Ignition System Theory and Testing
The 1980's ushered in the magic black box, Magnetron™ ignition coil, Briggs & Stratton's first truly electronic ignition system. Breaker point ignition systems for most small air-cooled engine manufacturers have totally disappeared over the last 20 years. In a point style system, the flywheel magnets rotate past the legs of the ignition armature. The armature itself is made up of two separate windings of copper wire - the primary and secondary - one wound on top of the other. Basic physics tells us that when a magnetic field (flywheel magnet) cuts through (moves past) a conductor (copper wire), a flow of electrons (electricity) is created. However, electron flow only occurs when we have a complete circuit. This means that the points must be closed. We're also told that the faster the movement between the field and the conductor, the greater the output.
Remember science class in grade school? At one point in your school career, an enterprising teacher wrapped a length of copper wire around a nail and hooked the wire ends to a dry cell battery. A handful of paper clips was instantly attracted to the nail and fell away when the battery was disconnected. Electron flow through a conductor then, causes a magnetic field. When the points close, electron flow causes a magnetic field to be created around the primary. This field also envelopes the secondary. The points now open, break the circuit and collapse the field through the primary. The field caving in on itself is movement just like the rotating magnetic field of the flywheel. This movement is at speeds much greater than the flywheel could spin - near the speed of light. The rapidly collapsing field tears through the secondary winding which has sixty turns of wire for every one turn of the primary - effectively generating 60 times the voltage created in the primary. The sum total of this is that the secondary winding can create up to 25,000 volts in some systems, which is used to jump the air gap of the spark plug and ignite the fuel/air mixture in the combustion chamber.
Now for the black box. Magnetron™ solid state ignition systems, in essence, replace the mechanical breaker points with a transistor. That is, we replace a mechanical switch with an electronic one. No moving parts, no arcing, no adjustments and solid state reliability.
Now that we've got an idea of how it all works, let's look at the meat and potatoes of what is required to create a good spark.
The flywheel magnet must generate a sufficient magnetic field to start the chain of events in motion. A fair test is to hold the flywheel on edge with the magnet facing up. Place the blade of a 10" #3 (1/4") straight blade screwdriver against the magnet. Release the screwdriver. The magnet should have enough strength to hold the screwdriver straight out. If we pass this test, assume the magnet is OK.
Remember speed is a factor. The engine must be pulled over at a minimum speed of 250 RPM before the coil will even think about firing. Thick oil on a winter day or a heavy parasitic load may cause problems. Customers come into play here as well. Shorter or elderly individuals may not have the leverage or strength required to reach the RPM required to activate the Magnetron's electronics.
The spark plug is a major element of the equation. A new spark plug may require around 10,000 volts to jump a .030" gap when the engine is cold. This drops to just 4,000 when the engine is hot as electrons are more easily emitted from a hot surface. That's one of the reasons the old vacuum tubes in radios had to warm up before the radio would work. Electrons are also more easily emitted from a sharp edge than a round one. A spark plug begins to require more and more voltage as the edge of the center electrode becomes less defined. And finally, an internal short or carbon/oil fouled plug simply shunts the high voltage burst straight to ground with no or insufficient spark.
The ignition coil is probably the easiest thing to check and therefore the first thing to check when embarking upon ignition system troubleshooting. Install the 19368 spark tester between the high-tension lead and a good engine ground. Spin the engine over (at least 250 RPM) and watch for spark in the tester window. As simple as it seems, this is a fairly comprehensive test. The tester electrode gap is .166" wide. Those wise in the way of electrons have calculated that it takes around 13,000 volts to jump this gap. We need 10,000 to jump the gap on a cold spark plug. Add it all up and we have voltage to spare. As coil temperature can aggravate minor coil imperfections that normally wouldn't be a factor, the same test can be done on a warm engine. Engine quits while running? Hook the tester up in line with the spark plug and start the engine. When the engine quits, monitor the window. If spark is present, the problem is not in your ignition coil. By the way, this test stresses the coil well beyond the demand it would see in operation. Think about it. We're asking the coil to build enough voltage to jump TWO gaps - the tester as well as the plug. If your engine starts and runs OK cold and hot, you've got a healthy ignition coil.
One additional test you can perform. Check the impedance (resistance) of the secondary circuit at room temperature. Hook an ohmmeter test lead to the spark plug terminal of the high-tension lead and another to the lamination stack (ground). Your resistance reading should range between 2,500 and 5,000 ohms. If infinite (no continuity), an internal open circuit exists. Replace the coil. If infinite and the engine runs, your problem is an internal break of the high tension lead, a poor attachment of the spark plug terminal or improper mating of the high tension lead to the coil. A pin within the coil body skewers the lead. If the pin does not contact the wire core, there will be no continuity. The coil will often have enough available voltage to jump the gap, so you see spark. The internal arcing that occurs within the high-tension lead will eventually create enough resistance that ignition system performance will suffer. If your resistance reading is much lower than 2,500 ohms, an internal short exists. Replace the coil.
Now, how about some of those old wives tales that just aren't true.
Rust on the flywheel magnets causes a loss of spark. Not true. A magnetic field does not care about rust. It has no effect on it.
A bright blue spark is best. A yellow/orange spark signifies weak ignition. Not true. Spark color determines virtually nothing. The hottest spark is ultraviolet which we can't see. Blue spark is cold in comparison to ultra-violet. Orange and yellow come from particles of sodium in the air ionizing in the high energy of the spark gap.
Laying the spark plug against the block and pulling the engine over can adequately test ignition coil output. Not true. The ignition coil will only generate enough output to jump the gap of the plug. When under compression, the plug requires twice the voltage to fire. This check is not an accurate test of the coil and can be misleading.
An armature air gap that is too wide will prevent spark. Not true. Well, sort of not true. Briggs & Stratton air gaps cannot be made too wide to prevent spark providing the coil is healthy and the engine is spun over fast enough. A wide air gap, say .030" will ever so slightly retard the ignition timing as the magnetic field takes longer to build within the coil windings.
Ignition coils, particularly Magnetron™ coils, rarely fail. If one is suspect, perform the outlined checks exactly as mentioned. MOST IMPORTANT: Be sure to isolate the coil from the equipment wiring harness as well as the engine's wiring harness. That's right, unhook the ignition grounding lead from the coil itself and use the spark tester. Many a technician is fooled into replacing a good coil because the coil grounding lead was shorting out against a piece of sheet metal. DO NOT attach the tester to the spark plug for this test. The engine may start. Without the grounding lead installed, you won't be able to turn it off. If the coil is properly grounded to the engine block, engine speed is at least 250 RPM and the flywheel magnets are OK, there should be spark present in the window of the tester. If not, repeat the test double checking your procedure. Still no spark? Then and only then, replace the coil.
A final bit of trivia - All Magnetron™ Ignition coils have the manufacturing date code cast into the coil body. The coil manufacturing date will usually be within a month of the engine's date code. That's an easy way for you to tell if the coil has been changed before. We use this information to match returned parts to the engine noted on a warranty claim as well as for internal tracking.