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John Deere Tractors Discussion Forum
Show Parts for Model:

Re: 70 ign problem update

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John T

11-18-2013 13:17:25
216.249.76.176



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"i thought the resistor was to keep the points from burning up. will it also cause the coil to go bad?"

The ballast resistor is there to drop coil voltage (from 12 down to 6) and reduce coil current (from say 8 without down to acceptable 4 amps with resistor). If its a 12 volt tractor with a 6 volt coil and theres no resistor, the coil will overheat PLUS the points will burn up prematurely........

John T

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Bob Harvey

11-18-2013 14:37:46
209.121.225.179



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 Re: 70 ign problem update in reply to John T, 11-18-2013 13:17:25  
My understanding of a 'Ballast Resistor' is: they are used to vary the resistance by the influx of heat. To make more sense, when the resistor is cool, almost all of the 6v goes through to power the points system with as much juice available. As the electron keep flowing through the wire coil of the resistor, the coil heats up, reducing the energy to the points system, primarily to lower the power input to the coil, thus reducing heat to the coil. (mostly for longevity I suspect)

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John T

11-18-2013 15:12:53
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 Re: 70 ign problem update in reply to Bob Harvey, 11-18-2013 14:37:46  
While that has some truth for all resistors since resistance increases upon warm up, what you describe reminds me more of a THERMISTOR. A typical old tractor ballast is more like 1.25 to under 2 ohms fixed, although sure it would be somewhat less when its cold. Its primary purpose is to drop 12 tractor volts down to 6 when using a 6 volt coil while the limited thermal characteristics can indeed improve cold weather starting

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John T

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Bob Harvey

11-18-2013 16:22:18
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 Re: 70 ign problem update in reply to John T, 11-18-2013 15:12:53  
Beg to differ. A 'reducing' resistor does just that with in a second or less, a 'Ballast Resistor' slows down the process with the use of a longer (lengthwise) wire coil, hence a slower response to the loss of energy that goes to the starting system = more juice to the coil/points system during starting. Seems to me the # series had that system hard wired into the "Ignition switch".



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John T

11-18-2013 16:32:40
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 Re: 70 ign problem update in reply to Bob Harvey, 11-18-2013 16:22:18  
Yep I think the 720 etc had the stand alone discrete white ceramic bathtub style of ballast resistors while the new generation had more like a wire link or fusible link looking thermistor on the ignition switch that acted more like the resistor/thermistors you described. My answer covers the old style two cylinder tractors with more of a fixed resistance voltage dropping current reducing ballast which isn't the same as what you're describing. I think youre more right for new generation and Im more right for the old two cylinder tractors.

Very fun chattin with ya, thanks for the info

John T

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Bob Harvey

11-18-2013 16:48:40
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 Re: 70 ign problem update in reply to John T, 11-18-2013 16:32:40  
Right back at ya. My limited knowledge of the subject is based on the 2 'N' Ford's I had. My '60' has a resistor as part of the 'ignition' sw., but I think it has to do with the lights.



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John T

11-19-2013 05:05:01
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 Re: 70 ign problem update in reply to Bob Harvey, 11-18-2013 16:48:40  
Youre right, many brands of tractors with that wire coil resistor on the ign/light switch THATS A LIGHT DIMMING RESISTOR.

Many other tractor light/ign switches have TWO RESISTORS on the switch. One (flat wound) is the generators field current limiting resistor for Low or High charge on tractors that used a cutout relay NOT a Voltage Regulator and the other coil of wire/resistor is for light dimming.

The older two cylinder Deeres (720 etc) used a stand alone discrete fixed bathtub style ballast ignition resistor and and a start by pass system so the coil got battery voltage while cranking but reduced (6 volt) when running.

Some of the 12 volt New Generation Deeres (that used a 6 volt coil) had a thermistor/resistor type device on the switch or wired to it (looks like a wire fuse link etc) AND THERE WAS NOOOOOOOOOO START BY PASS NEEDED because the resistance was low for starting but when it warmed up its resistance increased so the coil only saw 6 instead of the closer to 12 volts at starting. I THINK THATS THE TYPE OF DEVICE YOU ARE THINKING OF

Whewwwwwwwwww got all that lol post back an questions

John T

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