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8N Died In The Field
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Twice

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Alan M. (Va.)

06-22-2001 19:06:06




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This week my 8N stopped abruptly after mowing heavy grass and weeds without problems for 2-3 hours before it died. Temperature was in the 90s. I could see no spark when cranking while holding a spark plug terminal close to the plug. My 8N is a '49 with a front mount distributor, but someone in the past made a cover for the ditributor so it could be connected to an external side mounted coil. It has operated fine this way for the past two years since I bought it. I took this opportunity to return it to its original configuration by installing new points, condenser, rotor, cap, front mounted coil, and spark plugs. (This was last winter's project that I had purchased the parts for.) I found no real problem that I could point to for causing the shut down except the jury-rigged wire from the dist. to the coil broke off as I removed the distributor. Two days later I was back in the field and mowed for several hours and then the same thing happened. In my search of recent discussions on ingition in this forum I came across Dell's statement that the voltage at the coil should be 3.5v. All I had done in my troubleshooting was to confirm that there was voltage there - not how much. I went back and checked, and I had 6v there and in further checking found that the resistor had been bypassed by hooking the wire from the ignition sw and the wire to the coil onto the same terminal. So my first question is this : Could having full battery voltage to the coil fry it? Was my previous satisfactory operation only because on those occassions I did not run the tractor hot for long periods of time like I did this week? Unfortunately I have no way to confirm that the coil is now defective.
Today I bought a new transistor block and transistor from the FNH dealer. After installing it I still get 6v on the wire to the coil, but I think that might be because the coil and dist. have not been installed yet and there is no current being conducted until the points close. Is that correct? When and how should I see the 3.5v?
The only other problem I can find is that my points gap is less than half of the .015 it should be. I can not get more gap by turning the eccentric screw because the back of the stationary point terminal is up against the mounting screw. I think this problem is caused by the dist. cam being worn so that it doesn't push the moveable contact far enough away. Other than finding a new cam, the only thing I can think of is to file the points down to get the gap. Other than shortening their life, is there anything wrong with that? Also, could this .006-.007 gap be the cause of the loss of spark? I guess I've answered that question by the fact that the engine still would not start after everything had cooled down. Any and all advice will be appreciated and I thank you in advance.

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Dell (WA)

06-22-2001 19:51:01




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 Re: 8N Died In The Field - Twice in reply to Alan M. (Va.), 06-22-2001 19:06:06  
Alan..... ..no question about it, on a 6 volt frontmount ignition coil, you must see about 3.5 volts (2.5 min, 4.0 max) with the points closed and 6 volt battery volts with the points open. Its an Ohms Law electrical thing.

Sometimes, a well meaning but misguided shadetree mechanic, will "bypass" the infamous factory installed "ballast resistor" in the mistaken belief that by removing the resistor it will make a hotter spark for eazier starting. Sometime a little electrical knowledge is dangerious.

"Today I bought a new transistor block and transistor from the FNH dealer. After installing it I still get 6v on the wire to the coil, but I think that might be because the coil and dist. have not been installed yet and there is no current being conducted until the points close. Is that correct? When and how should I see the 3.5v?"

Thats a new one on me, "transistor block". I'm gonna make a wild guess that what you really mean is "ballast resistor".

Until you complete the ignition circuit by having the ignition point close, you will read 6 volts at the coil terminal. Its another electrical law called "Kerchoff's Law".

If'n the coil is dangling in mid-air, you definately have an incomplete circuit (no points) and you will read battery volts at eather the coil terminal or the springy thingy underneath the frontmount coil.

You must install the coil and you must correctly wire the ballast resistor into the circuit from the ignition switch to the coil terminal. If'n the ballast resistor which is mounted on the back side of the dash panel, is not in the circuit, you will get battery volts at the coil terminal and you will again burnout your coil.

As for the point gap. I've never known the distributor cam to wear that much, its usually the points rub block that wears, its designed to. The problem with such a small point gap is that electrons are lazy suckers and would rather jump a small gap than the big gap of the sparkplugs. It may work at idle speed but at higher engine speeds ya ain't gonna have enuff sparkies. Get a new set of ignition points.

Do it right so I don't have to explain it all over again..... ...Dell

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Alan M. (Va.)

06-23-2001 06:09:16




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 Re: Re: 8N Died In The Field - Twice in reply to Dell (WA), 06-22-2001 19:51:01  
Dell - Thanks for your help. By "transistor block" I was refering to the small block of insulating material that is screwed in place to hold the transistor. I thought I read somewhere that that is what it is called - anyway the contacts holding the transistor were rusted and I wanted to have complete confidence in the electrical hookup. If you don't mind I have one more question in trying to understand this: when the points close and current flows, I should see 3.5 volts at the coil. Is this a momentary thing where I have to be quick to see it? Once the condenser has been charged up, won't the current stop flowing and the voltage go back to 6V? Also, should I hook up the assembled distributor and hold it in my hand to turn it (I would ground the dist housing) to check for the 3.5v when the points close, or is it possible to put the tractor in gear and push it enough to turn the installed dist. to where the points close? Thanks again. - Alan

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Dell (WA)

06-23-2001 07:36:58




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 Re: Re: Re: 8N Died In The Field - Twice in reply to Alan M. (Va.), 06-23-2001 06:09:16  
Alan..... ..the 3.5 volts at the coil terminal when the ignition points are closed is a "constant" volts. Well, not really, as it will change about 0.5 volts as your "ballast resistor" heats up, but that takes minutes. Not something that you miss if'n you blink your eyeballs.

Since the condenser is across the points and the points are effectively shorting the condenser out when the points are closed, you won't see a condenser charge effect. When you open the points, the inductive reactance of the coil will mask any charging effect of the condenser.

Which by the way. if'n you had the condenser out in the open in your hand all by its self and put an ohm meter across it, you would see the charge effect of the ohm meter battery and on a good condenser would probably take less than 2 seconds.

And yes, you can mount the distributor, and turn the distributor points to open or close by pushing on the rear tires ...or... sometime you can make the fan belt tight enuff to turn the engine with the fan ....or.... stick a 1 5/15 socket thru the crankhole and turn the pully bolt clockwize and make the engine turn if'n you got long enuff arms (grin). Don't bother mucken' round with trying to turn the engine with the starter, you don't have enuff control.

Or you can do it in the air as long as you have the circuit complete. I ususally do it with the distributor mounted and watch the voltmeter on the coil terminal for changes..... .Dell

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Alan M. (Va.)

06-23-2001 18:02:39




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 Re: Re: Re: Re: 8N Died In The Field - Twice in reply to Dell (WA), 06-23-2001 07:36:58  
Dell - All went well and the 8N is running again. I'm using a used coil from a spare distributor. The new replacement really was fried - I put it back on and could get no spark. As for the points, I really learned that they are not created equal. Fortunately I bought a new set from the FNH dealer yesterday, and I had no problem adjusting them to the correct gap. The other new ones (not FoMoCo) I had installed are clearly worthless. Also, I was able to see the voltages change to the values you gave me both with the distributor in my hand and again when mounted using the fan to turn the engine over. Thanks again for your help. Alan M.

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glad its runnen . . . Dell (WA)

06-24-2001 19:34:07




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 Re: Re: Re: Re: Re: 8N Died In The Field - Twice in reply to Alan M. (Va.), 06-23-2001 18:02:39  
that pleases me..... .Dell, PS...unfortunately, frontmount ignition coils are not as robust as we'd like 'em to be. They can be bad right outta the box



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