Ingnition System Condensers and Testing (update)

Sat Jan 31, 2009 8:20 am

Condensers are somewhat of a mystery, but a necessary component of the breaker point ignition. The goal is understanding why this component is needed, how it effects the ignition system and how to test it.

Condensers are electrical storage device. In the breaker point ignition every time the points separate there is the potential for the electrical current to arc (or jump) across the point gap. The condenser limits the arcing or burning of the points. Without condensers the contact points would not last very long.

Condensers give the electrical current an easier path (electricity always takes the easiest path) to follow and eliminates almost all arcing.

Condenser construction is simply sheets of aluminum foil sandwiched between sheets of waxed paper. Attach a wire to the aluminum foil roll it up (waxed paper on the outside) and stuff it inside a can. The condenser is insulated, the tab is for mounting purposes only.

The more or less aluminum foil you use in your condenser will determine how much electricity or charge your condenser will hold. The term for this is capacitance.

The more or less capacitance your condenser has will reflect on the condition of your points or how long the points will last.

A condenser with to much storage or capacitance will cause a build up (metal transfer)on the mounting side of the points. While a condenser with not enough storage or capacitance will cause a build up (metal transfer) on the arm side of the points. Sometimes the right sized condenser will end up in your ignition system and no transfer will occur.

This is why you have to file a used set of points (eliminate the metal transfer) before you can gap them with a feeler gauge. You do not have to file points if you use a dwell meter. The dwell meter takes into account the metal build up, because it looks at the points electrical measurement rather than the mechanical measurement.

The condenser capacity is measured in microfarads. A microfarad is 1/10 of 1/1,000,000 of a farad (.0000001 farad)There are specialized testers to do this measurement. The typical condenser will have between .2 to .3 microfarads. The variation in condenser capacity can vary widely, (most condensers fall somewhere around .25 microfarad).This is why some contact points last longer than others. It is not the points fault but is the condensers fault instead.

Look for the metal transfer if there is none consider not replacing the condenser as it is matched to your particular ignition system. If there is metal transfer replace the condenser as you have nothing to lose in the process.

Condensers do not wear out, but will short out. Back in the day when point ignitions were common the majority of condenser failures were new ones. The saying was if it will last the first 20 miles then it will go the rest. New defective condensers would fail within the first few miles.I have also seen some intermittent shorting out caused by heat but again in my experience that is not very common.

I have used the same condenser through several sets of points on my tractors just because I stumbled on the correct capacity match that ignition system on that peticular vehicle.

Condensers can hold a good deal of high voltage. In my grandfathers shop you did not handle any condensers laying around as they were charged up ( he used a model T coil to charge them)and they would ZAP you. You would hear a SNAP and the person would have his hands i his pockets from then on.

Pictured below is a picture of me testing a condenser, this condenser has about .25 microfarads of capacity and is typical. ... 0_0311.jpg ... 0_0312.jpg

I have had this tester for years and it is old (it has vacuum tubes for diodes) and is very handy for the older electrical equipment testing.

This is the place to look for metal transfer on the points. ... 0_0316.jpg

Use a matchbook cover to hold the points apart. Set DVOM on Ohms and check for electrical integrity should be infinite or open.This set of points is good. Remove the matchbook cover and you should get a reading of NO resistance. Clean the contacts till all resistance is gone,(this is very true of voltage regulator contact points).
Also test condenser in the same way. Put one lead on the wire and the other lead on the condenser case.The Ohmeter should read infinite or open. ... 0_0320.jpg

Hope this helps

Mark LaFollette.
Last edited by cubguy's dad on Sun Feb 01, 2009 5:01 am, edited 1 time in total.

Re: Condenser testing

Sat Jan 31, 2009 9:01 am

Outstanding article. :thumbsup:

Re: Condenser testing

Sat Jan 31, 2009 10:22 am

cubguy's dad wrote:A condenser with to much storage or capacitance will cause a build up (metal transfer)on the mounting side of the points. While a condenser with not enough storage or capacitance will cause a build up (metal transfer) on the arm side of the points.

I didn't go look this up, But doesn't this depend on whether the system is positive or negative ground?

