Briggs Ignition

[Dutch]

I've received several emails asking questions about ignition systems. Rather than repeat the same things over and over, I decided to make this post. Besides, I'm no expert and maybe other members can correct any mistakes or add something I forgot. Anyway, when internal combustion engines were invented over 100 years ago there were primarily two (2) different ignition systems used to make spark plugs work. One system was the battery & coil. The other was the magneto system. In a "battery & coil" system (as used on K series Kohler engines), a low voltage battery (usually 12 volts) supplies voltage to a "coil". Through the mystery of electricity, the coil steps that low voltage up to high voltage (10,000 to 50,000 volts). That high voltage is stored in a "condenser" waiting to be used. There is also a set of "points". As long as the points are closed, the high voltage remains stored in the condenser. When the points open, the high voltage stored in the condenser is discharged to the spark plug. The spark plug has a gap between the electrodes, and when the high voltage jumps across that gap a spark is produced which ignites the fuel mixture inside the cylinder, and a controlled explosion occurs pushing the piston which turns the crankshaft. The points have to open at an exact moment (usually just before the piston reaches the top of it's stroke), that's called "timing". In a "magneto" system (as used on cast iron Briggs engines), a battery is NOT needed. The Briggs engines have a magnet built into the flywheel. The Briggs coil (aka an armature) is bolted very close to the flywheel. As the flywheel rotates, the magnet passes by the armature and produces a low voltage. The armature steps that low voltage up to high voltage. That high voltage is stored in a "condenser" waiting to be used. There is also a set of "points". The rest of the process is just like a battery & coil system. So what can go wrong with a Briggs ignition system? 1) The magnetism can leave the flywheel. This is not common but can happen if the flywheel is dropped or hit with a hammer. Check the flywheel by moving a piece of steel around the flywheel. The magnet should "grab" the steel as it passes by the magnet. 2) The armature (coil) can go bad from broken or shorted wires. The armature's wiring can be tested with an Ohm Meter. If the armature is bad, replace it. (TIP: The armature does not fit up tight against the flywheel. When I install an armature, I leave the two mounting screws loose. Then I rotate the flywheel until the magnet pulls the armature tight. I then pull the armature away from the flywheel and slide a common match book cover between the two armature arms and the flywheel, When the mounting screws are tighten, and the match books are removed, the gap is just about right on the nose) 3) The condenser can go bad. There are condenser testers, but a condenser can be tested another way. Take the condenser to your car or truck. Remove one plug wire. Stick the wire from the condenser into the end of the plug wire, and hold the condenser case to ground. Crank the car engine over a few times. Carefully remove the condenser from the plug wire. Then carefully move the condenser wire close to the condenser case. As the condenser wire comes close to the case, a high voltage spark should jump between the wire to the case. This is not an accurate test, but it will do. 4) Timing can knocked out of whack by a worn or sheared key holding the flywheel to the crankshaft. Replace the key 5) The points can go bad. Usually this is due to oil or corrosion on the points that prevents conducting electricity. Many times you can make a temporary fix by cleaning and filing the points. (TIP: I also use a common match book cover to set the gap while adjusting the points). Since the points open and close rapidly and continuously while the engine is running, the points require routine maintenance and replacement. Theoretically, since there are no moving parts in the armature and condenser, they should never good bad, but they do because of vibration and temperature change. So, you can see that the points are the weak link of the ignition system. About 25 years ago we entered the electronic age. The old fashioned tubes in our radios and TVs were replaced by transistors. Engine makers discovered a way to eliminate the troublesome points from ignition systems. The points & condensers were replaced by solidstate electronic "gizmos" that had no moving parts. Briggs offers two (2) ways to replace points & condensers with electronic ignition. 1) If your OEM armature is good, keep it. Install an "electronic trigger" and remove your points and condenser. Cost is between $10 - 15. Instructions are included. 2) If your OEM armature is bad, or you want a self contained unit, install a "Magnetron". The magnetron is an exact replacement for the OEM armature, and also contains an electronic trigger so you can eliminate the points and condenser. Cost is between $35 - 55. Instructions are included. (TIP: One word of caution when installing a Magnetron. Some of the cast iron Briggs flywheel magnets had the "South" pole facing out, others had the "North" pole facing out..If you install a Magnetron as per the instructions, but it doesn't work, you may have to send your flywheel to Briggs to be "re-polarized". If you install a Magnetron and it works, your polarization is okay and you shouldn't worry about the flywheel.) (TIP: If you remove the points and condenser, you should also remove the plunger that operates the points. Your Briggs dealer has inexpensive lead plugs that seal the plunger hole to prevent oil leaks.)

