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Discussion Starter #1
I have an STX 38 that has started giving me a problem. I have been reading here a lot and have seen some good ideas, but I would appreciate some reassurance before I begin to buy parts. I am not a complete novice, but I am not very proficient with small engines.

It started by shutting off after about 30 minutes of usage (no fire checked at the plug). A couple of days later it started and worked again for several mowings. The other day it finally died. I have not been able to get it started again.

To understand my readings to crank = starter turning flywheel over

If there is a safety problem (seat, pto, park brake, or other) the mower will not crank.

At this point I am thinking the coil is bad. Any help is appreciated before I spend any cash shot gunning the problem.

Thanks Ken
 

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Very possible Ken.

Disconnect the plug wire and use an in line spark tester (tester connected to ground, not plug) and disconnect the armature kill wire and see what you get.

If you have an intermittent spark due to heat, this will not necessarily tell you (well, unless you test it when it stopped running warm), but it may give you an idea if it is in the kill circuit perhaps.

More than likely however, it does read as if you may have a coil that has just given up. But you never know.

Good luck.
 

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Discussion Starter #3
Thanks but I do not know where the armature kill wire is on this machine. Can you give me a bit more information of how the armature kill circuit works.
 

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JD318
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If you didn't have spark, I'd say coil. Not sure if there is a kill wire on the STX38?
 

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Pull the shroud off the engine (if you haven’t already) and find the ignition armature / coil. There should be a small single wire going to the rear of the coil. I want to say a white wire labeled 940?

Disconnect it and test for spark.

You have effectively isolated the ignition module from any ground or safety switches in the circuit. WARNING: You also will not be able to cut the mower off with the ignition switch if you choose to leave the high tension lead pluged to the spark plug, but stopping the mower shouldn’t be a problem.

Again, if you have an intermittent failure due to heat related break down, you will not be able to tell unless you test this after it runs for a while, that is if it will run.

An inline spark tester (inexpensive device...well they can be expensive) will allow you to test for spark when the engine is running and when it is stopping (costing down).

The test mentioned above, is to isolate the coil from the entire mower’s electrical safeties and grounds to see if the coil is working at all.

I hope this helps some. :thThumbsU
 

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As this novice understands it:

Initially, coils begin to show symptoms when it gets hot. The hot coil will shut down the machine. Attempts to start it will turn over the motor, but fail to provide a spark. If the machine is allowed to cool for fifteen minutes or an hour or so, the machine can then be started again, only to shut down when it gets warm. Eventually, the coil will become so weak that it will not start a cold engine.

When mine went, it went gradually. It would shut down and not restart after almost completing my half-acre. I'd let it sit for a half hour and then it would restart right away. Then I'd finish the lawn. It gradually got worse until I could mow maybe a one-hundred foot strip before it would shut down.

My neighbor, an ex high school auto shop teacher, leaned over the fence one day and said, "You know, when a motor keeps shutting down when it gets warm, it usually means a bad coil." Genius me ignored that good advise and chased an imaginary fuel problem for too long before replacing the coil.

New coil R&R required dismantling the fan cage at the top of the engine and pulling a couple more parts. Install of the coil itself required dinking around with three bolts and two feeler gauges. Not difficult, but a PIA for a few minutes because I'd never done it before. I'm a duffus when it comes to this stuff and I was able to do it by myself in about 1.5 or 2.0 hours. I would recommend this as a do-it-yourself project to anyone who can change their own oil.

I think the part was about $70 from Deere. They don' call it a coil though. I don't remember what they do call it, but the counter guy should know.

If you need a shop manual on CD, send me a PM with your mailing address.
It's too big to email. Hopefully my description will help you decide whether or not this is a coil problem. Best of luck.
 
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