spark coil questions

I am still working on the 1930 Model A Ford doodlebug that I am trying to resurrect after it sat outside in the elements for 20 years. I did some preliminary checks today in hopes of trying to start the engine tomorrow. I think there is a problem with the spark. It was originally a 6 volt positive ground system, but was converted to 12 volt negative ground before I ever saw it. I am staying with the 12 volt negative ground. Question 1: With the key on but engine not running, should the voltage on the distributor side of the coil be the same as the battery side? Question 2: If the voltage on the dist. side is considerably lower than the batt. side, does that mean the coil is hooked up backwards? Question 3: Is there a difference between a coil designed for positive ground and one designed for negative ground? Thank you.

Question 1: The voltage should be very close to the same on both sides of the coil. Some minor voltage may be lost to the internal resistance of the coil.
Question 2: I was taught to connect electrical componants like batteries in series. ( i.e. from battery + to - side of coil, then + side of coil to distributor.) An engine will run with the coil hooked up either way but the spark will be hotter when hooked up correctly.
Question 3: I have never had to purchase a coil for a positive ground system, but as stated for question 2 I do not believe it really matters too much. HTH

I'm assuming the distributor is similar to 1940's and later tractor systems and uses the common can type coil as shown by many suppliers when searching for Model A coil

Question 1: With the key on but engine not running, should the voltage on the distributor side of the coil be the same as the battery side? It depends. With points open the voltage at distributor side should be near same as battery side. With points closed the voltage should be near zero.

Question 2: If the voltage on the dist. side is considerably lower than the batt. side, does that mean the coil is hooked up backwards? No, Most likely reason is that points are closed. The coil doesn't care which way it is hooked up. Coil hook up affects the polarity of the spark plug. With negative ground system, - on coil goes to side of distributor, with positive ground the + on coil goes to side of distributor. Hooked up this way, the spark plug will spit the spark from center electrode to the grounded tab. Reverse connection on coil and plug will suck the spark from tab to center electrode. Spark is more efficient if plug spits rather than sucks

Question 3: Is there a difference between a coil designed for positive ground and one designed for negative ground? No, see explanation in #2.

Thank you for the replies. That helps me figure out where to start tomorrow. This coil is labeled Bat & Dist rather than + and -. After watching a couple u-tube videos I realize the coil should be hooked up the same way regardless of system polarity.

The coil should be connected according to the ground polarity. If the coil was intended to be original with a positive ground, and it is labeled as stated, the bat should now be connected to the Distributor, and the Dist connected to the Key/source. Reversing it causes the spark polarity to be correct at the plug. Youtube is not always correct!!! Jim

4 cylinder engines almost always stop with the points closed. This is intended to ground the coil making it become saturated and ready for making a spark. With the points open, or a plastic knife stuck between them, the voltage will read the same on both sides of the coil. with the points closed, the voltage should be on the key side only. if closed and on both sides, the points are not closed, or are corroded. Jim

(quoted from post at 20:09:07 11/23/21) The coil should be connected according to the ground polarity. If the coil was intended to be original with a positive ground, and it is labeled as stated, the bat should now be connected to the Distributor, and the Dist connected to the Key/source. Reversing it causes the spark polarity to be correct at the plug. Youtube is not always correct!!! Jim

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X2 to what Jim wrote!

"Youtube is not always correct!!!


"The markings on a coil with the primary terminals labeled "BATT" and "DIST" will only be correct when it is used on a system of the same ground polarity as the machine it was designed/sold for."

It's easy to determine if the spark polarity is correct with an ignition oscilloscope (but you probably don't have one, LOL), the voltage spike that leads up to the firing of the sparkplug is "negative-going" when coil primary polarity is correct.

If you are a bit more adventurous you can use the "pencil test" to determine spark polarity!

"A quick simple check for correct coil polarity can be made with an ordinary "lead" pencil.

Place the lead of the pencil in place between the spark plug wire and the plug.

Start the engine and observe the spark (fig. 6-52).

If it "flares" or has a yellow tinge on the plug side, polarity is correct.

If it "flares" or has a yellow tinge on the wire side, polarity is reversed.



