I have a Snapper Model 281011BTE mower with a 10 hp B&S Model 254707 Type 0124-01 engine with magnetron ignition. It was running fine while cutting and unexpectedly just cut off - in middle of yard. I checked and no spark so first put a new plug in - still no spark. I then took off the cover and disconnected the shut off wire. Still no spark. I then figured it was the magnetron so I got a new one. I adjusted it to 0.010" gap between magnet on flywheel. I also cleaned the bottom of the magnetron and the engine to be sure it made a connection. Still no spark. I also checked the crankshaft key which seems fine, too. I finally got another new magnetron to check thinking the first might have been defective and still no spark. Any other ideas?
I was told by my local dealer than if there is no spark to the plug, it must be the magnetron but that there is no good way to check it without putting it on the engine. Engine turns over with no problem using electric starter. One issue I wondered about is that the part number for the magnetron has changed so I wonder if there might be a problem there. But the replacement number according to the book calls for the B&S like I got. Any help would be much appreciated.
#2
Fish
Yeah, give all of the numbers off of the engine, "code" numbers....
Also give the coil's part numbers
The thing that destroys these ignitions is voltage getting sent to the small "kill" wire that is plugged into the base of the coil, so
If you plugged in a new good expensive coil, and it sent voltage to that one too, then you have destroyed the new one as well.
As mentioned in the OP, I disconnected the "kill" wire before installing the magnetron to make sure that it was not shorting the spark. Also, I checked with switch on and with starter turning and had no voltage on this wire. The engine's code is 92082710 and the replacement magnetron is a B&S 398811.
The dealer said that this was a replacement for the original which is no longer available. It must be a somewhat generic one since the plug wire is over twice as long and the directions indicate how to cut it to correct length (which I have not done to this point). If I put a voltmeter on the spark wire, I do get a slight (0.06-0.08 volt) spike as I turn the flywheel/magnet pass the magnetron. The magnet seems to be pretty strong as far as I can tell, i.e., it squeezes the adjustment card tightly when adjusting the gap to the flywheel and has a very noticeable pull.
#4
Fish
the engine setup in it's basic form, with the magneto ignition system....
Every revolution of the flywheel past the electronic ignition coil over a certain crank speed will produce a spark that will run the engine.
#5
Fish
If the coil is good, and the spin of the flywheel is enough, you will have a spark. Provided the kill wire is disconnected or very less likely, the flywheel has lost it's magnetic charge......
#6
Fish
Like I said earlier.... If at any time you hooked up the small "kill" wire to any new coil, you likely destroyed it then. Get a decent
voltmeter.
#7
Fish
If you put on a new coil, leave the kill wire disconnected, but to shut it off, you will need to yank the plug wire!!!! But you will not destroy another ignition.....
What is frustrating is not being able to check the magnetron itself. As stated in the OP, "I then took off the cover and disconnected the shut off wire. Still no spark. I then figured it was the magnetron so I got a new one." I have not reconnected the "kill/shutoff" wire, but I have no idea whether someone might have tried this on a mower that was getting power to the kill connection and returned it to the dealer before I got it.
Not sure what your comment about getting a "good" voltmeter since I think mine is a "good" one. Plus I have checked the circuits with more than one anyway and they ALL get the same response. Thanks anyway. Back to ballgame!
#9
Fish
Well sorry about the voltmeter slur.... At any way at all!!! If you have an engine with a good flywheel, and you bolt on a "good coil", and do not hook up the small "kill" wire, and crank the engine you will generate a spark!!!!!
#10
Fish
If not, then the new coil is bad, or the flywheel is bad [lost magnets], but I have never ever seen that!
Never heard of using a voltmeter to check an ignition coil output!! Gap type or neon light test lead is all I know!! On a running engine I can check for average spark output voltage from the coil to the sparkplug with a non contact tach/spark output tester.
The vast majority of later model small engines have an ignition coil that has other electronic components internal to it. This is why they are best tested when installed on an engine.
Never heard of using a voltmeter to check an ignition coil output!! Gap type or neon light test lead is all I know!! On a running engine I can check for average spark output voltage from the coil to the sparkplug with a non contact tach/spark output tester.
The vast majority of later model small engines have an ignition coil that has other electronic components internal to it. This is why they are best tested when installed on an engine.
This thread reminded me that one of my adjustable spark checkers is not working, so I just went out to my truck and brought in my test equipment box and got out both of these testers and set them on my workbench to remind me to repair them. The ground wire where it attaches to the housing is a weak point and the wire breaks rendering the tester useless.