Re: Condenser testing

Sat Jan 31, 2009 12:07 pm


The resulting metal transfer and location would depend on whether it is positive or negative ground.

I just wanted to show that metal transfer was the fault of the condenser and why using the file on the points was mandatory before setting used points with a feeler gauge.

In the day we used to test condensers when I started out. But that changed years ago and replacing the condenser with the points was adapted as standard procedure. The condenser like most things, it is now either a pass or fail component.

Thanks for the help, Jim

Mark LaFollette

Re: Ingnition System Condensers and Testing (update)

Thu Feb 26, 2009 10:09 pm

Good post but has some inaccuracies that may confuse beginners.

A condenser or in the electronics world, a capacitor, is composed of two conductive plates separated by an insulator. In old style capacitors like an automotive condenser the plates are just two roles of metal foil separated by waxed paper or Mylar film. Each of the two foils are the conductors. One wire of a capacitor goes to one foil the other goes to the other foil. In an automotive capacitor, one lead goes out the center, the other lead is attached internally to the metal can or case. The mounting tab is for more than mounting and is not insulated from the capacitor, it is the second terminal. It is used at a low voltage so the mount and mounting screw should be cleaned prior to mounting in the distributor to insure good electrical contact.

A capacitor does reduce slightly the arcing from the points but that is not its main function. If you look at an engine with external points such as a Kohler engine and you will see quite a bit of arcing. The points will last a long time with or without a condenser. So why have a condenser at all? A capacitor in parallel with a coil and a collapsing magnetic field produces an oscillating circuit. This oscillating circuit produces a much stronger spark with both higher voltage and a much longer duration of spark. No capacitor and the effective spark is much less. That is a problem in battery powered systems but is absolutely critical in weaker magneto systems. Spin a magneto both with and without a condenser and the spark may be almost non-existent without the capacitor.

I don't worry about the build up on one side of the points or the other as related to the capacitance. What am I going to do anyway? I don't know the capacitance of the condenser I would currently have nor would NAPA, TM, or anyone else be able to sell me an auto condenser that had any capacitance marking. I would not know what I have and would not know what I was getting.

I have a capacitor bridge that I use for working on my antique radios but no need for fancy equipment to test an automotive capacitor. A capacitor will charge when a voltage is applied to it. The capacitance of an auto condenser is very small, you could put it on your tongue and there would not be enough stored voltage and current to notice. No reason to be afraid to touch. You need to test for 3 conditions, a short where the internal foils are touching, an open circuit where the internal foils are not connected to the external lead or to the can, and a leak where there is a leakage of voltage across the insulating Mylar. Take a cheap analog multimeter, put it in the OHMs mode to measure resistance, touch the leads together and it should show zero ohms. Take the capacitor lead and touch to the case to discharge anything still in the capacitor. Touch one lead of the meter to the capacitor lead, touch the other lead to the can or case. You should notice a kick in the meter needle, the needle should move slightly indicating charging of the capacitor. For the inexpensive meters the kick may be only 1/8 inch movement of the needle but that is enough. The needle should then move to infinity resistance and stay there. If there is no kick then the capacitor is open and should be thrown away, if the needle does not return to infinity then the capacitor is leaking or is shorted and should not be used. Just look for a brief small kick of the needle and a return to infinity. Keep your other hand off the capacitor case or lead otherwise the meter may just measure the resistance of your skin rather than the capacitor. Touching the capacitor lead to its case will discharge the capacitor so you can try again with the meter in case you may have missed the needle kick the first time.

The late Bob Shores wrote a great book on the theory of ignition coils and magnetos. His book is still available from his wife. Just do a search on model engines and Bob Shores and you should hit his website.

Re: Ingnition System Condensers and Testing (update)

Thu Feb 26, 2009 11:26 pm

Heitner: Condenser is connected parallel with the ignition points. When the circuit is broken the flow of induced current passes into the condenser, charging it. Thus momentarily diverts the flow of current from ignition points, allowing them to seperate with a minimum of following arc.
Owning a number of Kohler engines, all have condensers. When the condenser malfunctions the points have a significant yellow arc and the engine runs like crap. Points don't last very long with out a functioning condenser.

