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Clooney/John T JD 40 Will not start

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Richard Nieman

05-27-2003 07:42:03




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I want to thank you both for all the help,I was at the point of giving up.I went down the list of things to check that you said to do.All was ok so I only have two things left to do.I can buy a carb kit or a new coil.The coil was cheaper so I did it first.Well the new coil did it. She started right up.HOW can you check a coil to see if it is good? Seems years ago I heard of a way. Thanks again for all your help you guys are a life saver. Richard Nieman

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John T

05-27-2003 11:48:02




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 Re: Clooney/John T JD 40 Will not start in reply to Richard Nieman, 05-27-2003 07:42:03  
Congrats Richard, glad we helped you. As far as home testign a coil, theres really not much you can do, other than find out YES its definitely bad, cuz one of the windings (primary or secondary) shows an open or short, which means it is indeed bad.

HOWEVER, even if an ohm meter (which is low voltage, low current, and definitely does NOT warm up the coil where they are most apt to go bad if they are bad) shows the primary somewhere between 1.2 to 3 ohms and continuous and the secondary maybe 10K ohms, which is a good test, THEY CAN STILL GO BAD AFTER WARM UP AND WHEN THERES 10,000 VOLTS PRESENT due to high voltage insulation breakdowns that dont happen at low volts and low current and no heat up.

Best Wishes

John T

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Clooney

05-27-2003 09:04:27




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 Re: Clooney/John T JD 40 Will not start in reply to Richard Nieman, 05-27-2003 07:42:03  
Richard, there are some static tests you can do on a coil to tell if the internal wiring is open or shorted but in a lot of cases the coil won't act up until it becomes hot, or starts to produce a higher KV output. ~It sounds like you ran a good coil test while on your 40. It didn't work while on the tractor & a new one made it run so therefore the coil was ineffective. ~To do a proper coil resistance test you really need to know the coil specs. If it�s a Delco coil the coil number is probably stamped on the coil can & can be looked up in the Delco chart to find the specs. ~As a general guide line measure the primary resistance [between the 2 little coil terminals] & it should be somewhere between 1 & 3.5 ohms [probably closer to 1.5 ohms on that 6 volt 40 coil] higher resistance than about 3.5 ohms means it has an open primary & lower resistance than 1 ohm means it probably has a shorted primary]� Then measure the secondary resistance [between the coil's center tower & the distributor side little terminal [would be the positive terminal on that 40 coil if it's an original coil]. You should probably see somewhere between 8,000 ohms & 20,000 ohms, again it depends on the coil, so you could measure in that range & still have a bad coil. Higher could mean an open in the secondary & lower could be a shorted secondary winding�.
~Some auto parts stores have a heated coil tester that can load, warm, & test the coil�s output. Most will do it for free�.

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