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Howdy Brad, I am the classic coil symptom poster, I hope you don't mind if I try again. If Dave’s suggestion for a shorted wire between the coil and points contact does not pan out, read on. You have established there is no spark from the coil, points are functioning correctly and you have tested the coil by replacing it with a known good coil You have not indicated if your 640 is original 6-volt or if it has been converted to 12-volt. I will address your tractor as if it is 12-volt. You speak of an external capacitor; I will take it to actually be a resistor. (Although some people, myself included, have modified tractor wiring to include external capacitors in the primary ignition electrical circuit, it is not very common at all.) For your tractor to die suddenly and not restart until a cool down period, would indicate a temperature related electrical problem. The classic 1 to 3 hours run time with a 30-minute to an hour cool down is generally the coil. There is no simple electrical test available to the common person to prove such a failure. The usual means to troubleshoot a coil under such conditions is to manipulate the thermal conditions, i.e. chill the coil after the shutdown to see if the coil is actually experiencing thermal breakdown of the insulating compound. The other means is to use another coil in substitution to see if the symptoms disappear. My recommendation is to exchange with the BIL's coil, much cheaper than buying your own. I hope you borrowed the BIL coil. The other component that suffers a thermal related breakdown is the keyed ignition switch. However the normal symptoms are shutdown in ~5 to 15 minutes, startup after ~10 to 30 minute cool down. Using a test lead with alligator clips at both ends to jumper the ignition switch you can perform a quick check. If the switch is faulty, you should then be able to operate the tractor as normal with the switch jumped. This in fact is a common anti theft technique. If you decide to utilize this as an anti theft device then only you, your BIL and every farm boy in my home county will be carrying jumper clips on a regular basis, to hotwire the tractor. Any loose, corroded or dirty connection can suffer thermal degradation of their performance, usually indicated by gradual decline in performance. The usual justification for conversion to a 12-volt electrical system, is poor performance of the electrical system. Translated to “12 volts will push through my loose, corroded or dirty connections when 6-volt will not.” You would be wise to check all connections on a periodic basis. Springtime annual tractor tune-up, oil change and lube is a good time for “clean, bright and tight”. If the above mentioned procedures do not start the tractor, it is time to start troubleshooting the electrical system with test equipment. Remove the distributor cap, rotor, rotor retaining spring clip and distributor dust cap, so you can see the points. Roll the engine over so the points are closed. Now block the points open with a match book cover or other suitable clean oil free piece of paper, folded if you must. I trust you have a multimeter and test light. If not copy, paste and search here:>Link >Link We still have not determined if your tractor is 6 or 12 volt so I will reference you to two more web addresses Link Link We will be performing voltage test to troubleshoot the ignition system on the tractor. Connect your test light or voltmeter to a good ground, put your other test lead on the battery hot terminal to verify your test ground connection, your test equipment and your battery is good. If not, correct the fault. Now that you have verified the test equipment and battery are good, connect the other test lead to the coil primary terminal, key switch side of the coil. You should indicate voltage here with the keyswitch on. At this point we are just concerned with voltage to the coil. Your test light should be energized or your test meter should be indicating a voltage. If not indicating voltage on the keyswitch side of the coil primary, start working back towards the battery from the coil, you will find the external resistor, terminal block, keyswitch and associated interconnecting wiring. Test both sides of each component going back towards the battery until you have a good voltage indication. At that point you have identified either a faulty component or faulty connection. Correct the deficency. Once you have established voltage on the keyswitch side of the coil primary, remove the paper blocking the points open as installed previously. You can now test for spark from the coil by opening the points with an insulated clean oil free utensil and allowing them to spring closed. Go ahead and hook up your test lead to the coil primary terminal, distributor side of the coil. With the points open, you should observe voltage indicated either by the test light energizing or the test meter indicating voltage. If you do not indicate voltage, either your coil is open, or your wire to the distributor points is shorted to ground. But since we know you have installed a known good coil, see the second paragraph first sentence, it has to be a grounded wire going to the distributor points, see Dave’s suggestion. Correct the fault. With the points closed you should indicate no voltage, the light goes out. If you indicate voltage, your light remains energized, with the points closed you have a loose, open, dirty or corroded connection between the points and coil terminal. Correct the fault. Reinstall the distributor dust shield, rotor spring retaing clip, rotor and distributor cap. After all of this your 640 Ford should fire up. If not, look in your pocket for that little piece of paper you used to block the points open with. As always, any questions, comments or pics, please post back. ATW/WA
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