This should be a simple question, I think. Is it possible for the ignition coil on a B&S 18.5hp engine to not completely go bad? I will get a spark every once and a while with it. Sometimes it will start up like nothing is wrong and it will keep running like it should. Other times it wont start up at all. Its kind of a hit or miss deal. Any ideas? The engine model is 42A707.

 

Disconnect the coil ground lead and see if it runs properly then. You will have a very hard time shutting it down, unless it has a functioning fuel shutoff solenoid on the carb.

 

It is not often that electronic ignition modules operate sporadically. They usually either work or they don't.
However, that is really only in regards to the trigger mechanism. The "coil" part of the coil could have a break or a short in it. These types of failures usually only exhibit symptoms once the coil has warmed up and expanded slightly.
I have seen cracked coils that get moisture in them that causes intermittent spark failure.
But, Jared is dead-on......Unhook your kill switch mechanism at the coil by pulling the single thin black wire (usually has a white connector) off the terminal on the cylinder side of the coil. That will completely isolate the coil. If you still have intermittent spark, replace the coil. If it is steady and strong, you probably have a bare spot on the kill switch wire or perhaps the insulated terminal that isolates the kill switch itself from ground is loose, cracked, or otherwise defective.
If the engine runs fine with the kill switch disconnected, you can usually stop the engine by choking it out or by popping the spark plug wire off with a plastic handled screw driver.

 

Ok, I tried disconnecting the ground wire and it still did the same thing. The next test I did was to hook up a test light to the ground terminal on the coil. With the engine cranking, the light flashed, indicating that a spark was being created and was being grounded out, correct? However, when I took that light off the coil and tried to just crank the engine, I got no spark at the plug.

Its kind of a wierd deal. I went back a while later and got a spark from it. If the engine is cranking, I should be getting a spark right? Safety swithces should not be a problem now since it is turning over. Am I correct in thinking that? Maybe I'll get a new coil and see if that fixes it.

 

I buy my China made coils on E-Bay from Tulsa Warehouse, cost about $12.

Remove the coil, run sand paper over the flywheels surface, then the coil surface, and regap the distance again.

 

A magneto ignition coil is nothing more than a transformer running on DC.
In short: when the magnets of the flywheel align with the legs of the armature, (the iron laminations part of the coil assembly) the magnets' proximity causes the armature to become magnetized. If you think of the armature as having an "A" shape, you can envision about 250 wraps of relatively thick (I'd guess about 22 ga) wire around the cross member of the "A". One end of this coil of wire, called the primary winding, is attached to ground. The other end is connected to the electronic switching device consisting of a trigger coil and silicon controlled rectifier (among other small electronic components). When the magnets magnetize the armature, the magnetic field also passes through the primary windings. Since both ends are grounded (the electronic switch is closed), electricity flows in the primary coil, (couple of hundred volts) thus turning it into a focused electromagnet, building a magnetic field of its own around itself.
Surrounding the primary windings are the secondary windings. This consists of many thousands of wraps of very, very thin wire. I can't even guess at the gauge, but it is thinner than human hair. One end of this winding is connected to ground. The other end is connected to the spark plug wire. As the magnetic field builds around the primary windings, electrical flow is induced in the secondary windings.
A word about electrical generation:
Voltage (electrical pressure) is generated when a magnetic field passes through a piece of copper wire. Electrons actually will flow if the voltage created is greater than the resistance (Ohms) in the circuit. It takes a minimum of about 15k volts to overcome the resistance of the compressed air/fuel mixture across which the spark must jump at the electrode end of the spark plug.
The amount of voltage created depends on:
Strength of the magnetic field being used and the speed with which the magnetic lines of flux "cut" through the copper wire, as well as the number of locations at which the copper wire is being "cut" by the magnetic lines of force (wraps of wire).
The voltage created by the "buildup" field created in the primary windings by the magnets in the flywheel is not sufficient to jump the gap of the spark plug.
That intense voltage is created when the primary circuit is broken by the electronic ignition module (electronic switch-formerly the mechanical breaker points system).
When the primary circuit opens, the magnetic field that was created around the primary windings collapses at speeds approaching the speed of light. The collapse of the field causes the magnetic lines of force to cut through the many, many wraps of wire in the secondary windings at a very high speed, inducing the minimum 15k voltage required to push electrons across the spark gap.
Hopefully, you understand that part.
Intermittent spark can occur if their is a fault in the secondary windings that shortens the effective length of the total amount of wire in the coil. This results in a voltage strong enough to still shock the the bejeezus out of you, yet not strong enough to jump the gap of the spark plug.
It is really a very simple exercise in electromagnetic theory.
Gotta run to work. Be glad to add to this post later, but I have to run now.

 

Daggone. . . I love that kind of talk. (Above!)


INCREDIBLE POST!!!!!!!!!!!


Rep points deserved and sent!!!!!!!
(If DOUBLE Rep Points were allowed. . . you'd get them!!!)


That post is what FTE is about.

