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Old 08-19-2011, 07:37 PM   post #1 of 7
Leonard1818
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Default STX38 no spark (but coil tests out fine)

OK I realize this is a very long post. If you don't want to read it all but still want to help, please skip to the "Where I stand now" section and read that part and also read the final 3 questions at the bottom. This is MUCH appreciated!

Hi Everyone,

I'm new here seeking some expert advice!

So here goes:

Technical specs:

-- John Deere STX38
-- 12.5 HP Kohler Command Pro
-- Black Deck
-- Made in early to mid 80's
-- meticuliously maintained by previous owner (I mean he wrote down EVERYTHING)

History of the problem:

Several weeks ago, my mower started backfiring under part choke. This didn't seem to be too big of a deal cause I usually run it at full throttle and do a "cool down" at the end (when it would backfire). One day as I was mowing, I bent over to snag a branch (using the brake) and low and behold i hear a sound and see something in front of the mower -- The muffler had busted the weld and fallen completely off!

I ordered a new muffler online (next time I'm gonna have my father-in-law weld it back together) and put it on. While putting it on, the old man said "now's a great time to wash off the engine". Against every morsel of judgement in my body, I conceded. I taped off most everything I could and let him have his way with my mower. Well, we couldn't get it to start for a while after he washed it off so we let it sit. And sit. and sit. After a couple of hours I went out and cranked it and it fired right up. No backfiring (like it was doing before the muffler replacement) and spinning like a top. I concluded the backfiring had something to do with the muffler (perhpas the resistance the muffler provides is an important part of the combustion process?) and went ahead and cut the grass.

Everything went well and I was happy! A $57 muffler fixed my mower. Let's go fishin in Canada...

After my 9 day vacation in Canada I came home and of course my lawn needed some TLC. Pull the recently fixed mower out and go to start it up. It started really hard and was hardly turning over (just slow enough to keep it going for a few seconds) then it backfired and died. It hasn't started since then.

I pulled the plug after many attempts to get it started (figured it was flooded) and noted that it was wet so the plug is getting fuel. I had JUST changed the air filter and foam piece when I did the muffler so I knew that was good.

Air (check)
Fuel (check)

On to spark... At this point I should mention that I'm not a tech of any kind. I work with computers -- not engines. I know very little about even these basic engines and how they work.

I did some reading on here and found that if I pulled the plug out, then connected it back to the wire and touched it to the engine block, it should spark when the motor is turned. I did this. no spark. Tried a different plug. no spark. Reading on here, I found that it's perhpas an ignition coil. In reading the notes from the previous owner, I see that he replaced the ignition coil after about 9 years and here we are about 11 years later so it could be that these things have ~10 year life span....

Oh... also... had battery tested -- tested OK. Will hook up to charger tonight to make sure it has full charge but Autozone or wherever I took it said that it was OK as far as holding a charge and CCA's and whatnot.

Where I stand now:

At this point I've done the following test:

-- Take cover off of motor
-- Remove kill switch/spade terminal from ignition coil
-- Pull spark plug
-- Touch to the engine block (to ground)
-- Turn motor

I should see a spark. My question with this test: Would I see a spark EVEN IF the problem lies elsewhere? In other words, does the absence of spark DEFINATELY indicate a bad coil? It's my understanding that by disconnecting the kill switch, I'm essentially isolating the coil and ONLY testing the coil. Since I'm not getting spark, I could conclude that the coil is bad.

HOWEVER...

I read in the Kohler engine manual the following:



I did exactly as fig 8-4 outlines (i even removed the module from the motor) and it tested at 14,000 ohms which lies within the spec. So according to this test, the ignition module is OK.

So what's going on here? I am hesitant to go buy an ignition module cause I don't think I can take it back and the local shop wants $55 for one or something. Could my problem still be the ignition module even though the resistance tests out OK?

My Thoughts:

OK so my thinking was "what else could this be?" I've read about the PTO switch, the seat switch, etc.

My thoughts on these:

If I sit on the mower and pull the blade engagement, I can hear the switch near the front of the mower. Additionally, if the blades are engaged, it won't even turn over -- mine is turning over just won't get spark.

Also, if the brake/clutch pedal is not fully depressed, it won't turn over -- Mine is turning over, just no spark.

The only thing that will allow it to turn over but will kill spark is the seat safety switch. I've read about bypassing this but haven't seen any step-by-steps and I'm not mechanically/electrically inclined so I haven't been able to isolate and test this variable on its own.

So in conclusion, I need some expert opinion to assess where I'm at and my (seemingly) conflicting test results of the ignition module.

Does unplugging the kill switch isolate the ignition module so that I should DEFINATELY get spark? Because I don't get spark does that mean that the coil is DEFINATELY to blame? Why is it still reading 14,000 ohms if it is bad?
Thanks in advance!

