12 v to coil?

I recently bought an 8N that someone has converted to 12 V negative ground. When the ignition is switched ON, should I have +12 v at the terminal atop the ignition coil (front mount distributor)? Right now, I'm not getting voltage there.

(quoted from post at 17:06:23 11/09/08) I recently bought an 8N that someone has converted to 12 V negative ground. When the ignition is switched ON, should I have +12 v at the terminal atop the ignition coil (front mount distributor)? Right now, I'm not getting voltage there.

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Only when the points are open. When closed for current flow, much of the 12volts is dropped across the resistor(s).

Thanks. Main thing I was asking is, there's supposed to be a path for positive voltage there, which mine does not have. Must have an open connection somewhere, because it's zero, regardless of where the points are.

You're on the right path by doing the electrical detective work w/ your meter.

I have discovered that 75% of “no spark” problems on frontmounts are as a result of poorly aligned parts or wires & not parts failures. Remember, you are dealing w/ parts made over 50 years ago that have seen a lot of wear & tear. They just don’t fit together like they used to.


First, turn the key on & crank the engine while you look at the ammeter. What is it doing? If it deflects, that means your points are opening & closing. If it shows a constant discharge, or doesn’t move at all, that means you need to do some more checking.


Next, check for voltage at the top of the coil. Battery voltage w/ the points open & about half of that when they are closed. If you do not have voltage to the coil, then move upstream to the resistor then to the ignition switch. As you check each component, wiggle it & the wires attached to it & watch you meter. Resistors break or get shorted & wires corrode.


If the voltage never drops when the engine is cranking, that means the points are not closing or you have lost contact between the pigtail & the brass screw or the tab & the cap.

Next, check for continuity inside the distributor w/ a meter. It is very easy to ground the tip of the condenser wire to the body of the distributor when you replaced the points. Also, make sure the condenser wire does not go through the same opening in the distributor that the coil pig tail does. The condenser wire goes the opening on the top right. Look at the points; are they burned, pitted or mis-aligned? Check the point gap, .015 on all four lobes. Make sure you are using quality parts; the points should have a phenolic rubbing block & not the cheap white plastic crap. Make sure you have the star washers under the screws on the points.

Next, with the distributor still off the tractor, install the coil. Look at the pigtail on the coil; is it touching the brass screw w/ the concave head inside of the distributor? Don’t trust your eyes; test for continuity from the top of the coil to the points. If you do not have continuity, stretch the pigtail a bit until you do. (some people would rather put a small washer under the brass screw) Make sure that the pigtail is not grounded to the clip that holds the distributor plate in place; sometimes you can move the plate while setting the timing & actually ground the pigtail. Your meter should tell you this even if your eyes don’t! Also, look at the little tab opposite of the pigtail; bend it a bit also to insure that it will contact the distributor cap. Another common failure point is the insulator in the tab on the distributor plate where the brass screw w/ the concave head goes; it holds the copper strip from the points. Again, your meter will usually tell you if you have a problem, but you might have to wiggle the copper strip a bit. Once you are sure that you have good continuity inside the distributor, you should be good to go.

At this point, I just put the distributor, coil & cap all back on the tractor as a unit (it helps to loosen the fan belt). The reason I do this is because it is real easy to get the cap or coil mis-aligned trying to put it back together one piece at a time & the result is something gets broken or you get a ‘no spark’ problem.

As to the resistor, p/n 12250 or 12250B (the resistor on the terminal block) it should read .3 ohms cold & 1.7 ohms hot.

Post back w/ results & any other questions.
50 Tips

Can only be open circuit (break) in wires somewhere, bad contact on connections, failed resistor(s), bad ignition switch, dead battery or wires not correct. Those are the only things between the battery and the top of the coil in the ignition circuit. Any open circuit downstream of the top of the coil will give you battery volts at the top of the coil, and a short would still give you some voltage there depending on how much resistance you have.

Carl,

With a front mount 12 volt conversion, there are several possible ways the previous owner could have done it. You could have an original 6 volt coil or a 12 volt coil. You can't tell by looking at them. You have to measure with a volt/ohm meter. Off hand I don't remember the readings for each.

With a 6 volt coil you should have TWO resistors inline from the switch and the coil and about 3.5 volts at the top of the coil with the points closed and battery voltage with the points open.

