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John Deere Tractors Discussion Forum
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720 Starter Switch

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Raysflhs

01-13-2020 09:21:56




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Hello, I am replacing a starter switch on a 1958 720 (gas engine). A wire connects from a terminal on switch to ignition resistor on the coil side. What is the purpose of this connection? It seems redundant since the coil will get voltage when the ignition switch is turned on. If the coil gets activated by pressing the starter button without ignition switch on, then the engine could start and then immediately stop again when button is released. What's the point of that? My service manual doesn't address this switch operation. Thanks, Ray

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John T

01-13-2020 19:30:20




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 Re: 720 Starter Switch in reply to Raysflhs, 01-13-2020 09:21:56  
Ray, that extra small terminal on those starter switches is hot ONLY when the starter is engaged. What it does is to provide unballasted voltage direct to the coil ONLY while cranking which improves cold weather starting due to voltage drop.

The normal when running coil voltage is via the ignition switch and ballast resistor but ONLY when the starter is engaged the ballast is effectively by passed.

John T

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Weirddeere

01-14-2020 16:39:03




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 Re: 720 Starter Switch in reply to John T, 01-13-2020 19:30:20  
JohnT

This probably sounds like an odd request, I was wondering if you have copies of the various JD wiring diagrams on your computer and might be able to email them? I don't know if there are a bunch or if it would be pain? If so don't worry about it.

I remembered they were posted a lot by M Man back in the day but the photobucket he used for that is basically useless now to see them. It all came up because I realized I forgot to save the ones I needed on my computer way back when, and since I often work on weird projects it'd be nice to have everything LOL

Anyway, thanks
Brandon
moparwhs@hotmail.com

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Dan in Ohio

01-15-2020 06:26:41




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 Re: 720 Starter Switch in reply to Weirddeere, 01-14-2020 16:39:03  
Brandon,
Pretty sure I have an original dealer copy of that manual (SM 2029) if you need something from it shoot me an email. Away from home now but be back when the snow leaves but I am pretty sure I have that manual among some of my auction finds.



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Weirddeere

01-15-2020 12:32:56




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 Re: 720 Starter Switch in reply to Dan in Ohio, 01-15-2020 06:26:41  
Thank you Dan, I managed to find what I needed, appreciate the offer

Brandon



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John T

01-15-2020 04:26:27




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 Re: 720 Starter Switch in reply to Weirddeere, 01-14-2020 16:39:03  
Hi Brandon, haven't seen you since a New York Two Cylinder Expo yearsssssssssssss ago.

Here is where I find my Two Cylinder Diagrams. Years back one could down load or copy but I think that's changed grrrrrrrrrrr They are taken from JD 2029 (via M Man) and somewhere at home I have copies but none here in Florida or on my computer

http://www.johnnypopper.com/forums/topic/14556-two-cylinder-electrical-diagrams/

John T

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Weirddeere

01-15-2020 12:43:25




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 Re: 720 Starter Switch in reply to John T, 01-15-2020 04:26:27  
Thank you John I knew they were online originally somewhere. I found them again in a different spot

An internet archive of www.jdhpubs.com!!

https://web.archive.org/web/20101125061402/http://jdhpubs.com/

Here's one specific. It all works just fine!

https://web.archive.org/web/20101125062758/http://jdhpubs.com/pdf/WiringDiagrams/JD-R.pdf

Brandon



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Pete in MD

01-14-2020 20:32:35




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 Re: 720 Starter Switch in reply to Weirddeere, 01-14-2020 16:39:03  
Brandon,

Section 80 of the JD Electrical Systems manual, SM-2029, has the wiring diagrams for all the 2-cyl models up to the 840 industrials.

Hope this helps.



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Weirddeere

01-15-2020 12:35:03




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 Re: 720 Starter Switch in reply to Pete in MD, 01-14-2020 20:32:35  
Yes you're right, I knew I'd seen them online before but couldn't find them...but now I found them so I'm all set.