Last edited by Leonard1818; 08-19-2011 at 07:38 PM. Reason: Long post...
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Old 08-19-2011, 07:44 PM   post #2 of 7
Brad
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Default Re: STX38 no spark (but coil tests out fine)

I think I might consider taking the coil off and giving it the "baking" treatment. Put it in the oven at 200 degrees, very loosely wrapped in foil. You might still have some moisture in there that is causing your issue. Also, while removing and replacing the coil check for rust buildup or moisture where it attaches to the engine.

I think that you are on the right track with your troubleshooting. By the way, I replaced my coil with an aftermarket one for something around $22 if I am not mistaken. Might not be the best in the world, but worked much better than the dead original!!
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Old 08-19-2011, 07:51 PM   post #3 of 7
Leonard1818
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Default Re: STX38 no spark (but coil tests out fine)

Quote:
Originally Posted by Brad View Post
I think I might consider taking the coil off and giving it the "baking" treatment. Put it in the oven at 200 degrees, very loosely wrapped in foil. You might still have some moisture in there that is causing your issue. Also, while removing and replacing the coil check for rust buildup or moisture where it attaches to the engine.

I think that you are on the right track with your troubleshooting. By the way, I replaced my coil with an aftermarket one for something around $22 if I am not mistaken. Might not be the best in the world, but worked much better than the dead original!!
Hi Brad,

Thanks for your reply. In reasearching on here, you seem to be one of the most knowledgable so I was kinda hoping you would chime in!

I'll give the baking treatment a whirl. What confuses me is that it started fine and ran for an hour or so, spinning like a top after my dad hosed it off... I don't doubt that there could still be some moisture in there but I would think that it would dry up on its own eventually!

I considered an aftermarket piece. Unfortunately it's now been 17 days since my lawn was cut and I'm on a small time crunch. I don't mind paying the local JD dealer their asking price if I'm sure that it will correct my problem.

What are your thoughts about my testing and results? How would I be getting correct resistance but no spark when I unplug the kill lead?

Also, does unplugging the kill lead remove all other variables and isolate the ignition coil or is it possible that something else is still coming into play?

Thanks for your time!
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Old 08-19-2011, 11:37 PM   post #4 of 7
Leonard1818
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Default Re: STX38 no spark (but coil tests out fine)

Oh no!! My thread is about to leave p.1!!!! This forum is way more active than I anticipated. I figured it wouldn't be so busy on the forums cause more folks would be at the National Tractor Pull Championships here in my town!!!
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Old 08-20-2011, 04:19 PM   post #5 of 7
Leonard1818
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Default Re: STX38 no spark (but coil tests out fine)

Well, I figured I'd bump this and let everyone know that I bit the bullet and bought an ignition coil for $55 and sure enough, that's what it was.

So note to anyone who may search this thread out in the future, it *is* possible that your coil will test within spec (8,000-18,000 ohms) but still be bad. To accurately test you should do a resistance test AND pull the kill wire off of it and look for spark.

Thanks to those who helped me out with this problem. On to the crack in the hood
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Old 08-20-2011, 09:25 PM   post #6 of 7
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Default Re: STX38 no spark (but coil tests out fine)

Leonard1818--So glad you got the machine up and running. Sorry about not responding earlier. Just checked back on this thread.

I am not a very good candidate to ask regarding the testing of the coil with the meter. There are others that are much more informed on that procedure. I will say that when you remove the kill wire, that should remove all variables in the system. The safeties, if I am not overlooking something, will only control the solenoid/starter activation once the kill wire has been eliminated.

I understand about needing to get the part "now". I just paid $54 for a coil for my Lawnboy that has been giving me fits for a couple of years now. All the best intentions of finding a used one or an after-market were overshadowed by the possibility of getting one that wouldn't fit. Sometimes, original is the best way to go, plus it helps keep the dealer there for the next time an issue arises.

OK, now what's going on with the crack in the hood? I'll be watching for that thread.
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Old 08-21-2011, 06:06 PM   post #7 of 7
Leonard1818
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Default Re: STX38 no spark (but coil tests out fine)

Quote:
Originally Posted by Brad View Post
Leonard1818--So glad you got the machine up and running. Sorry about not responding earlier. Just checked back on this thread.

I am not a very good candidate to ask regarding the testing of the coil with the meter. There are others that are much more informed on that procedure. I will say that when you remove the kill wire, that should remove all variables in the system. The safeties, if I am not overlooking something, will only control the solenoid/starter activation once the kill wire has been eliminated.

I understand about needing to get the part "now". I just paid $54 for a coil for my Lawnboy that has been giving me fits for a couple of years now. All the best intentions of finding a used one or an after-market were overshadowed by the possibility of getting one that wouldn't fit. Sometimes, original is the best way to go, plus it helps keep the dealer there for the next time an issue arises.

OK, now what's going on with the crack in the hood? I'll be watching for that thread.
Well, like an idiot, I left the hood up while I was tinkering with it in the shed. I moved behind the mower and thought "why don't I roll this out of the shed real quick for more room" so I did (very slight incline) and as soon as it did, the hood came down with a bang and cracked on the top of it

It's just about 5-6" and purely cosmetic but it really makes me mad!!! I will be looking into ways to fix it here within the next few weeks...
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