Re: Ingnition System Condensers and Testing (update)

Fri Feb 27, 2009 6:11 pm

Landreo wrote:Good post but has some inaccuracies that may confuse beginners.

I am now confused about your post. :roll:

Re: Ingnition System Condensers and Testing (update)

Fri Feb 27, 2009 8:14 pm

I was just trying to explain why you had to first file a used set of ignition points prior to gapping them with a feeler gauge and pitting is a function of condensor size rather than a problem of the contact points.

Condensors are an often overlooked component of a contact point ignition system. The right condensor or the application of the correct size capacitance in the primary ignition circuit makes a huge amount of difference in the overall operation and life of the contact points. You can not buy a condenser by its capacitance rating nor can anyone tell you what rating any particular condenser has. However, there are telltale signs of the under or over sized amount of capacitance in the system (see article below)

I personally have had little success in operating a battery or magneto ignition circuit without a condensor (capacitor).The close contact point gap required to achieve any coil saturation will cause arcing and burning of the contact points that will severely shorten their life along with reducing the coil output to nothing.

Below is an excerpt from my Delco Remy Ignition handbook concerning ignition systems on off highway equipment, most notably tractor ignition systems. I hope this clears up rather than adds any further confusion.

Basically the function of a condenser in a coil ignition circuit is to reduce the spark at the contact points as they open in the distributor and thus minimize burning and pitting of the points. Arcing is caused by the effect of self induction in the coil as the points interrupt the flow of current. The resultant collapse of the magnetic field produces a high voltage to be generated in the primary winding which then tends to flow across the points, thus causing burning or pitting. This current flows into the condenser and charges it as the points open. The rapid collapse of the magnetic field produces this high voltage in the primary windings, which can be as high as 250 volts. This further charges the condenser and the consequent collapse of the field causes a high voltage to be induced in every turn of both primary and secondary windings of the coil. As the secondary winding has about 100 times the number of turns of the primary winding, the voltage can reach as high as 25000 volts. Normally this voltage is not reached as it is limited by various factors such as point gap, compression, engine revolutions, so only enough voltage is produced to make a spark at the spark plug. As the spark is produced at the plug gap the energy in the coil, stored in the form of magnetic flux, begins to drain from the coil through the secondary circuit thus sustaining the spark for a fraction of a second or several degrees of crankshaft revolution. During this interval the condenser discharges back through the primary winding producing an oscillation of the current flow in the primary circuit for the brief interval that is required for the primary circuit to return to a state of equilibrium. The condenser DOES NOT DISCHARGE UNTIL AFTER the spark has occurred at the spark plug.
Burning of the points results from high voltage, presence of oil or other material at the points, defective condenser or too small a gap in the points. High voltage can be caused from high voltage setting of the regulator or high resistance in the charging circuit or the third brush in the generator (voltage) is set too high. On third brush generators too small a gap at the points allows the points to stay closed longer resulting in the average current being high enough to allow the points to burn rapidly.
Contact pitting results from an out of balance condition in the system which causes the transfer of tungsten from one point to the other so that a tip builds up on one point and a pit on the other. The direction in which the tungsten is transferred gives an indication for correcting the situation. If the tungsten transfers from the negative to the positive point, one or two corrections may be made. Increase the capacity of the condenser, shorten the condenser lead, separate high and low tension leads between the coil and distributor or move these leads closer to the engine block. If the transfer is from the positive to negative point, reduce condenser capacity, move low and high leads closer together and/or away from the engine block, or lengthen condenser lead.

I do not routinely change out my condensors if the pitting on the points is minimal, as I have lucked out in having found the right capacitance or the combination for my particular ignition system.

For most people replacing the condenser along with the points is a good practice to follow and could save time and problems down the road.

Mark LaFollette

Re: Ingnition System Condensers and Testing (update)

Mon Mar 02, 2009 9:04 am

cubguy wrote:
Landreo wrote:Good post but has some inaccuracies that may confuse beginners.

I am now confused about your post. :roll:

No need to be confused.

The practical parts:
1. The condenser case is part of the electrical part of the condenser and needs to be treated the same as all other electrical parts, it should be clean and attached to a clean area. Clean the mounting tab and the plate that it touches to to get a good electrical connection.