Thank you,
Brandon



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James Howell

01-13-2020 16:19:10




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 Re: 720 Starter Switch in reply to Raysflhs, 01-13-2020 09:21:56  
"A wire connects from a terminal on switch to ignition resistor on the coil side. What is the purpose of this connection?"

Your tractor has been set up with the popular ballast resistor bypass for starting only option.

Take a look at the diagram below.

Note the black wire that is connected to the starter switch and ignition resistor.

Take a look at the photos below.

Note the side terminal on the starter switch.

Note the black wire connected to the side terminal on the starter switch leading to the ignition resistor.

Note the black wire from the starter switch connected to the "output" terminal on the ignition resistor.

When you press down on the starter pedal, 12 volt power temporarily goes directly from the battery to the ignition resistor and coil at start up.

Take a look at the diagram below.

If the ignition switch is ON, then once you release the starter pedal, the power comes from the ignition switch to the ignition resistor and coil.

Hope this helps.

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PJH

01-13-2020 18:05:08




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 Re: 720 Starter Switch in reply to James Howell, 01-13-2020 16:19:10  
And the brown wire from the ignition switch to the coil is the one that RVirgil is talking about. That must ground the coil when the ignition switch is off? I have a 620 and a 50 - neither of them have the brown wire.



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John T

01-14-2020 04:28:25




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 Re: 720 Starter Switch in reply to PJH, 01-13-2020 18:05:08  
You ask " That must ground the coil when the ignition switch is off? "

Correct. HOWEVER That doesn't mean over the years things stayed that way. I was a used tractor dealer and found that out grrrrrrr

John T



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PJH

01-13-2020 13:11:41




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 Re: 720 Starter Switch in reply to Raysflhs, 01-13-2020 09:21:56  
"If the coil gets activated by pressing the starter button without ignition switch on, then the engine could start and then immediately stop again when button is released. "

As others have said, the circuit from the starter switch to the coil bypasses the ballast resister while the starter is engaged. If the coil is not somehow grounded when the ignition is off, the engine could run as long as you had the starter engaged.

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ArnieinChesterfieldVa

01-13-2020 09:47:16




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 Re: 720 Starter Switch in reply to Raysflhs, 01-13-2020 09:21:56  
I don't remember back that far when I put my new switch in my 60. All I know is I followed through connecting up the new switch exactly the way the old one was....and all worked out just fine. Sometimes we can look too deeply into a situation. The manufacturer must have had a reason. They know more than us.
Arnie



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rvirgil_KS

01-13-2020 09:42:32




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 Re: 720 Starter Switch in reply to Raysflhs, 01-13-2020 09:21:56  
Safety factor

According to wiring diagram I have, there is a wire from starter switch that supplies 12 volts to input of coil when starter is activated to bypass the resistor for hotter starting spark.

The wire from the switch goes to the points side (output) of coil to ground the coil when switch is off. This will prevent engine from starting if starter is engaged when the switch is off.



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olddantucker

01-13-2020 13:18:46




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 Re: 720 Starter Switch in reply to rvirgil_KS, 01-13-2020 09:42:32  
Yes exactly when I was a boy I just got off the school bus and dad had just got a 60 well like all little boys I had to see the new tractor I got on and thought that is a funny looking gas pedal.so of course I pushed it and it jumped about a foot being in gear and all the only thing saved me from it starting and going through the shed was the switch wasn’t on.it scared me to death I got off and went to the house.so wire it back the way it should be you might just save a nosying little boys life

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PJH

01-13-2020 12:42:10




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 Re: 720 Starter Switch in reply to rvirgil_KS, 01-13-2020 09:42:32  
"The wire from the switch goes to the points side (output) of coil to ground the coil when switch is off."

That's interesting. I've never worked on a 720.



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Raysflhs

01-13-2020 10:19:43




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 Re: 720 Starter Switch in reply to rvirgil_KS, 01-13-2020 09:42:32  
OK thanks that makes sense now. Service Manual doesn't say anything about it other than it exists.



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