I'll leave it to you to figure out how to hold/support the pencil without getting a shock!

Source:

https://navyaviation.tpub.com/14329/Figure-6-52-One-Method-Of-Checking-Coil-Polarity-287.html

Summarizing, my view is that if factory proper positive ground system is change to negative ground battery & nothing else is done, the negative HV spark will simply become a positive HV spark. Same magnitude, reversed polarity HV out.
Then, if coil wires are reversed, the HV will now be negative, magnitude will decrease by the amount of primary voltage during spark (called 200v), IF this wire reversal results in changing the pri/sec common tie point inside the coil from 'connected at battery' to connected tie point being 'connected at distributor. Therefore, whether reversing coil wires results in HV increase or HV decrease, depends on where windings common tie point is located. The front mount square can coil has tie point at stud (battery), so such a wire swap on it will result in the HV decrease. If apply DC battery voltage to pig tail and stud (use resistor to reduce current) and connect a voltmeter to HV output and touch one at a time, the other meter lead to stud, then pig tail. Which ever shows ZERO voltage is common tie point. And finally, then with sq can coil, reversing wires in combination with battery ground yields a negative spark output and a lower voltage by ~200 out of 20,000 or ~1%. That might be contrary to my previous comments, but the way I see it today, since I have learned that the front mount sq can coil is start to start (finish to finish) type coil. Such wiring reversal isnt likely to be done with a front mount coil due to access of pig tail.
I contend that there is no significant energy loss, just the thermionic emissions deal, which isn't much either. Temperature may be a few hundred degrees in an engine, where in a vacuum tube it may be 2,000 degrees. Energy = L x I squared. Nothing here with regards to polarity! Example numbers of typical coil are, Energy = 3.75 mHenry x 4 x 4 = 30 mJoules or 30 Watt-seconds
The above pertains to the 8N/9N front mount coils which are start to start (finish to finish) coils. Most (if not all) round can coils are of the start to finish (finish to start) design. Flipping commections on this type of coil actually results in an increase in output voltage (tie point now at points), because with the start to finish windings, the primary voltage now ADDS to secondary voltage. Just the opposite of the 8N/9N coil, where it subtracts. The only situation that I have seen where factory install used this type coil in flipped arrangement is on positive ground Farmall SMTA, so its HV output is primary voltage plus secondary voltage AND is Negative HV spark!

This is a typical round can coil that was common in the 1950s and 60s. I have no idea which polarity it was intended for. There was NO ballast resistor on the bug when I got it. Maybe the coil was changed when it was converted to 12 volt? I have no way of knowing.

(quoted from post at 09:13:23 11/24/21) This is a typical round can coil that was common in the 1950s and 60s. I have no idea which polarity it was intended for. There was NO ballast resistor on the bug when I got it. Maybe the coil was changed when it was converted to 12 volt? I have no way of knowing.

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hat really matters is 1)points open you see 12v on both coil terminals. 2) Points closed you see zero volts on terminal connected to coil and 12v or at least 6v on side of coil receiving power from ignition switch.

Great questions, here's my response

1: With the key on but engine not running, should the voltage on the distributor side of the coil be the same as the battery side?

ANSWER NO the voltage on dist side to points SHOULD BE NEAR ZERO as the closed points (which is typical if engine not running) should create a short to frame ground. With key on the voltage on the coils input from ign switch should be battery voltage unless theres an inline series ballast and points closed.

YES it would be same as batt voltage IFFFFFFFFF points were open but see the above

Question 2: If the voltage on the dist. side is considerably lower than the batt. side, does that mean the coil is hooked up backwards?

ANSWER NO if the points are closed AS TYPICAL WHEN AN ENGINE IS NOT RUNNING there should be ZERO VOLTS on the coils output to
distributor terminal because the closed points creates a short circuit to frame ground

NO it DOES NOT MEAN coil is wired backwards


Question 3: Is there a difference between a coil designed for positive ground and one designed for negative ground? Thank you.

ANSWER NO However the coil should be wired at the correct polarity since due to the effects thermionoc emission its easier to arc jump current from a hotter surface to a colder one. I will still work wired backwards but its more efficient if wired correct.

