Ignition coil?

[MikeMl]

Older-style Kettering automotive point/condenser ignition coil:
Primary resistance?
Primary inductance?
Secondary resistance?
Turns Ratio?


I have in-hand some ignition coils from an older Natural-Gas powered 6 cyl industrial engine. It used one coil per spark plug. The spark source is a low-voltage Magneto, with a 1:6 low-voltage distributor that drives the primary side of the six coils. The coils have a primary resistance of ~5.7Ω, a secondary resistance of ~10K, and a turns ratio of 1:100. These are obsolete, and are no longer made. Cant find any NOS replacements, so am asking about automotive coils on the chance that they may work for the magneto output?

 

[b.james]

if I recall the output voltage was around 4000 volts so a turns ratio of about 4000/12 =333 to 1 would seem logical to me but I am often wrong mike as you know! Low voltage resistance was in the 1 to 4 ohm range high voltage in the thousands of ohms I think . If no one else posts I'll try to look it up in my old books .

A bit of a search-"How to test automotive coil "

Generally speaking, most automotive coils will have a resistance reading of about .7 - 1.7 ohms for the primary winding and 7,500 - 10,500 ohms for the secondary winding

Another

Ignition Coil Resistance Check

In addition to the test above, you may elect to perform an ignition coil resistance check as confirmation of the coil's condition.

1. Check the ignition coil primary coil resistance by connecting an ohmmeter between the positive (Black wire) and negative (Green wire) terminals on the coil. The resistance should be 0.4 to 0.6 ohms.
2. Check the ignition coil secondary coil resistance by connecting an ohmmeter between the coil output terminal and the ignition coil negative terminal. The resistance should be 5000 to 7200 ohms.

So depends on whose coil it is

As I learnt it the closed points allowed the 12 Volt system to charge the condenser (cap) ,when they broke open the capacitor let out a spark to the primary coil which caused a flow in the multicoil winding to the sparkplug. The gaps affected exactly when that happened . Funny how some explanations have it about turn

 

[rfranzk]

Hello All,
I assume you have a no spark condition and the engine won't run?? If that is the case I wouldn't suspect 6 coils to have failed. Does this industrial engine have battery for starting?? I am not real familiar with magnetos is it essentially a generator providing low voltage for the ignition primary? If a battery is available then an automotive ignition coil could be made to work. Could you post photos of the setup? Does the distributor have one set of points and condenser that controls the primary of all six coils? Do the coils all fire at the same time or are they sequential with firing order?

On a battery operated ignition coil when the points are closed the primary circuit of the coil is energized creating a magnetic field and when the points open the field collapses and a high voltage is induced into the secondary of the coil which follows the path of least resistance hopefully a spark plug. The condenser is used to absorb some inductive kick voltage when points are opened to help prevent arcing and burning of the points.

Thanks.
Rfranzk.

 

[MikeMl]

Engine runs, only one coil is breaking down and causing an intermittent miss in just that one cylinder when the engine heats up. The magneto has a built-in commutator, sort of like one in a car, but the rotating arm is distributing a ~300V primary pulse created by a coil, capacitor and points inside the magneto instead of a 30KV one...

 

[rfranzk]

Okay Understand, I would guess if you could find something automotive or industrial with close to the same primary resistance you would be alright. I don't think the turns ratio would be critical as the secondary voltage would be limited by spark plug gap. I don't know if a lower primary resistance would cause primary driver to heat or cause life span issues with the magneto. I was thinking something industrial or farm with a 24 volt system might have ignition coils with slightly higher primary resistance. Your original one is probably shorting internally when it gets warm.

 

[rfranzk]

Are the ignition coils the typical cylinder shape? Might look at this. https://www.denniskirk.com/2001761....m_medium=cse&gclid=CLu13vS_xcUCFZWCaQoddbYAuw

 

[MikeMl]

Good lead on that motorcycle coil.

Here is an update:

The engine in question is used to drive a pump for irrigation. The rough running (missing) happened on startup this spring after it had been sitting all winter. It turns out that the bad coil diagnosis was incorrect. I got the engine running smoothly by redoing the diagnosis, and finding the real culprit...

The problem manifest as a miss under load. The miss was intermittent to the ear (i.e. you can hear it). This was confirmed by using an inductive timing light as a spark indicator. There are six hv coils mounted on the top of the engine. It is easy to move the timing light inductive pick-up from one spark wire to the next. Initially, only cylinder #5 exhibited the occasional missing spark, which is where idea that the coil was breaking down internally came from. As I said, these coils are made from unobtainium, so getting a replacement is problematic.

After installing new plugs with no improvement, I repeated the timing light trick test, and this time I found that two of the cylinders (#1 and #5) were occasionally misfiring. I then swapped the coils on cyl#5 &6, thinking that the problem would follow a bad coil, but the misfire remained on cylinder #5, meaning that it was not the coil. The occasional misfire on cyl#1 also made me realize that two coil failures were very unlikely, so to go look at the only other thing where the problem could be: the magneto.

I took it apart, and sure enough, the magneto guts looked a lot like the innards of a normal Kettering automotive distributor. It has the rotor on the rotating shaft with the six cam lobes, the points that are opened by the high points on the cam, the "metal clad condensor" (capacitor), and different from an automotive distributor, the primary coil is inside the magneto.

Having owned several Volkswagen beetles as a youth, the problem was instantly obvious. The points were badly pitted, and were barely opening. I really need to replace them, but to get the engine running, I filed down the points to make two smooth, parallel faces, I reinstalled the points, set the gap to 0.025", and put the magneto back together.

The engine now runs smoothly as it did before. In hindsight, it turns out that the two cylinders that were misfiring are cam lobe neighbors, meaning that they are sequential in the firing order. This indicates a slight eccentricity in the cam rotation, so that side of the cam wasn't quite opening the points. Eventually, a third cylinder would have begun misfiring...

Sometimes you have to think about what you are doing when troubleshooting...

 

[rfranzk]

Good find!!!. Yes the basics of troubleshooting. Since the advent of electronic ignition those old school basics seem to slip or mind. Just a note for you. Changing the point gap i.e. dwell will affect the ignition timing as well and I assume that after disassembling the magneto a timing adjustment was required. Another thought is to apply a small amount of (High temp) grease on the cam lobe to lube the rubbing block on the points. This will help reduce wear on the rubbing block and hopefully keep the point gap stable for a longer time. The eccentricity problem is likely due to worn bushings on the shaft allowing a little orbital action.