[AT] question on use of resistor on coil

Dean VP deanvp at att.net
Sat Oct 23 11:12:06 PDT 2004


Cecil:

The 420 parts catalog calls out Delco Remy Coil # 1115379.  I would think it
would be a 6V coil although I cannot find a cross reference on the web. I
don't have a 420C service manual handy and I haven't owned or worked on one
but I will provide my experience on other JD tractors. 

Typically on 12V systems, JD inserted a resistor between the 12V source and
the coil. This dropped the voltage to 6V at the coil which was normal
running voltage. However, when the starter was engaged this resistor was
bypassed so that a full 12V was applied to the coil for starting spark
improvement.  This may be related to the resistor your material is talking
about. 

However, yours started out as a 6v system so I doubt JD would do this on
this particular tractor. A 12V coil will produce a much lower spark in a 6v
system. That may be part of your problem. 

However, in my experience with JD's, 99% of the time the problem is strictly
the points. Especially if the tractor has set unused for a period of time. A
kind of film develops on the point contacts that has to be removed to get
the contact resistance back to zero ohms. A point file may do that or a
folded emery cloth sandpaper slid in between the points to remove both sides
of the contacts corrosion. For some reason you need to be very aggressive at
this to get them clean. If you have an ohmmeter, check after each scrubbing
to see what the resistance is when the points are closed naturally. It has
to be zero or damned close. Just an ohm or two will cause problems. 

And yes, in my experience, once one has to start filing points it is
sometimes better to just install a new set. Once filing is needed they seem
to require it much more than a new set.

I would be the farm your problem is the points and I would re-install the
original coil.  

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: Saturday, October 23, 2004 4:19 AM
To: Antique tractor email discussion group
Subject: [AT] question on use of resistor on coil

	I've been trying to get my JD 420 crawler to start after sitting
all summer. At first I had no spark. With the loader down and the hood
on the tractor, it is almost impossible to get to the coil and to where
you can see to work on the distributor. I took the hydraulic connections
apart yesterday and got the hood off. Substituted a new coil for the old
one without success. Took the cap and rotor off the distributor and used
a "point file" I got at Sears to clean up the points. Now I have a sort
of yellow spark when I lay a spark plug where it is grounded and crank
the engine over. The yellow spark indicates to me it is a weak spark
or still bad contacts at the points.

	The question is - the coil came out of the box with a note saying
it must be used with a resistor but no resistor is there. I figured that
being as it is marked 12 volts on the coil and I'm using it on a 6 volt
system, I should not need a resistor. Is this true or do both 6 volt and
12 volt systems use the same coil with a resistor?

	Another question is that I can feel pitting on the points on one
side when I run the file over them. Would you just change out the points on
general principals?  I have already changed out the condenser as I initially
thought it might be the problem.

	In case someone wonders about fuel, I smell gas in the exhaust when
cranking so it is getting gas.

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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