Resistor in ignition circuit .... question ??

Crazy Horse · Jun 2, 2021

Someone has wired in a 1 ohm ceramic resistor into the ignition circuit of an 8N Ford (side mount distributor). I suspect the original wiring has been changed around a bit but that shouldn't make any difference to my question. So 6 volts of supplied power into an ON/OFF ignition switch and a line comes out from the switch, through the resistor and heads off to the ignition components (coil, distributor, points, etc). Any guesses as to why somebody wired this up in this way? What purpose is the resistor serving? The tractor starts and runs the same whether the resistor is part of the wiring or not.

kencombs · Jun 2, 2021

My guess is they were trying to extend the point life by lowering the current they have to handle.

Janicholson · Jun 2, 2021

Lowering 6 volts to 5 or so coupled with cranking voltage loss is not the best option, especially in colder weather. Were it mine and if I lived south instead of Minnesota, I would leave it in. Jim

Geo-TH,In · Jun 2, 2021

Measure the primary current. Should be close to 4 amps when points are closed. Resistor will lower current. If it starts fine with 1 ohms resistor leave it.

Geo-TH,In · Jun 2, 2021

After giving it some thought there's no one way a 6v coil will work with a 1 ohm resistor. Ohm's law. Amps times ohms equals volts.
4 amps x 1 ohm equal 4 volt loss. That only leaves 2 volt for the coil. No way.
Measuring ohms is the least accurate scale on all multimeters.

Crazy Horse · Jun 2, 2021

OK then George, what are you telling me? That the resistor must be somewhat less than one ohm? With a digital multimeter, it is maybe 1.3 ohms. Still impossible? If it is one ohm or say 1.3 ohms, sounds like I'm better off without it?

JMOR · Jun 2, 2021

(quoted from post at 18:24:52 06/02/21) OK then George, what are you telling me? That the resistor must be somewhat less than one ohm? With a digital multimeter, it is maybe 1.3 ohms. Still impossible? If it is one ohm or say 1.3 ohms, sounds like I'm better off without it?

f it starts and runs fine, do not monkey with success, is a good rule. Or if fine now, you can go ahead and "fix" it until it is broken..

Geo-TH,In · Jun 2, 2021

Digital ohmmeters typically aren't accurate.
If tractor runs fine don't mess with it.
If you can't start it then remove it. Won't hurt anything if you jump out the resistor for a short time.
Measure primary current with and without resistor.
Typically coils need 4 amps.
What is the voltage at coil when points are closed?

Crazy Horse · Jun 2, 2021

I will check those readings, the tractor isn't handy to me right now. You mentioned jumping out the resistor for a SHORT TIME being OK. That must mean that it should be kept in for regular long term use, at least that's how I interpret what you said.

jimtrs · Jun 2, 2021

(quoted from post at 16:00:36 06/02/21) ......Any guesses as to why somebody wired this up in this way? What purpose is the resistor serving?

My guess is that someone may have put a 12 volt battery in at some point, leaving the 6 volt coil.

Geo-TH,In · Jun 2, 2021

When cars went to 12v, the ballast resistor was shorted out when the starter was enguaged applyingthe full 12v to the coil.

So what I'm saying is you can short out resistor when cranking the engine and not hurt it because the cranking voltage is less than the running voltage.

Switchflicker · Jun 2, 2021

(quoted from post at 18:10:27 06/02/21)

(quoted from post at 16:00:36 06/02/21) ......Any guesses as to why somebody wired this up in this way? What purpose is the resistor serving?

Click to expand...

My guess is that someone may have put a 12 volt battery in at some point, leaving the 6 volt coil.

You've hit it right on the head.
My 2N has the same wire wound (large french fry shaped resistor) and has been converted to 12 volts. The starter is still 6volts so it cranks over like there's no tomorrow. If it's reluctant to start, I must discipline my self to not crank too long.
I'm sure the coil is still 6 volts also
Stan

Sprint 6 · Jun 3, 2021

Ford used ballast resistors on their 6 volt tractors and vehicles because they designed the ignition to run on 4V continuous. I didn't know any 6V systems ran a ballast resistor until I ran into a 9N that ran but had no power and would flood easily. When I traced the ignition power back, there was a resistor in the circuit by the steering column. An old timer told me it needed to be there or the points would burn. I know this setup to be true of the square bail wire coils of the N series and flathead V8s. I do not know if the practice continued once Ford went to traditional round oil filled coils.

