Piston Slap: Of Condensers and Resistors
Mark writes:
I have my very first car in my garage: It’s a 1940 Ford pickup truck that I started driving when I was 14 years old. It was in the early 1980’s, that time before computers, cell phones, and cameras were everywhere. I am sporadically resurrecting my truck, and have a question about the best way to go about setting up the ignition system.
It has the points and condenser that the 1949–53 flatheads were designed with. I have upfitted the 6-volt system to 12-volt, and installed a 12-volt coil that needs no ballast resistor. I know that this new coil is internally connected to a resistor, my question is what will that do to the points?
Will it be a situation where they are getting consumed more rapidly than normal, or will they operate correctly?
I don’t wish to fit a Pertronix system since what I have works fine. I’m looking long-term, when the truck is done, I don’t want drivability and reliability issues to hamper my roaming as it did so many years ago.
Sajeev answers:
I am far from an expert on ignition systems of this vintage, but this fantastic question deserves my best effort to point the conversation in the right direction. So let’s proceed with that in mind.
To answer your first question, the coil’s ballast resistor isn’t the only part of the ignition system that we need to discuss. There’s a condenser inside the distributor, and whatever juice the coil shoots to the distributor is touched by a condenser before it gets to the points. The ballast resistor and condenser operate on different principles. If the condenser is not the right spec and/or fails over time, damage to the points can happen.
My answer to your second question is both optimistic and ignorant of how this could all go sideways. If the condenser is both in good shape and the correct spec for your points, it should do its job and keep wear to the points at a normal level. But let me ask Kyle Smith, my esteemed co-worker here at Hagerty Media for a second opinion:
Kyle writes: I think you’ve got it. From my understanding, the condenser is just the shock absorber for the current flowing through the ignition system and exists to protect the points from arcing. The resistor in the coil is just to keep the coil running steady. It makes sure the coil is not seeing peaks and valleys of current coming in, which would shorten its life and make for potentially erratic spark.
And now let me make one final point: Keeping a spare set of points and a condenser (or perhaps multiple sets of spares?) is a good idea. This is important because replacement points and condensers made in the last decade have been pretty junky. It’s something we’ve discussed previously on Piston Slap, and I haven’t heard anything to change that assessment. The parts aren’t that expensive, nor do they take up much space in the cab of a 1940 Ford truck. What say you, Hagerty Community?
There is never anything wrong with keeping spares of a points-equipped ignition system along with you. As Sajeev mentions, poorer quality of parts is just a reality we have to live with nowadays, and points systems were never 100% reliable even in the best of times (due to many variables).
I agree with what the Hagerty brain-trust has suggested: your combination should not be a problem in and of itself (i.e. – internally-resisted coil and condenser combo should not degrade points any faster that systems with external ballast resistors). But points themselves may degrade faster than you want, simply because they are made of inferior materials compared to “in the good old days”. Bottom line to me is that points are a wear item, and should be maintained and checked regularly, and at the first sign of potential failure, swapped out. As Sajeev says, they’re not that expensive.
Last set of points I purchased was for an early Ford 8N with the front mount distributor. New points looked great, but had no spark at all after I installed them. Had to take the distributor back off and put back on several times trying to find the problem. Finally I used a points file on the “NEW POINTS” and it fired right up. Quality is very bad on that type of things that are available now.
I would suggest having a points file in the travel tool kit for anything that still has points.
Well yeah, that or a matchbook (I’m really old school). 😉
A matchbook is used to set point gap, it is paper and will not clean any crud or divots from the points to get them firing, that’s what the file does..
The striker strip on the matchbook will clean the contacts in a pinch!
The striker portion of the matchbook is abrasive. It will clean a set of points as a temporary fix.
Or a dime
Ah, a points file! Great suggestion and thank you for sharing it!
I’m a points guy to my very core. Their greatest advantages are simplicity, and that they fade away instead of stranding you suddenly without warning. I can’t speak to the alleged poor quality of newer parts since I haven’t bought any in decades, I just file and regap them every few years. They’re easy to diagnose with any meter or test light, and no component will be damaged if you apply power to the wrong contact or with the wrong polarity. And they will be the only vehicles running after an EMP!
The resistor limits the surge current through the points. In some cars, a ballast resistor further limits current, and is momentarily bypassed when the key is moved to the start position. Many coils were initially designed for 6V and measured 1.5 ohms. Instead of changing the coil design when they went to 12V, many autos simply went to a ballast resister or resitance wire of 1.5 ohms to limit the current.
The easiest way to “fry” the points is to leave the ignition switch on with the engine not running. If the points happen to be closed, continous current will heat them excessively. IMO this is the most common reason for points failure.
The word “condenser” belongs in Sajeev’s article about unusual automotive terms. It’s a capacitor.
This is a situation where I would run the 6V points and condenser and see what happens. It’s not like we commute 60 miles a day in our toys
If it doesn’t pan out (you’ll know right away with the condenser), then start looking into mod options.
Thank you for the advice and knowledge, the truck always fires up and runs well, I don’t want my lack of familiarity to incur problems down the road. It’s a situation of not fully understanding a system that pre dates me that always works until it doesn’t.
