens teeth and Unicorn poo are probably less scarce) Laughing
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47Jag



Joined: 26 Jun 2008
Posts: 1480
Location: Bothwell, Scotland

PostPosted: Fri Oct 09, 2020 6:17 pm    Post subject: Reply with quote

Ray,

That looks like it's just a screwdriver and a bit to stop the locking ring turning, Pliers perhaps? Try sticking a screwdriver in the hole and see what moves.

Art
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Ray White



Joined: 02 Dec 2014
Posts: 6286
Location: Derby

PostPosted: Fri Oct 09, 2020 6:56 pm    Post subject: Reply with quote

47Jag wrote:
Ray,

That looks like it's just a screwdriver and a bit to stop the locking ring turning, Pliers perhaps? Try sticking a screwdriver in the hole and see what moves.

Art


I can't shift the screw. There is a brass locking ring that also won't move.

I do have a tube with two square ended prongs on the end which I use for carburettor adjustment... but I made that from brass so it's too soft.

I am now making a tool from a length of steel tubing which I hope will be strong enough but the risk is I damage the little brass locking ring.

Other horns have an improved design which allow you to adjust with a spanner.

Perhaps heat is needed.?
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Ray White



Joined: 02 Dec 2014
Posts: 6286
Location: Derby

PostPosted: Sat Oct 10, 2020 1:36 pm    Post subject: Reply with quote

I gently heated the armature and it was enough to free the lock nut so I could move the push rod in and out.

I have tried different settings but no matter how it is adjusted I can't get a beep out of it.

Turning to the electrics I am now getting a reading of just 0.4 OHMs...so I don't know what is going on. Confused
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peter scott



Joined: 18 Dec 2007
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Location: Edinburgh

PostPosted: Sat Oct 10, 2020 2:24 pm    Post subject: Reply with quote

Sounds like the contact pressure is too strong and the pull in of the armature is not strong enough to open them.

Peter
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Peter_L



Joined: 10 Apr 2008
Posts: 2680
Location: New Brunswick. Canada.

PostPosted: Sat Oct 10, 2020 6:30 pm    Post subject: Reply with quote

peter scott wrote:
Sounds like the contact pressure is too strong and the pull in of the armature is not strong enough to open them.

Peter


I asked before if a magnetic field had been proved via connections direct to the coil A - B on the photo.
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Ray White



Joined: 02 Dec 2014
Posts: 6286
Location: Derby

PostPosted: Sat Oct 10, 2020 7:14 pm    Post subject: Reply with quote

Peter_L wrote:
peter scott wrote:
Sounds like the contact pressure is too strong and the pull in of the armature is not strong enough to open them.

Peter


I asked before if a magnetic field had been proved via connections direct to the coil A - B on the photo.


Above my pay grade and I'm not afraid to say so. All I know is it doesn't work.

I think my next option is to either try and get it rebuilt by a specialist or trade it in for one that works.

It's the wrong year for my car anyway.
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Ray White



Joined: 02 Dec 2014
Posts: 6286
Location: Derby

PostPosted: Fri Oct 16, 2020 11:49 pm    Post subject: Reply with quote

I have left the horn with our local auto electrician. He didn't seem very confident that he could fix it - but as they say - he knows man who can.!
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Miken



Joined: 24 Dec 2012
Posts: 544

PostPosted: Sat Oct 17, 2020 6:34 am    Post subject: Reply with quote

If you want to see a man that knows everything about old vehicle horns visit " Taff the horns" website.
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Ray White



Joined: 02 Dec 2014
Posts: 6286
Location: Derby

PostPosted: Mon Oct 26, 2020 2:43 pm    Post subject: Reply with quote

Miken wrote:
If you want to see a man that knows everything about old vehicle horns visit " Taff the horns" website.


Unfortunately Taff is no longer in business. Some parts are still available but no rebuilds.

I have spoken to the auto electrician who tells me that one of the "old boys" has managed to get the horn working. Apparently it could do with a new diaphragm because the old one is distorted.

I will be collecting the horn tomorrow. I am awaiting the return of the chrome bezel from the platers; with that fitted it will be one more job done.
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