[AT] question on use of resistor on coil

[Dean VP]

Cecil/Dudley:

The normal voltage of a fully charged 12V battery is higher then 12V,
something like 12.5 to 13 Volts. Then when under load it will pull down to
12V or less depending on the load and the battery's internal resistance. The
battery tester that Harbor Freight has on sale puts a temporary 50 amp load
on a 12V battery and measures how much the voltage drops. A sick battery
with high internal resistance will drop too much under load

The change in voltage at the battery, when running the starter, is why the
voltage at the connection between the resistor and the coil drops under
load. One has to assume that the points are closed when doing this
measurement or it will read the same as the battery voltage because no
current is being drawn in the battery, coil, resistor circuit. 

If the coil dc resistance is 1.6 ohms and the resistor is 1.5 ohms and the
battery is at 12V. The reading at the connection between the resistor and
the coil will read (12V/3.1) X 1.6 = 6.2V. Then when the starter is used it
will drop proportionally with the battery voltage under the starter load.

Since the coil and the resistor are such low resistance, any substantive
resistance at the point contacts will mess things up dramatically. 0.9 ohm
of point contact resistance would reduce the voltage across the coil by
almost 25% resulting in a significant reduction in spark voltage. That is
why points when closed should measure 0 ohms.  

Dean A. Van Peursem
Snohomish, WA 98290

I'm a walking storeroom of facts..... I've just lost the key to the
storeroom door 


www.deerelegacy.com

http://members.cox.net/classicweb/email.htm



-----Original Message-----
From: at-bounces at lists.antique-tractor.com
[mailto:at-bounces at lists.antique-tractor.com] On Behalf Of Cecil E Monson
Sent: Monday, October 25, 2004 9:48 AM
To: Antique tractor email discussion group
Subject: Re: [AT] question on use of resistor on coil

> Cecil, if you have a coil that says 12 volts on it I would be curious as
to
> what its' internal resistance is if you can readily take the measurement.
> 
> You also stated that the voltage at the coil dropped to under 5 volts when
> you checked it with the ohmmeter -- I am not sure what you meant by this.
>   I connected my meter to the coil (and to the tractor frame) on my T035
and
> turned the ignition on and read 6.1-6.2 volts on the meter.  I then hit
the
> starter and, while grinding, the voltage reading dropped to just under 5
> volts.  Is this anything like what you did?
> 
> Dudley
> Snohomish, Washington


	Dudley, I measured the coil that has the words "12 volts" painted
on the outside of it and it measured exactly the same as my other coils -
all of which are 6 volts. I used my Beckman digital VOM and got 1.6 ohms
on all four of the coils I have. Three are 6 volts for sure and the one
is marked 12 volts but is most likely a 6 volt coil. It has a resistor in
the box with it which I am sure is for use on a 12 volt system. This
resistor measured 1.5 ohms.

	FWIW, and I don't know if it means a thing, one of the coils is
one of the new Bosch extra high voltage coils. It measured the same as
the others.

	As to the voltage I measured that I referred to as less than
5 volts, yes, it was when I was cranking the engine. Up to then, it
measured about 6.3 volts as it was on the charger. Also, I left the
charger on when trying to start the tractor.

	So, I as soon as I can get to it, I am going to change out the
points and the spark plugs also and see if I can't put an end to this.

	Thanks again for your help.

Cecil
-- 
The nicest thing about telling the truth is you never have to wonder
what you said.

Cecil E Monson
Lucille Hand-Monson
Mountainville, New York   Just a little east of the North Pole

Allis Chalmers tractors and equipment

Free advice

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