 

Thanks, B.O.R.
I had to get to work before I could finish, but all I really had left to say is that I suspect a short in the secondary windings of the coil at a location that allows a fair amount of voltage to be produced, but not enough to jump the gap regularly.
To continue the explanation a little bit:
Testing a coil by laying the spark plug on the gap is an iffy test at best. If you have no spark, then ok, your ignition system isn't working, PROBABLY. You might have a bad spark plug and the electricity could be going straight to ground via the threaded shell at the bottom of the plug. You would not see a spark at the gap in this instance.
You might see a spark at the gap, but it takes significantly less voltage to jump a spark across a .030" gap of atmospheric air than it does to jump across the same distance through the much denser compressed air and fuel mixture. All those little molecules packed into a smaller space leaves less room for the spark to make it's way through them.
That is why one should use a bona-fide spark tester. And don't use those little neon ones, either. They will light at ridiculously low voltages-voltages too low to jump the gap. I was all proud of myself for buying one and using it only to be stumped by the fact that my tester was saying I had spark when I really didn't have strong enough spark to run the engine. Not until I put a Briggs tester on it (they have an air gap of .166"=the amount of space a spark must travel through air to equate to .030" of compressed air and fuel) did I realize that I had a leaky condenser (points system).
Consider how fast one can walk alongside a pool of water and compare that to the rate at which one can walk the same distance through waist deep water. See, the water is denser.
I am sure Earnhartracer has an electronic ignition coil and the opening and closing of the primary circuit is done via electronic switching.
In pre 1981 Briggs engines, breaker points were used opening up a bunch of other possibilities for causes of weak sparks. Breaker points are merely a mechanical switch for the primary circuit, timed to a flat spot on the crankshaft. Because they are mechanically timed to the crankshaft to ensure spark at the proper piston location, a partially sheared flywheel key would throw the whole works out of whack by closing and opening the points at the wrong time in relation to the magnets.
Electronic ignition will not be affected by a sheared key. It simply makes spark every time the magnets pass beneath the coil. If you have a sheared key, you could easily be making a spark at the beginning of the intake stroke instead of at the tail end of the stroke when it is supposed to occur.
With electronic ignition, the flywheel has to spin at least (I think) 300 rpm for the trigger coil to activate and open the primary circuit.
With breaker ignition, it only has to spin maybe the equivalent of 10 rpm to trigger a spark. I have been bit many a time just spinning the flywheel to feel for internal problems (loose rod, etc) while having my arm in contact with the sparkplug wire on the end of the plug. That is irritating.
Anyway, Earnhartracer, I believe that replacing the coil will take care of your problem.
Glad to have been of some educational value to others reading along.

 

Wow. I just learned more about ignition coils in this post than I thought I would ever know. And it all makes sense!!! Thanks for the explaination ckal704. I'm gonna stop by NAPA on the way home from school today and pick up a new coil. It just blows my mind that it will work awesome one minute and then not at all the next.

Now I just need to figure out why my Bronco II isnt getting spark and all of my problems for now will be fixed. Thanks again.

 

Well, your Bronco II has an battery ignition system.
Same principles apply, as far as the ignition system goes, except that the juice comes from the battery (then the alternator after the engine is running).
The coil looks different but is constructed similarly-ie: fewr turns of thick wire and many,many turns of very thin wire. The thin wire exits the coil at the top where that short "spark plug wire" looking wire is attached and goes to the distributor cap.
The thicker primary winding is energized by the vehicles electrical system and its continuity is broken systematically by either breaker points (usually located under the distributor cap) or an electronic ignition unit. Again, the difference is that points are a mechanical switch and electronic is well, an electronic switch.
But the fact remains that the current flows through the primary builiding up the magnetic field around the primary then the flow stops abruptly collapsing the magnetic field through the secondary windings sending the spark to the distributor cap which sends it to each spark plug in turn.
Automobile style coils can be tested by checking continuity. You simply put a continuity tester on the negative terminal on the coil. Put the other end on the positive terminal to check continuity of the primary windings or put the other end down into the end of the coil to check the continuity of the secondary winding, as both windings share a common ground; that being the negative terminal on the coil.
But so many other things can be wrong in auto style ignitions. I would not be afraid to say I am expert and many aspects of small engine repair, but would never claim as such when it comes to automotive. Both operate on identical principles, but there are more components in the equations for the various automotive systems and I simply do not have much experience with them.

 

My bronco II actually has a 5.0efi in it, not quite stock, but same type of ignition. The coil checks out fine. That was one of the first things I looked at for some reason. This whole deal started when I noticed it would just shut off randomly at idle when I was out four wheeling. It would then hard to start back up, acting almost as if something had a major drag on the engine (the starter is new). I thought maybe it was the ICM getting warm causing the stall. So I pulled the distributor out, marking the way it came out, and replaced the ICM. I put it back in the same way it came out, and now it doesn't work. I tried putting the old ICM back on, but it still won't work. It ran before I pulled the distributor.

On the same note, is it normal that the distibutor is hard to get out immediately after being run? I had to wait a while til the intake manifold cooled down and shrink some.

 

I feel very confident giving advice about mower and other outdoor power equipment engines, but not vehicular.
I defer to someone with more experience than I.

 

It sounds like either the distributor is not in the same position, or it is not getting a good ground. I chased a similar issue on my 84 F250 about a week after I swapped in the DSII system. After I blew out a muffler, a friend blew out the other one, and it sat for a few days, I finally tried a ground wire to the distributor body. It fired up and ran perfectly.

 

Well, I know the distributor is in the correct position. I have taken it out several times, set the engine at TDC, and re-installed the distributor so its pointing to cylinder 1. I too thought maybe it off 180* but it wasnt.

Ford_Six, did you just leave that ground wire attached to the distributor? I'll give it a try.

ckal704, I appreciate all of your help. That new coil fixed the problem.