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John Deere Tractors Discussion Forum
Show Parts for Model:

720 gas tractor

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WTM8

10-10-2018 14:06:55




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What causes fire at the points when the ignition switch is turned off on a John Deere 720 gas tractor?




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John T

10-11-2018 04:56:04




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 Re: 720 gas tractor in reply to WTM8, 10-10-2018 14:06:55  
IFFFFFFFFFFF you have the CORRECT Ignition switch that's wired CORRECT THERE SHOULD BE NO FIRE ON THE POINTS WHEN CRANKING WITH IGNITION OFF AND IF SO READ BELOW !!!!!!!!!!!!

(Copy and paste of my response above)

With all due respect to James and Lee, let me offer a somewhat different response, maybe right maybe wrong, its based on olddddddddddd recall lol so NO WARRANTY. I believe there's a safety related reason for FOUR wires to the coil NOT just 2 or three, and if wired correct to the correct ignition switch IT SHOULD NOT FIRE WHEN CRANKING WITH IGNITION OFF IE

ITS NOTTTTTTTTTTTTTTTT NORMAL AND DOES (or should be for safety and originality) NEED FIXED !!!!!!!!!!!!!!!

IFFFFFFFFFFF its wired CORRECT with the CORRECT ignition switch ..

a) With Ignition ON when cranking the ballast is by passed so there's unballasted battery voltage getting to the coil via the extra little terminal on the starter switch so the points would spark and the coil produce a spark........Thats the ballast by pass while cranking system used with a 6 volt coil on a 12 volt tractor to produce a hotter spark when cranking to help in cold weather starting.

b) With Ignition OFF when cranking if its wired CORRECT with a CORRECT ignition switch THE POINTS SHOULD NOTTTTTTTTTTTT SPARK That's a safety design so if a person engaged the starter with the ignition OFF it wouldn't spark when cranking

HERES HOW IT WORKS AND WAS DESIGNED AS I BEST RECALL

I believe there are FOUR wires TO the coil (even if one is from ballast where it wires to coils input).

1) On INPUT from ignition switch one wire is hot when ignition is ON 2) On INPUT (or ballast terminal that wires to coil) there's a wire to the small side terminal on the starter switch which is hot ONLY while cranking to by pass the ballast while cranking. 3) On the coils OUTPUT there's a wire to the distributor which leads to the points inside. 4) On the coils OUTPUT theres s wire up to the MAG/GROUND terminal on a CORRECT ignition switch so if its cranked with ignition OFF the coils output is safety grounded (effectively shunts across points) so there's no fire and the points should NOT spark.

Thats the reason for FOUR wires on the coil, 2 on INPUT (or one to ballast which wires to coil) and 2 on OUTPUT. Normally the OUTPUT just has one wire to the points but for safety to prevent fire when cranking with ignition OFF you need the extra safety wire to a CORRECT ignition switch which grounds the points (direct path from coil output to ground) when cranking if ignition is OFF

MY BEST GUESS FOR WHY POINTS STILL FIRE WHEN CRANKING WITH IGNITION OFF

A) Wrong ignition swtitch with no MAG/GROUND terminal down to the coils OUTPUT..... B) Wire is missing or disconnected FROM coils OUTPUT TO MAG/GROUND terminal on ignition switch. C) Ignition switch is bad or not well frame grounded..... D) Incorrect Ignition switch missing the Mag/Ground terminal I believe the correct switch had the Mag/Ground terminal which grounded out a mag if a mag was used instead of a distributor and the ballast and ballast by pass.....

TROUBLESHOOTING Insure you have the correct ignition switch,,,,,,,,,,,,,, with the extra Mag/Ground terminal that's connected to the coils output,,,,,,,,,,,Insure the coil has two wires on its OUTPUT

John T

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James Howell

10-10-2018 20:20:04




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 Re: 720 gas tractor in reply to WTM8, 10-10-2018 14:06:55  
"What causes fire at the points when the ignition switch is turned off"

Take a look at the GENERATOR, IGNITION, AND STARTING WIRING diagram below.

Note that power flows (1) from the battery to the cranking motor.

Then the power flows (2) from the cranking motor to the ammeter.

Then the power flows (3) from the ammeter to the combination switch.

Then the power flows (4) from the combination switch to the IGN terminal only if the ignition is turned ON.

These are the only four points that are "hot" at all times.

If there are any other additional "farmer tech" wiring at these four terminals as displayed in the diagram above, then remove the extra "farmer tech" wiring.

Take another look at the GENERATOR, IGNITION, AND STARTING WIRING diagram below.

Note that the combination switch (IGN) terminal is switched to the OFF position.

With the ignition switch turned OFF, using a test light, (1) check for power at the coil input terminal.

If there is power at the coil input terminal, then (2) check for power at the ignition resistor output terminal.

If there is power at the ignition output terminal, then (2) check for power at the ignition resistor input terminal.

If there is power at the ignition input terminal, then check for power at the IGN terminal on the combination switch.

If there is power at the IGN terminal on the combination switch, then there is a problem with the combination switch and it should be replaced.

If there is NO power at the IGN terminal on the combination switch, then (3) check for power at the two output terminals on the push button starter switch.

Check for power at the output terminal to the ignition resistor output terminal.

If there is power at the output terminal to the ignition resistor output terminal, then check for power at the output terminal to the cranking motor.

If there is power at the output terminal to the cranking motor, then there is a problem with the push button starter switch and it should be replaced.

Hope this helps.

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James Howell

10-10-2018 20:57:29




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 Re: 720 gas tractor in reply to James Howell, 10-10-2018 20:20:04  
Disregard the above information.

The diagram is for a 730.

Failed to read the "small" print.

Sorry about that.



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rick deere

10-10-2018 18:57:37




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 Re: 720 gas tractor in reply to WTM8, 10-10-2018 14:06:55  
take a test light, with switch off check to see if you have fire, without trying to start. If not you have no problem. Yes it should get fire if switch is off and you engauge starter. with switch off as soon as you get off the starter there should be no fire.



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susandiane

10-10-2018 16:05:01




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 Re: 720 gas tractor in reply to WTM8, 10-10-2018 14:06:55  
Help us out a bit. . . Are the points sparking when cranking with the distributor cap removed and ignition off? Does it continue to run when the ignition is turned off? Does it pop or backfire when the ignition is turned off? Are the plugs still firing when the ignition is off? Help us get on the trail-sounds like a challenging problem!



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WTM8

10-11-2018 08:59:41




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 Re: 720 gas tractor in reply to susandiane, 10-10-2018 16:05:01  
Yes, there is fire at the plugs when the ignition switch is off. I also unhooked the wire from the resister that goes to the started solenoid and there was no fire at the points or plugs with the ignition switch turned off.



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