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Garden Tractors Discussion Forum

John Deere 318 misses

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06-18-2017 20:19:59

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I have a John Deere 318 mower that has a weird problem that I can't figure out. The engine runs fine until I engage the electric clutch. Once engaged it will idle fine but miss badly once revved up. I will run fine at high RPM with clutch disengaged. I thought bad clutch and swapped with a known good clutch but it still acts the same. I then tested coil voltages to see if this caused the miss. With deck disengaged, + voltage on coil runs about 13.5, the same as battery voltage. - terminal on coil runs about 10.5 volts. Once I engage the clutch, the positive terminal on the coil will run the same 13.5 volts. With engine idling the - terminal will be about 10 volts. Once you rev engine up with clutch engaged, + coil voltage remains at 13.5 volts but the - coil terminal will fall to about 6 volts and the engine will begin to miss badly and backfire through the carb. What could be causing this? I'm thinking it almost has to be the ignition module but yet everything is fine with the clutch disengaged.

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07-29-2017 07:08:07

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 Re: John Deere 318 misses in reply to Sixfoot, 06-18-2017 20:19:59  
Problem solved. After removing the engine and tearing it down, I discovered a cracked flywheel at the key cut. The flywheel would tighten on to the engine and went just a little to far and was in the process of destroying the magnetic ring for the ignition.Im amazed that it ran at all.

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06-20-2017 12:48:12

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 Re: John Deere 318 misses in reply to Sixfoot, 06-18-2017 20:19:59  
Even though your machine has electronic ignition, it also has a condenser, attached to the + terminal of the coil. Not the - as a points system would have.

I'd change that first. Cheap and can cause a lot of weird issues. don't have to buy an Onan coil to test the theory. any old GM points ignition coil will work for a test.

Then test the ignition pickup, located under the flywheel but can be tested in place.

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Mule Meat

06-20-2017 11:25:53

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 Re: John Deere 318 misses in reply to Sixfoot, 06-18-2017 20:19:59  
If it has the original 18 hp Onan engine. The alternator should put out around 20 amps at operating speed.

1. Start with the Battery. Make sure that it is good.

2. Voltage regulator/rectifier. Could be bad and not "communicating" with the alternator telling it to produce more amps once the blades are engage at speed. If it does not produce enough amps at speed the engine will 99% of the time be the first to suffer, doing everything from missing badly to completely shutting down.

3. Alternator not producing the amps that it is capable of under load.

4. Wiring system including the key switch.

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06-19-2017 18:36:26

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 Re: John Deere 318 misses in reply to Sixfoot, 06-18-2017 20:19:59  
I follow up Jeffcat's remarks, which are possible causes, with the question: Does it act the same with the deck belt off the clutch?

If it clears up-and you say you don't have that problem without the clutch hooked up and working the deck, from what I understand-then you probably have either a fuel supply problem or else possibly a valve or valve seat problem. Because now the only additional load on the engine is the electrical draw of the clutch itself.

If the problem is still there, it is probably electrical, as Jeffcat says.

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06-18-2017 22:12:59

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 Re: John Deere 318 misses in reply to Sixfoot, 06-18-2017 20:19:59  
I tryed to follow your diagnosis. Your electric clutch pulls around 5 amps. when ingauged. If you have a digital meter hooked across the battery terminals when it is running, turn on the deck. If she is showing 14 volts I bet she will drop to just a little over 12.5 volts. That is what my big Husqvarna does. It you are getting really wild readings you could have crappy battery cables or even if stranger a bad ignition switcy. Have had both. New switch fixed several problems. Sat out in rain way toooo much and was not good for it. That 5 amps. going through a junk switch can cause a multitude of sins. Keep us informed.

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