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testing a horn coil
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Ray White



Joined: 02 Dec 2014
Posts: 6285
Location: Derby

PostPosted: Thu Sep 24, 2020 10:46 am    Post subject: testing a horn coil Reply with quote

I can't remember doing this before so perhaps someone can confirm if I am doing things correctly. I want to test the coil in my Lucas Altette horn as I suspect it may have failed.

I set my multimeter to Ohms 2K.

I then put the probes on the terminals as shown.





There was no reading. Does this mean one of the following:

a) the coil is dead or

b)there is a break-in the wiring or

c) there is poor contact across the points?
( which I believe should be closed at rest.)

Does no reading mean there is an incomplete
circuit? I presume it doesn't make any difference if the armature is fitted or not if just testing the coil. ?
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traction39



Joined: 19 May 2009
Posts: 399
Location: South Wales

PostPosted: Thu Sep 24, 2020 11:23 am    Post subject: Reply with quote

This may be of interest:

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

The expected reading for the coil are in the text.

Alistair
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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Thu Sep 24, 2020 1:09 pm    Post subject: Reply with quote

Hello Ray,

What does your meter display if you just connect the two probes together?
Your photo appears to show 1 ohm. Try it on the "200" range for a more accurate reading.

Peter
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Peter_L



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

PostPosted: Thu Sep 24, 2020 1:32 pm    Post subject: Reply with quote

Ray.. To get an idea of how and what your meter displays at different settings, put the leads across say a 21W, or 52W Bulb and something like a 5W. You will see that the higher wattage bulbs will display a lower resistance.

For example if the power is 100 watts and the voltage is 50 volts, the current is 100 / 50, or 2 amps.
Calculate the resistance, in Ohms, by dividing the voltage by the current. Ohms law states that voltage = current x resistance, so by rearranging the formula resistance = voltage / current.

A 12volt horn draws about 4A according to the information in Alistair's link. (Great info there)

Those connections look a little dirty.
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Ray White



Joined: 02 Dec 2014
Posts: 6285
Location: Derby

PostPosted: Thu Sep 24, 2020 1:32 pm    Post subject: Reply with quote

peter scott wrote:
Hello Ray,

What does your meter display if you just connect the two probes together?
Your photo appears to show 1 ohm. Try it on the "200" range for a more accurate reading.

Peter


I thought someone might question that. I don't know why by the meter always reads 1, it just does. If I connect the probes together it reads 0 (zero).

It doesn't move from I whatever the setting.
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Peter_L



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

PostPosted: Thu Sep 24, 2020 1:37 pm    Post subject: Reply with quote

Ray.... 1, open circuit. Put it on 20M, wet your fingers and hold the bare ends of the probes, you should get a reading if the meter is working.
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47Jag



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

PostPosted: Thu Sep 24, 2020 1:44 pm    Post subject: Reply with quote

Ray,

It sounds more like dirty points. The way the horn sounds is the points are normally closed, you press the the button, the coil pulls the diaphragm in and opens the points, the diaphragm pops back out closing the points again and if the button is still being pressed the sequence repeats thus creating the sound.

Of course if the coils is open the preceding is redundant.

Art
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Ray White



Joined: 02 Dec 2014
Posts: 6285
Location: Derby

PostPosted: Thu Sep 24, 2020 1:47 pm    Post subject: Reply with quote

Peter_L wrote:
Ray.. To get an idea of how and what your meter displays at different settings, put the leads across say a 21W, or 52W Bulb and something like a 5W. You will see that the higher wattage bulbs will display a lower resistance.

For example if the power is 100 watts and the voltage is 50 volts, the current is 100 / 50, or 2 amps.
Calculate the resistance, in Ohms, by dividing the voltage by the current. Ohms law states that voltage = current x resistance, so by rearranging the formula resistance = voltage / current.

A 12volt horn draws about 4A according to the information in Alistair's link. (Great info there)

Those connections look a little dirty.


Hi Peter. I have tried the meter on a couple of bulbs and I think there might be something wrong with it - or perhaps it needs a new battery - because it seems unable to hold a number and will either go back to 0 or 1.

I will try it with a new battery and see if it makes any difference.
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Ray White



Joined: 02 Dec 2014
Posts: 6285
Location: Derby

PostPosted: Thu Sep 24, 2020 1:49 pm    Post subject: Reply with quote

47Jag wrote:
Ray,

It sounds more like dirty points. The way the horn sounds is the points are normally closed, you press the the button, the coil pulls the diaphragm in and opens the points, the diaphragm pops back out closing the points again and if the button is still being pressed the sequence repeats thus creating the sound.

Of course if the coils is open the preceding is redundant.

Art


I have been cleaning the points but as mentioned above I may need a new meter.
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Ray White



Joined: 02 Dec 2014
Posts: 6285
Location: Derby

PostPosted: Thu Sep 24, 2020 1:50 pm    Post subject: Reply with quote

traction39 wrote:
This may be of interest:

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

The expected reading for the coil are in the text.

Alistair


Thank you. I already have it.
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Peter_L



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

PostPosted: Thu Sep 24, 2020 1:54 pm    Post subject: Reply with quote

Yes Ray, "iffy" meters are of less use than a chocolate teapot, at least one can eat the teapot. Faulty leads have been known to happen.
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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Thu Sep 24, 2020 3:33 pm    Post subject: Reply with quote

Ray White wrote:
peter scott wrote:
Hello Ray,

What does your meter display if you just connect the two probes together?
Your photo appears to show 1 ohm. Try it on the "200" range for a more accurate reading.

Peter


I thought someone might question that. I don't know why by the meter always reads 1, it just does. If I connect the probes together it reads 0 (zero).

It doesn't move from I whatever the setting.


That's fine! Now when you measure the horn on range 200 Ohms what does it measure?

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



Joined: 02 Dec 2014
Posts: 6285
Location: Derby

PostPosted: Thu Sep 24, 2020 3:44 pm    Post subject: Reply with quote

peter scott wrote:
Ray White wrote:
peter scott wrote:
Hello Ray,

What does your meter display if you just connect the two probes together?
Your photo appears to show 1 ohm. Try it on the "200" range for a more accurate reading.

Peter


I thought someone might question that. I don't know why by the meter always reads 1, it just does. If I connect the probes together it reads 0 (zero).

It doesn't move from I whatever the setting.


That's fine! Now when you measure the horn on range 200 Ohms what does it measure?

Peter


I have just fitted another battery and now when I test across the points (which I have tried cleaning) it reads 0.00.


When I test a 21 W bulb it also goes to zero.

I guess there could be a fault with my equipment.?
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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Thu Sep 24, 2020 4:10 pm    Post subject: Reply with quote

If you connect a battery and bulb via the horn does the bulb light?

If not then try making the same test but directly via the coil connecting wires inside the casing. If that works then try cleaning up the rusty outer of the diaphragm so that it makes a good ground connection to the mating face of the case.

P.


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Last edited by peter scott on Thu Sep 24, 2020 5:29 pm; edited 2 times in total
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MVPeters



Joined: 28 Aug 2008
Posts: 822
Location: Northern MA, USA

PostPosted: Thu Sep 24, 2020 4:50 pm    Post subject: Reply with quote

It's a bit late now, Ray, since you've dismantled the horn, but a couple of times in the past I've re-tuned horns of that type.
I've just cleaned the contacts, connected 12V & gently tweaked the adjusting screw. Slowly turning the screw makes the horn rattle, then beep, then squawk & finally parp the way it should.
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