[JOHNP]

GOOD POST DUTCH, THANK YOU. FOUND IT HELPFUL IN SOME AREAS THAT I WASN'T SURE OF. JOHNP

[roma3112]

yes thanks for the post you reassured mo on some fears i had about the magnetron i installed on my cast iron 300421

[HubbardRA]

Dutch, I just wanted to make one comment on your description of the Battery-points type of ignitions. The condenser does not work as you say. An engine will fire and run without a condenser. The spark is generated by applying power to the coil which is an inductor. Once the inductor is charged, a spark is created by breaking the input circuit by the points, to produce a fly-back or reverse voltage from the inductor. This produces the spark. The condenser is used to stabilize the spark across the points "only" when the circuit is being broken, and prevent point burn. If the condenser value is matched exactly to the rest of the ignition, the points will last for years. I had one condenser on a car theat I had which would give 28000 miles on a set of points. I didn't replace that condenser till it dies (3 sets of points) As I said earlier , an engine will run without a condenser, but it will not run smoothly because the fire across the points will be erratic. This also happens if you have a condenser fail by going open circuit. If they fail in a short circuit, the coil will not break causing the engine to not fire till you turn off the ignition. A shorted condenser can cause the weird problem of no fire while cranking but a single fire when you turn off the engine. Since this single firing is not timed to the engine rotation, but to your turn off of the ignition, it can at times produce a dramatic backfire. Your other descriptions of the electronic ignitions is great! You know much more than I do about those systems. Keep up the good advice! Rod H.

[Spyder]

I was about to write and addendum until I read on down to Rod H.'s comment. You are exactly right about the spark creation. From my memory the equation is E = L * ( di/dt ) where L is the inductance (measured in units called millihenries, or microhenries) of the coil or inductor and ( di/dt ) is the rate of change of the current (mearured in milliamps) in the coil with respect to time. E is the voltage generated across the coil. So, for a fixed inductance coil the faster you can make the current change, the higher the voltage generated. The condenser in the circuit is really a "capacitor". The capacitance or storage value of the capacitor is measured in units called "farads" and the capacitance of these tractor condensers only about 10 microfarad. The capacitor also has to have the property of withstanding the high voltages on the points when they are opened. Capacitors most commonly fail electrically shorted (result: no spark) but sometime they can fail completely open circuit (result: points weld shut). The real purpose of the capacitor is to "snub" or suppress the arc across the points as they open. The arc you see when the points open is in exact time synchronism with the spark conducted to the spark plug. Without the capacitor in the circuit, or a failed one, the points would quickly weld shut from the arcing and the engine is dead for sure. To make the engine spark work, the points are closed and then there is a current flowing in the armature coil. When the current is interrupted by opening the points, a high voltage is created using the above equation. Typically, this is at least 10,000 volts for a small engine. The condenser as you say is to protect the points from burning because of the arc which is created by the high voltage which is created when the points open. The same high voltage is conducted to the spark plug to ignite the fuel/air mixture.....DaveG

[powerking_one]