For NEG ground the coils + goes to ign switch and - to distributor

NOTE if ign is on and engine slowly cranked over a test lamp on coils output to dist should flash ON when points are open but OFF when closed

Read through my Troubleshooting Procedure which can help understand and better answer this

http://www.ytmag.com/cgi-bin/viewit.cgi?bd=farmall&th=5745


Hope this helps, any questions???

John T
John Ts Ignition Troubleshooting

After watching a couple u-tube videos I realize the coil should be hooked up the same way regardless of system polarity

WRONG WRONG WRONG

Question 3: Is there a difference between a coil designed for positive ground and one designed for negative ground? Thank you.

ANSWER NO However the coil should be wired at the correct polarity since due to the effects thermionoc emission its easier to arc jump current from a hotter surface to a colder one. I will still work wired backwards but its more efficient if wired correct.

For NEG ground the coils + goes to ign switch and - to distributor
For POS ground the coils - wires to ign switch and + to distributor


John T Electrical Engineer

(quoted from post at 10:18:12 11/24/21) After watching a couple u-tube videos I realize the coil should be hooked up the same way regardless of system polarity

WRONG WRONG WRONG

Question 3: Is there a difference between a coil designed for positive ground and one designed for negative ground? Thank you.

ANSWER NO However the coil should be wired at the correct polarity since due to the effects thermionoc emission its easier to arc jump current from a hotter surface to a colder one. I will still work wired backwards but its more efficient if wired correct.

For NEG ground the coils + goes to ign switch and - to distributor
For POS ground the coils - wires to ign switch and + to distributor


John T Electrical Engineer

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e need not worry about polarity and really can not since he said his coil does not have polarity markings, but rather BATT and Distr markings.

Maybe the coil was changed when it was converted to 12 volt? I have no way of knowing.

Heres a way to possibly find the answer:

If the LV primary resistance from the little + and - or BAT to DIST is around 2.5 to under 4 ohms (many around 3) ITS LIKELY A 12 VOLT COIL

If the LV primary resistance from the little + and - or BAT to DIST is around 1 to 2 ohms (many around 1.5) ITS LIKELY A 6 VOLT COIL

Hope this helps, any questions??

John T

I checked the coil voltage again with the points open and got 12+ volts on both sides. We also verified that the dist. terminal of the coil is connected to the distributor and the batt. terminal is connected to the switch. We tried to start it today. It putted a few times but no joy. It also has a carb. issue. Thank you for the comments. I think I unstand this better now.

JMOR is correct. There may be a slight output voltage change based on the internal wiring but is small. The thermionic emissions itself has only a small effect at spark plug temperatures.

I only remember one study that looked at fuel air ratio, compression ratio, type of electrodes on the spark plugs, turbulence inside the cylinder, and spark plug polarity. The study found the effect of switching the polarity was around 30 percent. So the 10,000 volts or so needed to jump the gap goes to 13,000 volts. That sounds like it would be a problem but a coil should be able to produce at least 20,000 volts so producing 13,000 volts will not be a problem.

There are lots of duel spark coils on cars and motorcycles that fire one plug positive center and one plug negative center and they still seem to be able to run just fine. Magnetos for multi-cylinder engines typically will fire every 180 degrees of magnet or rotor rotation. They will fire with the magnet in a certain position relative to the coil and produce a spark of a certain polarity. The rotor will then turn 180 degrees and now the magnet is reversed from the first spark position so the polarity of the spark is reversed. They still manage to run correctly even with that alternating polarity and the supposed large effects from thermionic emissions.

The effect from thermionic emission is real but small and not significant for coil or magneto that is in good condition.

I suppose you could have a failing coil that produces just enough voltage to initiate a spark with one polarity but that may not be enough for the other 30% more voltage for the other polarity.

I am missing the theory behind why one polarity would be more efficient than another. Both polarities would be the same as far as efficiency, i.e. power in vs power out.

Even though the effect of thermionic emissions is small you still have to wire the coil in one direction or the other some way so you may as well wire it so the terminal listed as positive is connected in some way to the positive side. Coils with terminals labeled dist and ground or grd are typically for neg ground systems and therefor the dist terminal would be positive and the grd terminal would be negative.