How to test a ignition condenser

Paul Dobbin · December 31, 2023

Years ago I had a friend who would test old condensers with a lawn mower engine.

I didn't pay close enough attention to duplicate that test today. My 34 Fords take a

condenser that fits into the coil housing to form the top of the distributor.

Does anyone on this forum know how to test one???

Edited January 1 by Paul Dobbin (see edit history)

nat · December 31, 2023

With the condenser isolated and using an analog ohm meter, connect the leads to the condenser. Reverse the leads and do it again. The needle should quickly bounce to a high reading and return at a slower speed.

Usually, a condenser is either good or bad. The sensitivity setting of the meter will affect the degree of bounce but my experience has been that any bounce is good.

This test and use in cold weather is one of several reasons I consider an analog meter superior/more useful than a digital one.

Edited December 31, 2023 by nat
. (see edit history)

dodge28 · December 31, 2023

I used a test light . Ground the body to any battery post and feed the wire with the test light. It gives a fair indication . It may be on the brink so instead of a test light use a heavy (10) guage wire. A condenser may be good but it is important to know whether uf (farad) is adequate for the job. For that you need Data.

playswithbrass · December 31, 2023

Or use the correct tool

Paul Dobbin · January 1

1 hour ago, playswithbrass said:

Or use the correct tool

Figures there would be an antique meter for this. Never seen one, Thanks

Larry Schramm · January 1

Here is what I use to check capacitors/ condensers. And yes, I put this tester together from the kit back in the day.

zepher · January 1

56 minutes ago, Larry Schramm said:

Here is what I use to check capacitors/ condensers. And yes, I put this tester together from the kit back in the day.

That is a very nice tester.

If you don't have a spare magic eye tube for the indicator, you may want to find one now.

They are getting very hard to find.

Farmallregular · January 1

An analog ohmmeter can tell you if the condenser is open or shorted, but can't directly measure capacitance. As a quick check, you can compare the meter response with the condenser in question to the meter response with a known good condenser. I've seen people claim to be able to do a similar test with a digital multi-meter, but that always looked suspicious to me, although a digital meter will find a short.

Different meters may require different range settings. Start with something in the 10K range. Try the measurements a couple times with a known good condenser to get a feel for how the meter responds.

Ohmmeters measure resistance by applying a known voltage and measuring the current. When you first connect the ohmmeter to an uncharged condenser, it shows relatively low resistance as the current from the meter starts to charge the condenser, and current decreases, i.e., shows higher resistance, as the condenser charges. Swapping the leads charges the condenser in the opposite direction. Once the condenser is charged, it should show high (10's of thousands) ohms resistance

If there's no "bounce" in either direction, and the meter shows a high value of resistance, the condenser is open. If the meter shows a low resistance value after the "bounce", or a low resistance with no bounce, the condenser is either shorted or leaky.

Some higher end digital multimeters have a capacitance measurement function, if you happen to know someone who has one. You can also buy relatively inexpensive digital capacitance meters, although I have no experience with them. Ignition condensers are typically around 0.22 microFarads (uF).

Keith

edinmass · January 1

I guess I should pull out my Herbrand 660 coil and condenser tester and do a video.

alsancle · January 1

I apologize for slightly hijacking this thread but it seems the perfect place to ask this. The typical auto ignition points condenser values are in the 0.22 to 0.3uF range. Can someone explain the tradeoff between the lower rating and the higher rating? My simplistic understanding was that you needed a lower rating for generating more sparks (higher RPM) and the higher rating for a stronger spark. Completely wrong?

alsancle · January 1

8 hours ago, edinmass said:

I guess I should pull out my Herbrand 660 coil and condenser tester and do a video.

