Part 1. The basics
When giving seminars at tractor shows or answering electrical related questions on some of the antique tractor websites, I have noticed there exists a degree of confusion regarding ignitions systems. I have, therefore, written some instructive articles which I hope may serve as a guide for beginners, as well as experienced tractor restorers. It has long been my belief and practice that if one understands how an ignition system functions, he can perform routine troubleshooting and repair without ever having access to any sort of wiring diagram whatsoever. Besides, “real men” like John Wayne and me throw away instructions and never ask directions, right?
A simple battery powered continuity tester and a 6/12 volt test lamp is all that is necessary to troubleshoot most common ignition problems. This first series of articles covers battery powered coil ignition systems with magnetos to be later discussed and whatever else the publisher may desire. (Feel free to send any comments, only if favorable, on the back of $20 bills.) This Part 1 installment covers basic theory with troubleshooting and repair to follow in the future. I realize some of this may be dry, but an understanding will help later on when we begin troubleshooting.
The term HOT spark may have different meanings to different people. If voltage across two points is of a sufficient level, it will cause current to jump (arc) across an open gap and current flows through that gap. However, it is heat (not voltage) which ignites the fuel/air mixture and it’s the flow of arcing current across the plug’s gap which produces that heat. A blue current arc is of higher amperage than a yellow one, which is why a good blue (hot) spark is the one we prefer. When you think of voltage, consider it as the potential difference between and in reference to TWO different points. There is no such thing as just voltage; voltage exists and is relative to two terminals, such as the positive and negative on a battery. Conversely, think of current as current through a conductor which is caused to flow if a voltage difference exists between two terminals which are connected by a conductor. So, it’s voltage ACROSS and current THROUGH which will be used to understand what follows.
The components in a typical battery powered coil ignition system are the coil, distributor (which contains contact points, condensor, cap and rotor), ignition switch, spark plugs and wires. I will first explain the function of each such that in future articles, the whole system can be examined. I will save the ballast resistor for a future article since it and its associated circuit can be more complex than one may think.
Ignition Coil: The ignition coil’s purpose is to serve as a step up voltage transformer to increase the low battery voltage to several thousand volts necessary to arc (jump) across the spark plug gap and ignite the fuel charge. It is also an energy storage device, which stores energy in a magnetic field when electrical energy (volts times amps) is supplied to it from a battery. An ignition coil is actually two coils of wire (low voltage primary and high voltage secondary) wound tightly and closely together around a common iron core. When wire is shaped in the form of coils, it forms an inductor which exhibits certain electrical properties. If one passes current through those coils, a magnetic field is generated, such as in grade school science class when a battery was connected to a wire circled around a nail that became magnetized. Now, when another (secondary) coil of wire is wound around that first (primary) coil but has many, many more turns of wire, it is possible (when the primary’s magnetic field collapses upon opening of the ignition points breaking the circuit) for a voltage to be induced in the secondary winding that is higher than in the primary’s. This is how the coil serves as a voltage step up transformer. The coil’ s low voltage primary winding circuit consists of several hundred turns of wire connected between the small positive and negative terminals. The high voltage secondary winding consists of thousands of turns of smaller wire wound around the primary (more volts but less amps) connected between the small positive terminal and leading out the big center high voltage tip that connects to the distributor via the coil wire. Current passes through the coil’ s primary winding from the ignition switch and goes through the distributor’s ignition contact points to ground, but when the path is broken upon opening of the points, high voltage is induced into the secondary causing current to flow out the coil top, through the distributor cap, through the rotor tip, to the spark plug wire terminal and finally to the plug, where it arc jumps current across the gap. I hope you readers understand all this because there will be a pop quiz.
