Understanding and Testing Ignition System Condensers
Condensers are somewhat of a mystery, but a necessary component of the breaker point ignition system. The goal is to understand why this component is needed, how it effects the ignition system and how to test it.
Condensers (also known as Capacitors) are electrical storage devices. In the breaker point ignition system, 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 micro-farads. A micro-farad 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 micro-farads. The variation in condenser capacity can vary widely, (most condensers fall somewhere around .25 micro-farad).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 particular 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 micro-farads of capacity and is typical.
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.
Checking the Points for Metal Transfer:
This is the place to look for metal transfer on the points.
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 Ohmmeter should read infinite or open.
Hope this helps