EVERY system works until it doesn’t. It’s the nature of the beasts we call machines! 😉
I agree with most of the advice you have received, might pick a nit here or there, but in general the Kettering ignition system is about the most reliable system ever made with only a lack of preventative maintenance tarnishing its reputation. Regarding your concerns about lack of family can be cured quite easily. I strongly recommend you find a decent copy of an excellent textbook by Martin W. Stockel titled “Automotive Service and Repair.” Look for an edition from the 1960s which will provide an excellent foundation of knowledge on how automotive systems work and how to repair and maintain them. I recommend looking on a website called Alibris which will provide you with specific information on editions and dates. My oldest copy is dated 1968? As for the quality of replacement parts, sadly most new items do suffer this problem. When working on older machines, I recommend searching out NOS items from the big E, Hemmings, or club resources. Earlier factory parts for Fords were branded either Ford Motor Company, of a bit later Autolite. Newer parts branded Motorcraft can come from poor suppliers in the far east. Store branded parts are, in my opinion, the last resort choice. Their suppliers seem to have no understanding of quality control, and the sellers appear to leave their quality assurance up to the customer.
Always have a spare condenser, the points can be filed if needed. I once chased a bad condenser around and around. The car would shut down for a few cycles, then fire up and backfire through the exhaust system. Many tries and parts later we replaced the point and condenser and the problem went away. All for a intermittent condenser. The new parts are not as good as the old parts of yesteryear were.
I never carried spare points and or a back-up condenser. The one time my condenser failed found me within easy walking distance of an auto parts store and in a position to push my car to a safe place for diagnosis and repair. But…I’m not 18 anymore and no longer live life like Alfred E Neuman (Google it). Carry the spare points and condenser with you, and whatever tools you’ll need to change them out. Just make sure you’re in a safe place to do the work-a tow is well worth the money if you or your car are at risk.
I’d been wrenching on cars for 54 years, and never realized that the distributor-mounted condenser was vital to the operation of a conventional coil/points/condenser ignition system. Until I found out the hard way, when my car quit on me for seemingly inexplicable reasons.
I knew the condenser served to “buffer” the current coming from the coil to extend point life, but didn’t know that a dead condenser would prevent any current from reaching the points. No juice there = no spark to the plugs. Guess I was lucky in that in those previous years, I’d never had a complete condenser failure.
Pertronix solved all of my worries on points. It will also bypass the problem of shaft wobble if that exist. No one will know unless you tell them.
I agree. I’ve had a Pertronix on my straight 8 ’51 Buick for probably 25 years now without a problem. I did have to regap it maybe 10 years ago, but I think that was a loose hold down screw problem vs. a system problem. Had 1 on my ’32 hiboy with a ’68 Buick 430 until the stock distributor wore out. I also have 1 on my ’56 Tbird with a 312 in it. Most of their systems cost less than $150 & require NO spare parts to be carried
Somebody already explained what the ball resistor does-limits voltage, therefore current through the coil when the engines running versus cranking the engine at lower voltage. GM vehicles have an internal resistor wire to accomplish the same purpose. The condenser is just a capacitor- when the points open the electric field primary ignition coil collapses >transformer effect which gives the high voltage surge(secondary wiring in the coil). The condenser creates a gradual collapse versus an abrupt collapse of the field, creating a longer duration spark in a more powerful spark. (Square wave versus sinewave). Condensers have several fail modes; Including intermittent failure. Bottom line always have a spare condenser! The little scratcher thing on a book of matches makes a great points file. points closed draw it through flip it over and draw it through again filing both points. you can run an MSD box with or without the condenser the points become a very low current switch and last forever-pertronics systems are really good just never leave the ignition on-It will fry very quickly. I do have one car left with a stock point system. It’s a 68 Firebird and I do have the pertronic module ready to go. I guess that makes this conversation pointless ha ha
Sanjeev and Kyle you are correct about the role of the condenser which is to mitigate the electrical arc across the points as they open, which causes the electromagnetic field in the ignition coil to collapse which fires the spark plug.
The ballast resistor’s purpose is to limit the current through the points and coil for longevity of both. Doing so, the coil runs at about 6 to 7 volts, and the balance of the supply voltage is across the ballast resistor, hence the term “ballast”.
So what was the point(s) of this article? Hopefully this article will receive no resistance! :^)
I’ll be here all week.
Quick, somebody get the Big Hook!!!
The condenser/capacitor serves 2 purposes. First, it reduces the spark generated when the points open which at least slows down the pitting of the points contact surface. Second, it forms a resonant circuit with the inductance of the coils primary winding which increases the secondary output voltage ie a bigger spark. The condenser needs to be fairly close to the specified value of capacitance which is why there are different values depending on the coil.
PS I also meant to mention that if the ballast resistor fails, the car will crank and start normally because the ballast is bypassed, but as soon as you let go of the key the engine will stop. My dad showed me this a LONG time ago, and 2 or 3 times I’ve looked like the smartest guy in the room by instantly diagnosing a failed ballast from these symptoms. Yet another debt I owe my Dad!