If you look thru the service manuals from various small engine manufacturers you will see coil testers. They are fairly expensive and not universal to a particular engine manufacturers entire line of ignition coils, in addition to the fact that they are sensitive to damage from dropping and can get lost easily, thus the reason why many servicing dealers don't have them and prefer to pull a new ignition coil out of stock and put it directly on the engine in question. However just because a part is new doesn't mean that it works, so get another one from the parts room!!! On a twin cylinder engine I generally replace both coils even if only one is confirmed bad and I always check the coil harness to make sure the diodes are not shorted or open electrically and most time replace it along with the coils. Although Briggs claims that rust on the magnet surfaces has no bearing on spark, I have proven that rust can be a factor as it affects the gap of the coil to the flywheel when using a nonmagnetic material to make the gap adjustment. I use brass or plastic shim material to correctly set this gap and rust on the flywheel magnets can affect this adjustment.
I also checked the battery in my capacitance tester while I was in this toolbox and it is almost out of date, however not leaking!!
Fish, do you have a Briggs coil tester?????
I like Husqvarna saws, just traded my 390 XP for a new T 540 XP, very clean 390 XP, extra chains and blades. The 390 was too much for this old phart, but nice saw. Still have two 455s.
Just some thoughts from
Mad Mackie retired mech!!! :laughing::biggrin::smile:
Had the same problem with a 6.75hp briggs.Went through the same thing u did with 3 different coils, sanding and cleaning flywheel and still no spark. I ended up changing the flywheel even though it seemed to have a strong magnet. I have spark now but i still don't understand what made the difference. Im lucky, i work on small engines all the time so i have extra coils, flywheels and such so i can experiment with it.
Interested parties should go to Briggs Magnetron web page for a good cutaway and dynamic video of the magnetron coil in theory and action. The transistors used inside are probably asian (not nec bad) and probably not best commercial grade. This means they have not been screened at temperature to weed out the "infant mortality." So what? So you the consumer will weed them out. Lucky owners will sail through. It is doubtful most dealers know how iffy "new" product is and should all have testers IMO to support customers. I thnk I saw a figure on commercial transistors that they will live up to 259F. So I guess that temp or below is maintained where they sit near the flywheel and fan.
Just babbling some may find interesting and others irrelevant, as usual.:laughing:
#19
Fish
Yeah, it is pretty simple to check for spark, just plug the loose sparkplug in and lay it against the block, and pull the rope, or turn the key.... Will the kill wire disconnected of course. If you have no spark, you either have a bad plug a bad coil, or a bad flywheel. And in all of my years, I have never came across a flywheel with bad magnets....... It could happen, I guess....
Hi All,
First of all, thanks to all who made suggestions. I finally was able to get back to this engine to try it out and yes, I do know how to check for spark since I have been doing that for over 50 years at the spark plug. Nowadays, I use one of the spark plug testers like you can get at HF for about $5 which makes it easy to put between the plug wire and the plug. You don't have to hold the spark plug threads tight against a ground, turn engine and look for the spark at the same time.
Anyway, I was reading about the primary and secondary circuits on magnetos which got me to thinking that there must be a sequence to which side of the magnetron senses the magnet first. Bottom line was I took it off and flipped it upside down, turned the starter and had spark! I then tried to start it, and it cranked right up. I shorted out the kill wire with a screwdriver and then attached it. Working fine now. I looked back at the instructions that came with the new B&S magnetron and it never said which side was the top. There is a little sketch, but it doesn't say whether it is top or bottom view. BTW, I had made sure I put the replacement magnetron on with the spark plug wire and kill wire in the same positions as the old one. In fact, I looked back at the old one again, and you could tell by the crud around the screws which way it had been on. However, the original magnetron has been replaced by another part number so I suspect the wiring is different with the new part. That's the only thing I can think of because the engine had run fine for years. I had never taken the cover off before all of this and had never touched the magnetron before.
So after reading all of the stuff on-line about how if the engine turns fast enough, magnet is good, gap is right, kill wire is not connected, you should have spark from the magnetron. The missing info is that the magnetron has to be right side up! So if you put a new one on and don't get spark with everything else okay, you might want to flip it over since it will mount either way, or at least mine will.
Thanks again for everyone's suggestions.
#21
Fish
Well. sorry about that..... Us poor guys cannot think of every way the posters here can farq things up!!!!
#22
Fish
Briggs does put on the top of their coils, "this side up"
As many times as I took off the magnetron and had it in my hand, I had not seen the black writing on black until after your post when I looked for it. For the benefit of others, it actually has "THIS SIDE OUT" on the top side and "CYL SIDE" on the bottom. Note that there is nothing about this on the installation directions though so it was easy to overlook. Nice to know that my kill wire works, too.
