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JohnF2015, Small Engine Technician
Category: Small Engine
Satisfied Customers: 1132
Experience:  Owner/Service Technician at S & J Small Engines
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I have a FC420V DS00 engine in a Deere mower, that just

Customer Question

I have a FC420V DS00 engine in a John Deere mower, that just would not start one day. Checked the ignition and found no spark. Change the coil, as it checked out bad per the resistance checks I found on line. After I changed the coil, I still did not have any spark, so I changed the ignition module. Still nothing - I thought. With the spark tester I can see a very weak spark, but with a spark plug grounded, I cannot see anything. It appears I have a very week spark. I have disconnected the grounding wire to the coil with no difference. Any thoughts?
Submitted: 1 year ago.
Category: Small Engine
Expert:  JohnF2015 replied 1 year ago.

HI, my name is***** will be assisting you.

On this system you can't disconnect the small wire going to the ignition coil because it is actually a feed to trigger the ignition coil to fire. There is a small trigger unit mounted to the outside of the engine block that has a wire going to it and then from it to the ignition coil. Most likely the trigger unit is bad causing your problem. The trigger unit or module is about 1" square and normally gold in color. If you haven't already replaced it then this is where I would start since you have already replaced other parts with no needed results.

Thank you it has been a pleasure to assist you.

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Expert:  JohnF2015 replied 1 year ago.

I am including a diagram of the ignition system to maybe help you better identify the trigger module.

It is the part that is referenced with 21119A in the picture.

Customer: replied 1 year ago.
I also changed the ignition module - or trigger unit. It did not make a difference.
Expert:  JohnF2015 replied 1 year ago.

Have you checked the condition of the magnet that is on the flywheel? Will it hold a medium sized screwdriver straight up or not?

Customer: replied 1 year ago.
I did check it. It will hold a 7/16 wrench with no problem
Expert:  JohnF2015 replied 1 year ago.

Ignition coil is gapped at .010 inches to the flywheel?

Customer: replied 1 year ago.
I actually gapped it at .012 - I found that number on line. I can certainly reduce it to .010 if you think it will help.
Expert:  JohnF2015 replied 1 year ago.

In my experiences .010 normally works better. That is where I become accustomed to setting all coils in my 28 years of small engine repair. It just seemed like I had better dependability and life from the coil.

Customer: replied 1 year ago.
OK - I will do that and try it.
When I said that I disconnected the grounding wire to the coil, I disconnected the wire that goes back to the keyswitch. the wire from the ignition module to the coil was still connected.
Expert:  JohnF2015 replied 1 year ago.

OK - that should have eliminated everything backwards in the rest of the system then.

I would readjust the ignition coil gap and see if that doesn't help. If not it is possible that you may have received a weak or bad coil, at which point I would request a replacement from the place that you purchase it from.

Customer: replied 1 year ago.
I just gapped the coil down to .010 - with no change. Still extremely week spark.
I was also thinking the new coil may be bad??
If I get a replacement coil in a day or two, can I continue this conversation?
Expert:  JohnF2015 replied 1 year ago.

Yes, all you need to do is come back here and we can continue from here.

Customer: replied 1 year ago.
Expert:  JohnF2015 replied 1 year ago.

You are welcome. I will be waiting to hear back from you in a couple days.