This one is pretty good.

dodge28 · January 1

Let us start with the basics. The condenser is a device to store current and to release it when desired. With the points contacts opening and closing has a ready supply of current to to the spark plug, Too small a condenser or too large a condenser can be a problem. Look at the pattern of wear on the contact points itself will indicate that . The build up of material from one side to other is an indication, either too much or tooo little current. There comes a time when the dielectric between the foil breaks down and causes the foil to touch each other, rendering the unit useless. At best condensers or capacitors are difficult to test 100 %. New condensers are rated at plus or minus 10 0r 15 percent. Electrolytic caps are usually used to smooth out AC ripples to DC. The device for checking condensers are like a battery load tester.

oldford · January 1

Here is the one I use. Small enough to carry in your pocket and only $26 on Amazon...

Frank

JACK M · January 1

I worked at a dealership in my youth where one of the tune-up guys would load up used condensers and put them somewhere where it might get picked up.

I only picked up one condenser when I worked there.

edinmass · January 1

35 minutes ago, JACK M said:

I worked at a dealership in my youth where one of the tune-up guys would load up used condensers and put them somewhere where it might get picked up.

I only picked up one condenser when I worked there.

That would have been a capacitor…….usually for an electric motor. We did it all the time at our motorcycle shop. Put a 3hp capacitor out of a walk in beer cooler all charged up on the counter with a sign that said danger, do not touch. The screams were a riot. Fantastic entertainment for the boys in the shop. Then again, the acetylene bombs are much more fun!

JACK M · January 1

I stand corrected.

Frank DuVal · January 2

8 hours ago, edinmass said:

That would have been a capacitor

AKA, condenser! 😮 🛠️ They are the SAME part! Just different terminology. Why two different names for the same part? I do not know the historical reference.

You can charge up a .22 uF condenser/capacitor and get a reaction out of people. They did it all the time in the radio/tv shops.... I was wise, so never got me!

Now that refrigerant to air heat exchanger in front of the radiator, that is a condenser, never a capacitor.😉

❄️☃️🎄🍾🍾🐖🍯

zepher · January 2

Back in HS, in electronics class we would charge up huge capacitors with wire lugs on top and call someone's name so they turned around.

We then tossed the capacitor to them and your first reaction is to catch it.

Always good for a laugh but people catch on after a few rounds.

But the new guys in class always got initiated.

We even built a massive Tesla coil in class.

It was a ton of fun in that class.

Learned everything from basic electronics theory to troubleshooting to radio to microwave transmission.

We even did a little bit of wave guide study over the fours years I was in the class.

edinmass · January 2

12 hours ago, JACK M said:

I stand corrected.

Myself also.

nat · January 2

19 hours ago, oldford said:

Here is the one I use. Small enough to carry in your pocket and only $26 on Amazon...

I see it has lots of uf ranges. How is the test performed?

Lee H · January 2

On 1/1/2024 at 8:59 AM, alsancle said:

I apologize for slightly hijacking this thread but it seems the perfect place to ask this. The typical auto ignition points condenser values are in the 0.22 to 0.3uF range. Can someone explain the tradeoff between the lower rating and the higher rating? My simplistic understanding was that you needed a lower rating for generating more sparks (higher RPM) and the higher rating for a stronger spark. Completely wrong?

If the condenser is exactly the right size for the coil resistance you are running, you will get equal wear on both sides of the point contacts. That rarely happens, and over a long time, metal will transfer from one contact to the other. That is controlled by the capacity of the condenser. Nothing else magic about it.

Ben Bruce aka First Born · January 2

AND what happens if no capacitor [ condenser ] is in the ignition circuit?

Ben

oldford · January 2

2 minutes ago, Ben Bruce aka First Born said:

AND what happens if no capacitor [ condenser ] is in the ignition circuit?

Ben

Your points will burn prematurely and will need replacement

oldford · January 2

2 hours ago, nat said:

I see it has lots of uf ranges. How is the test performed?

Simply select a range that corresponds to the capacitor being tested and connect the leads. If polarity is critical, red to + black to -. Read the value in uf or pf whatever is appropriate. It's like using an ohm meter. Directions are in the box...

That is, microfarad or picofarad, etc

Frank

Edited January 2 by oldford (see edit history)

Ben Bruce aka First Born · January 2

30 minutes ago, oldford said:

Your points will burn prematurely and will need replacement

OK. Prematurely meaning a few minutes or miles? Or how long?

