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?
My first car a 65 Rambler had a coil with an internal resistor. I found a coil on sale and put it on the car thinking one more thing that won’t fail on me. From that point forward my points were burning up constantly until a mechanic looked at car and noticed that it did not have a coil with internal resistor. After installing a ballast resistor no more burnt points.
That precisely what I do- carry a fresh point set and condenser.
If you can buy NOS or American made (NAPA). I have had early fails with Imported parts
The resistor is to limit the current flowing through the points. Not all systems use a resistor. Some coils have a primary that is high resistance by itself. External ballast resistors serve a second function.when the engine is cranked, particularly when cold, the battery voltage drops, making the coil output drop significantly causing hard starting. By designing the coil to operate BELOW the cold cranking voltage so it produces a HOTTER spark when cranking, then switching the ballast resistor into the circuit to drop the running voltage/current to the coil design voltage/current.
The condenser is used to “snuff” the current when the points open.
The resistor is used to limit the current through the primary curcuit.
OUCH! I don’t enjoy correcting people, but Mr. Smith is quite wrong, as I’m sure others have mentioned. The Ballast Resistor is there to limit the current that flows through the coil. It’s normally bypassed when the ignition is turned to “Start” so the coil gets all available voltage to give a stronger spark when cranking. The extra current the coil will draw when the Ballast is bypassed would normally get the coil HOT, but cranking is (hopefully!) such a short duration event, that the coil can absorb the extra heat. Since the coil is being fed by a D.C. source, there really won’t be any “Peaks and Valleys” to worry about, unless he meant the “valley” that’s caused by the lower voltage normally encountered during cranking.
I’d be very leery of a “No Ballast Required” coil, as if the ballast is indeed built-in, then you will not get the stronger spark while cranking. Might matter, might not, depending on vehicle.
Mr. Smith is mostly correct on the “Condenser”, which is an old term for a Capacitor. It’s there to (mostly) absorb the “Inductive Kick-Back” that occurs when the current flowing through a coil (any coil) is interrupted. It’s common to use a Condenser/Capacitor in this way in electronic circuits, and it’s called a “Snubber”. And yes, it greatly reduces the arcing the points experience.
It sounds like you are describing an external resister coil and referring to an internal resistor coil. I have yet to see an internal resistor coil that has a bypass for cranking. Not sure how the coil would know if the engine was in a cranking or running condition. Happy to be educated though.
Yes, I was speaking of a non-resistor coil, with an external ballast. The bypass function was usually provided by another wire that ran from the starter solenoid directly to the coil. When the solenoid pulls in, it bypasses the ballast resistor until the key is released from the Start position. The coil “can’t tell”, and really doesn’t care, where the voltage comes from, as long as it’s enough to produce a spark.
My 57 Ford stopped on the way to a funeral. In a pinch I used a finger nail file borrowed from a woman with me to clean up the points. Got me back on the road.
The main problem I’ve had with points type ignition is wear of the rubbing block. In the old days (good/bad take your pick) you were able to buy special lube for the distributor cam that provided lube for the rubbing block without contaminating the points. I haven’t seen that in years. Add this to the list of problems that a pertronix system will solve. I’m thinking that along with the other quality issues with new parts the plastic rubbing block might also be a. weakness. I’ve been running a Pertronix ignition in my Austin Healey for 25 years with no failures and no need for adjustment. I do keep a set of points and condenser on a spare baseplate in the boot just in case…
Thanks for all the information from everyone.
Great comments .
Some older test instruments have a built in condenser.