Carlmac 369 · Jun 3, 2021

(quoted from post at 22:19:31 06/02/21) When cars went to 12v, the ballast resistor was shorted out when the starter was enguaged applyingthe full 12v to the coil.

So what I'm saying is you can short out resistor when cranking the engine and not hurt it because the cranking voltage is less than the running voltage.

Terminology is important. The resistor doesn't get "shorted out" it gets bypassed when the starter is engaged.

bc · Jun 3, 2021

There may or may not be a number on the resistor but there may be a series of colored lines around the resistor depending upon the type. The color codes will then tell you what its rated resistance is. Let us know. The ohms readings on those multimeters depend upon which scale you are using.

1st color of brown stands for 1. Then it depends upon if it is a 4 or 5 band resistor. The second is another number of ohms and on a 5 band the third will be another ohm number. Last color is the tolerance such as +or- 1 or 5%. Second to the last color is the multiplier such as .1, .01 or .001 etc.

So a 1 ohm 4 band would be brown (1), black (0), gold (.1), and gold (+-5%) which equals 10 x .10 = 1 ohm with a +-5% tolerance or the last color could be black with a +-1% tolerance.

A 1 ohm 5 band would be brown (1), black (0), black (0), silver (.01) plus the color of the last band for tolerance which could also be a silver (+-10%) which equals 100 x .01 = 1 ohm at 10% tolerance. There are other tolerance codes as well.

Crazy Horse · Jun 3, 2021

I remember those colored stripes but this one is what might be called an automotive-type ballast resistor .... the LINK BELOW shows one which is 1.5 ohms (which I measure this one to be with a digital meter). Most of the ones I see for sale on the web are around that value for resistance. Then there is also the possibility of additional resistance in the coil from what I read, I have no idea what type of coil is on this tractor, it might even be a 12V coil for all I know, I will check it out later today. Right now I would say that this 6V system on the 8N has full time reduced current heading from the ignition switch to the coil full time, whether starting or plain running.
Typical auto ballast resistor .....

bc · Jun 3, 2021

I suppose the way to test the resistor is to see what the voltage drop is after the resistor. Might have to disconnect the negative from the coil but check the voltage leaving the ignition switch which should be the same as the battery (12+). Then check the voltage after the resistor or where the wire runs to the coil which will tell you what voltage the coil is getting. Ground the black test lead to the tractor to get your readings. If it reduces the voltage down to 6 then you would likely have a 6 volt coil, I believe or maybe it just drops it down enough to save voltage going across the points.

Crazy Horse · Jun 3, 2021

I'm going to do that BC like you suggest. Remember though that this is totally a 6V system, but your suggestion still holds regarding the voltage drop .

Hdonly · Jun 3, 2021

Does it start and run good? If so, leave it alone. Why create a problem that doesn't exist?

Crazy Horse · Jun 3, 2021

Well, starting was a bit of an issue but we reset the point gap (it was too wide) and that improved a bit, and I'm picking up a new cap and rotor today, the old ones were in pretty bad shape, well the inside terminals in the cap for sure were corroded and eaten away. I agree with leaving it as is.

Repairman60 · Jun 3, 2021

Ken I believe you hit the nail on the head. Resistor is for current limiting purpose.

Sprint 6 · Jun 4, 2021

You have to make the voltage measurement after the resistor, with the engine running. There will be no voltage drop shown without current flowing in the circuit. Yours should show about 4 to 4.5 V while running.

Resistor in ignition circuit .... question ??

Well-known Member

Well-known Member

Well-known Member

Well-known Member

Well-known Member

Well-known Member

Well-known Member

Well-known Member

Well-known Member

Member

Well-known Member

Member

Well-known Member

Member

Well-known Member

Well-known Member

Well-known Member

Well-known Member

Member

Well-known Member

Member

Well-known Member

Similar threads