With a 12 volt coil you should have just ONE resistor inline from the switch and the coil and about 10 volts at the top of the coil with the points closed and battery voltage with the points open.

Good luck trying to determine how the conversion was done!

Danny

(quoted from post at 23:23:15 11/09/08) Carl,

With a front mount 12 volt conversion, there are several possible ways the previous owner could have done it. You could have an original 6 volt coil or a 12 volt coil. You can't tell by looking at them. You have to measure with a volt/ohm meter. Off hand I don't remember the readings for each.

With a 6 volt coil you should have TWO resistors inline from the switch and the coil and about 3.5 volts at the top of the coil with the points closed and battery voltage with the points open.

With a 12 volt coil you should have just ONE resistor inline from the switch and the coil and about 10 volts at the top of the coil with the points closed and battery voltage with the points open.

Good luck trying to determine how the conversion was done!

Danny

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I hate to rain on the parade here, but no need in a guy burning things up.

I would say that the ONE/TWO and 6v/12v looks backwards to me. I would also say that any square can/front mount coil with 10v (points closed) will melt. 3.5v (points closed) for 6v coil is higher than Ford spec sheet, but probably survivable.

JMOR,

How is it backwards? If you have a 6 volt coil in a 12 volt conversion, you need an additional resistor to drop the voltage before the 6 volt coil to keep it from melting. The voltage at the top of the coil should be between 3.2 and 3.5 volts with the point closed. I get 3.4 volt with my 6 volt coil after my conversion 6 years ago. No problems.

With a 12 volt coil in a 12 volt conversion, you still need to cut the voltage to the coil. You can't run full 12 volts. I fell 10 volts will be fine for the long run.

In either case, the lower the voltage at the top of the coil, the worse the sparks. In my opinion, you want the max volts without melting the coil. Yeah, you can drop the volt mode, but I believe you will have starting problems in cold weather.

Respectfully,
Danny

Danny........yer voltage "quotes" are spot-on, for the reasons you cited. Yes, the squarecan coil top terminal volts will be different depending upon whether the coil is the OEM 6-volt or the modern 12-volt replacement. And as you pointed out, there are NO EXTERNAL markings to differentiate between the 6-volt and 12-volt squarecan ignition coils.

JMOR is also correct for the 6-volt coil ONLY. I think he miss-read yer original answer.......Dell

(quoted from post at 00:36:11 11/10/08) JMOR,

How is it backwards? If you have a 6 volt coil in a 12 volt conversion, you need an additional resistor to drop the voltage before the 6 volt coil to keep it from melting. The voltage at the top of the coil should be between 3.2 and 3.5 volts with the point closed. I get 3.4 volt with my 6 volt coil after my conversion 6 years ago. No problems.

With a 12 volt coil in a 12 volt conversion, you still need to cut the voltage to the coil. You can't run full 12 volts. I fell 10 volts will be fine for the long run.

In either case, the lower the voltage at the top of the coil, the worse the sparks. In my opinion, you want the max volts without melting the coil. Yeah, you can drop the volt mode, but I believe you will have starting problems in cold weather.

Respectfully,
Danny

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Sorry, Danny, as Dell said, I did mis-read your answer. I read your "With a 6 volt coil you should have TWO resistors inline .." and "With a 12 volt coil you should have just ONE resistor ..." as though you were saying 'with a 6v system....' and as "with a a 12 system..'. No reason why I should have, you were perfectly clear, as it is now to me in the light of day......guess I was just up too late. Glad I didn't really jump your case over it!
Sorry, JMOR

LOL, no problems! My old eyes don't always see the right things and I jump to conclusions!

Danny

Guys....May I interject a thought here?

I believe it's a mistake to suggest either one or two resistors are required based on the percieved voltage of any front mount coil. My recent 12v conversion uses two resistors and a 12v marked coil from the "land of almost right". After reading many posts on this subject I measured the primary resistance of the old 6v coil (1.05 ohms) and the new 12v marked one (1.3 ohms) on a GR 1650 bridge. The conversion kit came with a second resistor.

Unless someone knows of a source of higher resistance 12v FM Coils then I believe you will always need two resistors...the factory one for higher initial starting current and a second one for limiting run current 3-4a. JMHO.

Ike