620 ignition coil

I need to replace the original coil in my 620. Here is the question: Are coils application specific ? Doing a search, I see some coils are listed as 6 and 12 volt.?.? Can I use any 12 volt coil for external resistor? Or do I need a external resistor coil for a 620 John Deere ?? Thanks in advance...Dan
 
You ask: Can I use any 12 volt coil for external resistor?

If the coil is in reality a 6 volt rated coil of which some are labeled 12 Volts for use with ballast

ANSWER Yesssssssssssssssssssssss HOWEVER dont use a full true 12 volt coil no ballast required coil PLUS a ballast, that would yield low coil voltage and a weak spark !!!

Or do I need a external resistor coil for a 620 John Deere ??

ANSWER

1) Yesssssssss you need a 6 volt coil PLUS an external ballast resistor IFFFFFFF YOU WANT TO KEEP IT ORIGINAL WHERE THE BALLAST IS BY PASSED WHEN STARTING TO IMPROVE COLD WEATHER STARTING

2) Yessssssssssssss you need a 6 volt coil PLUS an external series voltage dropping (12 down to 6) for use on a 12 volt tractor or else the coil would overheat...


NOTES


If its the 12 volt 620 that uses a 6 volt coil plus an external series voltage dropping (12 down to 6) ballast resistor and you want to keep it that way (original used a ballast by pass when starting for improved cold weather starts) ........YOU NEED A 6 VOLT RATED COIL Well Duh thats what's left for a 6 volt coil after the 6 volt ballast drop.

NOTE What some call an internally ballasted coil does typically NOT actually contain an internal stand alone discrete resistor tucked away inside the can, its necessary resistance is made up strictly by the LV primary windings IE HAS NO INTERNAL BALLAST RESISTOR

A full true (NO ballast required) 12 volt rated coil has a LV primary resistance of around 3 ohms
A 6 volt coil has a LV primary resistance of around 1.5 ohms

A coil labeled 12 Volts for use with external ballast is in reality more like a 6 volt coil IE it needs the series ballast LIKE IT SAYS to drop the 12 volts down to 6 or it overheats

NOTE Sure its possible NOT to use a 6 volt coil plus series ballast but instead a full true 12 volt coil no ballast required and ditch the ballast and ballast by pass and it will still work, BUT I DONT DO IT THAT WAY If you use a full true 12 volt coil DO NOT ADD BALLAST it would be a weak spark


John T
 
John, thank you for the reply and info...to be clear (I don't profess to be of brilliant mind) this 620 is 12 volt. I have owned it since 1966, so I am guessing it the original coil but I have no idea the voltage of it...620 has an external ballast resistor. It seems confusing go me to put a 6 volt coil on a 12 volt tractor.?.? But maybe that is what the original was.. I don't care if it original, this is a working tractor, not a show queen.......so....I really want a 6volt coil then ?? Correct ??? Thanks...sorry I am not smarter about these things...Dan
 
Yes, the 6 volt coil with a resistor will give you better spark when cranking if the tractor is wired as original according to what John has said. I am an IH guy but many of them used the same configuration on 12 volt systems. To get to the information that woreout posted you need to copy his link and paste it into your browser address bar, hope you can follow that or at least that is what I had to do. Very useful information.

This post was edited by used red MN on 04/25/2023 at 09:46 am.
 
You're welcome glad to help and thanks for the kind words

YES on that 12 volt tractor that has a 6 volt dropping ballast resistor in series between ignition switch 12 volts output and the coils input, YOU NEED A 6 VOLT COIL so the coil sees 6 volts and doesn't overheat

Its done that way so in extreme cold the ballast can be by passed to give a hotter starting spark. When cold cranking battery voltage may drop drastically so with a 6 volt coil on a 12 volt tractor with ballast by passed when cranking you may momentarily have 9 to 11 volts FOR A HOT SPARK only temporary when cranking then the ballast drops 6 again for a 6 volt coil

John T

Some coils are labeled 12 Volts for use with ballast which are in effect 6 volt coils

John T
 
Thanks guys...I appreciate all the responses and advice..this is a lot for an idiot to digest...still working on the link, want to read that information.... Been driving this tractor for a lot of years and look forward to having it running great again!!...Thanks agsin...Dan
 
I had similar problem with other tractor (orange thing) that ended up being coil.. Ran great when cold, longer it ran (maybe an hour or so), then engine began stumbling, missing now and then, then backfiring then stalling. Don't restart til cold. Put in points, plugs,wires...no fix...asked an old friend in his eighties (who ran a auto electric shop for 40 years for help...he said-runs good cold, runs bad hot mean coil..I put on coil, problem fixed....so I thought I would save time and money and go straight to coil...that was same symptoms and coil fixed that problem.
 

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