Ben

oldford · January 2

My guess would be that depends on how many times the points open or close. There is only an arc when the points open, and it's the arc that causes metal migration. Why not just replace the condenser when it needs to be replaced?

Frank

Ben Bruce aka First Born · January 2

OK. I am just trying to clarify something. On this forum as well as others, the prevailing "wisdom" is if there is no spark, change the condenser. Looks like if the condenser does not prevent the points firing, only prevents early point failure, changing the condenser will not help. Am I wrong?

Ben

dodge28 · January 2

Let us start with the basics. The condenser is a device to store current and to release it when desired. With the points contacts opening and closing has a ready supply of current to to the spark plug, Too small a condenser or too large a condenser can be a problem. Look at the pattern of wear on the contact points itself will indicate that . The build up of material from one side to other is an indication, either too much or tooo little current. There comes a time when the dielectric between the foil breaks down and causes the foil to touch each other, rendering the unit useless. At best condensers or capacitors are difficult to test 100 %. New condensers are rated at plus or minus 10 0r 15 percent. Electrolytic caps are usually used to smooth out AC ripples to DC. The device for checking condensers are like a battery load tester.

Frank DuVal · January 2

If the condenser shorts, then the points electrically never open, so no spark, since the condenser is in parallel with the points.

If the condenser opens then the spark is weak, as the tuned circuit of the coil/capacitor (L/C circuit in engineering speak) is out of tune, you might think of it as turning your radio knob off station, radio is working, but not delivering the expected result. Way simple explanation, but lets run with it. Same with change of capacitance, out of tune.

dodge28 · January 2

There are dozens of types of capacitors and condensers for different applications. Comes in different shapes and sizes . Some are ceramics. Condenser and capacitor means the same. One is an English term and one is American. Valves (English) and tubes (American)

The type of capacitor that can be charged and shocks if touch the leads are called Electrolytic. It is used for smoothing AC ripples to make the end result DC in Radio Applications. Some radios work on both AC and DC. Some on DC only and some on AC only.

Ronnie · January 2

3 minutes ago, Ben Bruce aka First Born said:

Looks like if the condenser does not prevent the points firing, only prevents early point failure, changing the condenser will not help.

Although the condenser won't keep the points from triggering the coil to fire, the points arcing will effect how the engine runs.

I recall installing new points and condenser in my brother-in-law's Ford. When I started the engine it ran terrible and sometimes backfired. It took me a long time to figure out that I had connected the lead from the condenser to the points on the wrong side of an insulator washer so it wasn't making contact with the points. Basically it was doing nothing. I guess the points arcing was causing the coil to fire erratically. When I attached the condenser lead to the points correctly the engine ran smoothly.

My point is the condenser does more than just keep the points contacts from pitting.

dodge28 · January 2

Frank, those are tuning capacitors, usually inserted in a can, part of tuning radio with a tuning generator to Kilo cycles or Megacycles on (radio waves) the receiver is tuned. Those capacitors are usually a coil of fine wire on a tube with a threaded movable slug. By moving the slug one way or the other the desired wave is achieved.

nat · January 2

26 minutes ago, Ben Bruce aka First Born said:

Looks like if the condenser does not prevent the points firing, only prevents early point failure, changing the condenser will not help. Am I wrong?

You could write what I know on your thumbnail with room left over but I do know that no/bad condenser = pitiful spark at best. Hovever wrong it is, I've used small engine condensers on auto engines and vise versa to get the #&*@! running or verify the source of trouble.

I didn't know how too much or too little capacitance effected the contact surfaces. This thread has some good information in it.

21 minutes ago, dodge28 said:

One is an English term and one is American.

I was told that "condenser" indicated a DC circuit and "capacitor" indicated an AC circuit.

Lee H · January 2

2 hours ago, Ben Bruce aka First Born said:

OK. Prematurely meaning a few minutes or miles? Or how long?