2. To test a condenser you need to test for 3 conditions, open, short, and capacitance, i.e. the ability to store electricity. An old fashoion multimeter with a needle works, hard to do with the newer digital meters. The test for a short is easy, zero or a low resistance is bad. An open condenser will read infinity and is bad. However, the problem is a charged condensor will also read infinity and that is normal. The way to tell the difference is to look for a charging kick of the needle. An ohm meter will charge a condensor and will initially show decreased resistance as the condensor charges but then the needle should return to infinity once the condensor is charged. The charging movement of the needle is a brief kick of the needle which depends on the meter and size of the condensor but is likely going to be less than 1/8 inch movement but is still noticable. I touch the lead of the condensor to its case, this will discharge any charge still left in the capacitor. I hold one lead of the ohm meter on the capacitor lead and hold the other meter lead over the case but not touching the case. I then watch the meter closely and lower the meter lead to touch the condensor case. At that instant there should be a kick or small movement of the needle. That is the charging kick, no kick equals a bad condensor or bad eyes. You can touch the condensor lead to the case to discharge and repeat the process if you think you missed the kick the first time. You can't assume a condensor is good because it reads infinity resistance, it may be charged and good or it may be truely open inside and bad. You need to look for a charging kick.

3. The condensor is for more than protecting the life of the points, it is critical to making a spark through generation of oscillating currents in the coil. A bad condensor will give little or no spark, especially true in magneto systems.

Some theory:

A capacitor or condensor is just two conductors separated by an insulator, typically two sheets of foil separated by an insulator all rolled into a tube. One of the foils is connected to one lead, the other is connected to the other lead. For engines, one of the leads may be connected to the condensor metel case, some older magnetos actually have two wires and are not connected to the case but that is not the norm.

An ohm meter measures resistance by running a current through a resistance and measuring that current. That current is what charges the condensor and produces the initially kick of the needle. Once the condensor is charged, there is no more space for current storage or flow, and the needle reads infinity resistance.

A simple circuit like a spark circuit actually has some interesting physics behind it. The capacitor causes an oscillation of the voltage in the spark coil. The intensity of the spark depends on several things including the strength of the magnetic field and the time it takes for the magnetic field to collapse. The faster the collapse, the more intense the induced spark. Arcing of the points slows down the collapse so if we reduce the arcing we increase the spark. We can place a 100 ohm resistor in place of the condensor and the arcing is reduced and the spark will increases slightly. This method was used with some vibrating power supplies used in antique car radios. The vibrator was set to produce AC current so there was not a whole lot to be gained by using a capacitor. A point system in an ignition system, by itself, has little oscillation and using a condensor instead of a resistor causes a set of good strong oscillations and increases both the intensity and the duration of the spark until the oscillations die out. The primary voltage can reach 100 volts during the occillations, much greater that the 6 volts it started with. Decreasing the arcing of the points does increase the life of the points but that would also occur with a resistor instead of a condensor. A condensor is used to both decrease the arcing of the points to some extent but it greatly increases the spark quality.

Bob Shore's book has a few chapters devoted to the theory of ignition circuits but there are also several web sites out there that have the scope tracings to look at.

Another thought, the spark goes through the spark plug to the ground, what happens to the other side of the secondary coil and its 15K volts, where does it go, the points are open so it does not go there. It has to go to the ground somehow.

I was lucky to have a father that taught physics and a much older brother that has a PhD in physics so I learned physics and electronics from a young age. I set myself on fire at age 4 using a large battery and a small wire, luckily my father was coming home from work at that time and put the fire out. I tell people I was trying to understand ohm's law but I really was just trying to find out how hot I could get the wire!

Re: Condenser testing

Sat Mar 14, 2009 11:13 am

Jim Becker wrote:
cubguy's dad wrote:A condenser with to much storage or capacitance will cause a build up (metal transfer)on the mounting side of the points. While a condenser with not enough storage or capacitance will cause a build up (metal transfer) on the arm side of the points.

I didn't go look this up, But doesn't this depend on whether the system is positive or negative ground?

For what it's worth, I always remember this by the "minus, minus, minus" rule. If there is "minus" material (pitting) on the "minus" (negative) side of the points, then you have "minus" (not enough) capacitance.