Ignition contact points: The points serve as a simple mechanical contact switch, opening and closing the ignition coil’s primary circuit. Its rotation of the distributor’s cam opens the points at a high cam lobe on the rotating center shaft. The points’ silver coated tips are designed to make good low resistance electrical contact when closed and withstand the heat and small current arcs produced when they initially open. They eventually deteriorate (burn up) and become pitted, burned, uneven matched, carbon coated and/ or lose their coating, such that the increased resistance does not allow sufficient current flow causing system failure. When the ignition switch is turned on, current flows from it, to and through the coil’s primary, through the points (when closed) and then to ground completing the circuit. It’s when the points open (when the cylinder is at TDC on compression stroke and the rotor tip points to that wire’s cap terminal) that high voltage is induced in the coils, high voltage winding sufficient to arc current across the plug gap. Setting the points to open the proper gap (usually around 0.015 to 0.020) affects timing, spark efficiency, point longevity and overall system performance. It is possible and good to clean them (not overly abrasive which removes the coating) to remove any oxidation, uneven surfaces or build ups, but once they begin to pit and their coating is lost, the system is prone to failure. So, the points are a simple switch which maintenance such as gap ping and non abrasive light cleaning can help increase longevity.
Condensor: The MAIN function of the condensor is to protect, preserve and prevent premature burning up of the ignition contact points. They also increase system efficiency, but how is beyond the scope of these articles. The system would work if there were no condensor at all (one way to check for a dead shorted out condensor which causes complete system failure) but the points would burn out quickly and the spark across the plugs would be of less energy. A condensor is comprised of two thin metallic plate surfaces (that never make direct electrical contact) separated by an insulator (non electrical conductor), then wound (plate, insulator, plate) in that small cylindrical package. The external wire lead goes to one plate and the other plate is connected to the metallic outer core, but there is no connection between them. There should be an open circuit to a battery powered continuity tester; otherwise you have a bad shorted out unit which renders the system inoperative. The condensor is wired in parallel with the points right at the base of the distributor to ground (through its metallic outer case). When the points open, the energy stored in the primary’s magnetic field is dissipated in the form of volts times amps, which causes that arc of current to jump across the points when they start to open. However, with the condensor in place, most of that energy is discharged into the condensor (it is also an energy storage device) which causes only a low energy with less heat, smaller arc across the points to prevent their premature burn out. So, the condensor saves the points from premature burn up and the system would work if there were no condensor (but only until the points burned up). But if the condensor is shorted out, the system fails.
Cap and rotor: The cap and rotor are merely devices to mechanically and physically direct the coils’ high voltage output current (via the coil wire) to the correct cylinder’s spark plug at TDC on its compression stroke to ignite the fuel charge at the right time. When the points open, the coils’ high voltage secondary wire will arc current to ground when not connected to the cap (which is how you test the coil independent of the cap and rotor). However, if the coil wire is connected to the distributor cap, current flows through the rotating rotor tip, which is then lined up (pointing to) with the inner connector leading to the external spark plug wire terminal. Things that go wrong in caps can be cracks or carbon paths which may cause the current to flow to ground versus the correct external terminal.
Distributor: The hopefully obvious purpose of the distributor is to DISTRIBUTE current to the correct spark plug terminal at the right time. It contains the points, condensor, cap and rotor, all as described above. When you time the distributor, all you are doing is rotating it which causes that cam to open the points either sooner or later to fire the plug sooner or later. Remember, if the ignition is on and the points close and then open, the coil can produce high voltage, but it’s the cap and rotor which direct it to the correct plug and timing (rotational position) which controls when the spark occurs.
Plugs and wires: The plug wires distribute high voltage to the spark plugs and since they contain voltage at a high level of potential different from ground (tractor’s steel frame), they need to contain no cracks or excess wear which could cause a bleed off of current to ground as opposed to arcing it across the plug gap as intended. The plug is, of course, the device which contains the gap inside the combustion chamber where arcing current flows produce heat to ignite the fuel.
I hope this has provided some insight into theory of how the individual ignition system components work such that future articles on the entire system and troubleshooting may be readily understandable.
aka Ol John T
Bloomington, Indiana Editor’s note: John Nordhoff is a retired electrical engineer (BSEE Purdue University 1969) and now a county lawyer who resides in Bloomington, Indiana. He collects and restores antique John Deere tractors and has given electrical seminars at Gathering of the Green in Moline, Illinois both in 2000 and 2002.