I haven’t tried it, but probably more like minutes than miles. The condenser isn’t there to really control a 6 or 12 volt spark; those would be pretty insignificant. But when a coil is charged, and then the magnetic field collapses as the points open, you might get say, 25,000 volts to the plugs from the coil secondary. However, that collapsing magnetic field also induces a higher “back-EMF” on the primary side of approximately 250 volts (depending on the coil turns ratio…in this example 100:1), and that’s what wants to spark across the points if the condenser is missing.

This is also why a true high voltage tester is needed to fully test an automotive condenser. Your run of the mill ohmmeter doesn’t apply a high enough voltage to detect a condenser that might be only breaking down in actual operation.

Frank DuVal · January 2

43 minutes ago, dodge28 said:

The type of capacitor that can be charged and shocks if touch the leads are called Electrolytic.

Even non-polarized (i.e. not electrolytics) capacitors can be charged to give a shock. Electrolytic just is a description of what the chemicals are inside, they are still layers of foil with insulators in between. A .22 uF charged to 400 to 600 VDC will give quite a shock! Just use one of the old Sprague or Heathkit Capacitor checkers. Google TO-5, IT-22 or C-3. SImilar to one posted earlier.

Today with modern insulators they make large value capacitors which are not electrolytic in small physical size that 50 years ago could not be made except using the electrolytic method or gargantuan sizes.

Frank DuVal · January 2

40 minutes ago, dodge28 said:

Frank, those are tuning capacitors, usually inserted in a can, part of tuning radio with a tuning generator to Kilo cycles or Megacycles on (radio waves) the receiver is tuned. Those capacitors are usually a coil of fine wire on a tube with a threaded movable slug. By moving the slug one way or the other the desired wave is achieved.

ANY coil and capacitor forms an L/C circuit, whether the parts are in series or parallel. This is a tuned circuit. Not a variable tuned circuit, just tuned. This one is tuned for best high voltage output of the coil.

$f_{0}={\frac {\omega _{0}}{2\pi }}={\frac {1}{2\pi {\sqrt {LC}}}}.$ f is frequency in Hertz, (little 0 indicating fundamental frequency), L is inductance in Henries and C is capacitance in Farads.

BTW, a coil of wire wrapped around a form with an iron slug inside is a coil, aka inductor, not a capacitor. If the iron slug moves, it is a variable inductor. Commonly used in auto radios, aka permeability tuned, more stable under rough riding conditions. A capacitor is "plates" separated by an insulator. The common tuning capacitor being the aluminum plates separated by air, half of which rotate through the other plates, varying the capacitance.

Small capacitors are a roll of very thin aluminum foil wrapped up with an insulator, typically plastic (Mylar [Polyester] being very common now), in the old days it was paper and wax as the dielectric (insulator).

The paper gets wet from humidity in the air, causing electrons to go through the insulator, aka "leaks" through, becoming a resistor in parallel with the capacitor, upsetting the circuit. This is a common failure of automotive condensers.

Now that you all are asleep from reading about electronic theory in an automotive forum....🤣

Bloo · January 2

11 hours ago, Ben Bruce aka First Born said:

OK. I am just trying to clarify something. On this forum as well as others, the prevailing "wisdom" is if there is no spark, change the condenser. Looks like if the condenser does not prevent the points firing, only prevents early point failure, changing the condenser will not help. Am I wrong?

Ben

Here is a grossly oversimplified version.

The coil is an autotransformer, meaning it only has one winding, well sort of. The windings that take battery power are bigger wire, but then the winding just continues in smaller wire, a LOT of smaller wire and a lot of turns.That is why there is continuity between all 3 coil terminals. There is no DC isolation between a "primary" winding and a "secondary" winding like there would be in an normal transformer in a power supply for your stereo or computer etc. The end of all the smaller turns is connected to the coil wire.

Important: There is no ground terminal on the autotransformer (coil), nor is it grounded to the housing.

When the points close, the large wire part of the winding is powered by the battery. The iron core becomes an electromagnet. That electromagnet charges with magnetism, and hopefully it charges to complete saturation. It will if there is time. If the engine is not turning really fast, there is time.