Dave, Rod, others: You are both correct. The condenser is indeed there to neutralize or counteract the inductive component of the current flow throught the points from the primary of the coil winding to prevent arcing. Most all Kettering (point/condenser) ignitions use a condenser in the .18 to .30 microfarad range (primary coil inductance dependent). Static testing of a condenser on a capacitance tester may not show it is good or bad compared to it's operation under true higher voltage operating conditions (100-300V breakdown range). This is why it has always been historically prudent and recommended to replace it with the points. It has nothing to do with storing energy for firing the coil. Herb, As far as your testing method of the condenser (attaching to the spark plug wire), well that's insane/very crude and destructive. It will probably destroy the die-electrict properties/function of the condenser. They are normally rated at 100-400 volts breakdown range. If you "zap" them with 10-20 KV of secondary spark voltage, it will have it's insulation breached and will breakdown now under normal operating conditions. Just my 2 cents worth, Tom(PK)

[HubbardRA]

Tom, I agree with what you are saying. Back before electronic ignitions were being used on cars, one of the electronic engineers that I work with put together a small device to measure the capacitance needed on a specific ignition. We then started matching the capacitance on our autos. I had a capacitor on a Ford back then that allowed me to get the 28000 miles that I mentioned above. Lots better than the 3000 that I was getting on the Chevy that had the capacitor integral with the points. Rod H.

[Dutch]

Guys, When I made the first post to this thread, the intention was to help those with limited mechanical knowledge to understand how an ignition system works. I sacrificed complete technical accuracy for basic simplicity. I don't believe it is necessary to understand the theory of electro-magnetic fields or induction to diagnose a faulty coil or a set of points. How many journeyman mechanics have ever measured a coulomb, microfarad, or microhenry? I suspect if anyone owned such a measuring device, they wouldn't be reading a basic "how to". Every electrical manual I have defines a condenser (capacitor) as an electrical device that accumulates and holds a charge of electricity. To me, that means storage. If someone asked how to relieve a headache, I wouldn't tell them to take 650 milligrams of C₉H₈O₄ (acetylsalicylic acid). I'd tell them to take 2 aspirin. If I attempted to explain how and why aspirin worked, they'd probably need more than 2. Tom, In the days when autos used Kettering ignition, and I didn't have much money, I would take condensers from the trash, test them as I stated, and use them in my own car for tens of thousands of miles. You have obviously read technical manuals. I suggest you read a book on etiquette so you can learn to express disagreement in a more gentlemanly manner without resorting to using insulting and inflammatory words and terms.

[HubbardRA]

Dutch, I didn't mean to cause this squabble. I only meant to say that points and condenser is not a "Capacitive Discharge" ignition. Sorry. Rod H.

[thedaddycat]

I guess I missed this post a month ago when it was first posted. It is my understanding of electricity, electronics, and ignitions that lead me to make some observations here. The points act as a switch on the primary side of the ignition coil and are subjected to 12 volt DC supplied by the battery, not the high voltage of the secondary(spark plug) side. The secondary voltage is dependant only on the ratio of primary windings to secondary windings and applied primary voltage. The ignition coil is really just a special application step up transformer. Capacitors(condensors) will charge when voltage is applied and discharge trying to maintain voltage when power is no longer applied. The condensor discharges to maintain the 12 VDC that was applied to the primary side of the coil, hence minimizing the difference in voltage across the points when they open. Remember that one side of the points is connected to the battery, while the other side (along with the condensor)is connected to ground through the primary side of the coil. This is how they reduce arcing across the points. When power is applied to the primary side of the coil a magnetic field is induced in the primary windings. When the points open and power is no longer applied, this field collapses inducing the secondary voltage(moving magnetic field cutting across secondary windings). The electronic gizmo is just another kind of switch(non-mechanical) for controlling primary side current flow. I have way too much time on my hands when stuck in the control room. I must have spent 5 or 6 hours just tonight on this site.

[HubbardRA]

Kirk, I like the way you described the ignitions. Rod H.