How To Test Ignition Coil
Ignition Coil:
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.