When the points open, the magnetic field in the iron core collapses, and it collapses suddenly, much faster than it can be charged with a car battery. This generates a voltage in the winding, the entire winding, both parts. This voltage is in the reverse direction, in other words positive and negative are reversed.

If this were any ordinary transformer, you could know what voltage to expect by comparing how many turns of wire in the large wire (battery powered) part of the coil to how many turns are in the whole coil. In this case though, when the points open the magnetic field collapses faster than you can charge it. You get a LOT more voltage than expected across the whole coil.

OOPS. There is no ground.

More properly stated, there is no path back to complete the circuit. The electrical current must find it's way back to the other end of the coil winding to discharge this high voltage. But, we have opened the points, and that other end of the winding is not connected to anything now. It was connected to the block some microseconds ago, where the spark plugs are grounded, but we opened the points so now it isn't.

The voltage rises to absurd levels in the coil. On one end of the winding, it must initiate a spark across the rotor gap and the spark plug gap. On the other end, the only path back is through the positive coil terminal, the ballast resistor, and the battery to get back to the engine block. This is a better path than you might think, as a battery has fairly low internal resistance, but the rise time of the coil voltage is extremely fast, and this path back through the battery is *extremely slow*. In no way can it get the job done, so the coil tries, and probably loses some energy there, but the voltage continues to rise extremely fast, and an unwanted spark initiates across the points to complete the circuit.

Some people will tell you a capacitor (or condenser if you wish) passes AC current. That is not quite true, but in practice it APPEARS to be true. It can fake "passing" a pulse by absorbing a bunch of electrons. If we put a capacitor across the points, it completes the circuit, or at least appears to. It can't keep up the act for long, but it doesn't need to. It needs to react fast, and that it can do by collecting a bunch of electrons. Since it reacts fast, the coil does not to need to waste energy trying to get back through the battery, nor does it need to initiate an unwanted spark across the points. This leaves the capacitor charged, but when the points close, they short out the capacitor and discharge it. The capacitor is then ready to do it's job again the next time the points open.

If the capacitor is not present, or open circuit, the spark will be extremely weak. More often than not the engine will appear to "have spark" but not run at all. The spark will be weak if it is present. The engine might actually run, but badly.

Edited January 3 by Bloo (see edit history)

Frank DuVal · January 2

1 hour ago, nat said:

I was told that "condenser" indicated a DC circuit and "capacitor" indicated an AC circuit.

Interesting, but no. We call those plates that rotate in a radio "tuning condenser". They tune AC aka RF signals. Also hear automotive people use condenser (and that is an AC circuit, as it is in the primary of a transformer [the coil]) and electronics people use capacitor, then there is that tuning condenser term again! OK, let's just call them the original term, Leyden Jar. 🤣

Bloo · January 2

Capacitor and condenser are the same thing. Condenser is just an older word, like Mechanician.

In the radio and electronics world, "condenser" was pretty much gone by 1934 or so, except for the one with movable plates used in radios, it was sometimes called a "tuning condenser" instead of a "tuning capacitor" as late as the mid 50s. In the electronics world it is well understood that the two are the same.

Capacitors, or Condensers if you wish, come in many forms and with many different characteristics. Their value is called "capacitance" (technically not "capacity" though you hear that a lot), and is measured in Farads. Usually not whole Farads, because Farads are so big. Micorfarads (uF) and other small portions are more common.

MCHinson · January 2

2 hours ago, Ben Bruce aka First Born said:

OK. I am just trying to clarify something. On this forum as well as others, the prevailing "wisdom" is if there is no spark, change the condenser. Looks like if the condenser does not prevent the points firing, only prevents early point failure, changing the condenser will not help. Am I wrong?

Ben

I will let Bloo or someone else with more technical training explain why, but...

No, You are wrong. The condenser is the cheapest, easiest to change and most likely cause for a failure in the ignition circuit. Personal experience tells me that if it quits firing, try changing the condenser and you have a good chance of having fixed the problem. I once did a tune up on a 1937 Buick and it would not run. I removed the "new out of the package" imported condenser and reinstalled the old condensor and the car would run again.

How to test a ignition condenser

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