B&S twin cylinder spark troubleshooting

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  #1  
Old 10-02-15, 09:14 AM
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B&S twin cylinder spark troubleshooting

So I picked up a parts tractor last week.

In repairing the ignition system (someone had made a mess out of it) I accidentally sent +12v down to the ignition coil kill switch wire and the smoke came out.

So, I grabbed a "this worked when pulled" coil out of my parts pile and strapped it in, and got nothing.
So I ordered in two brandy new ones- one for now, and one for next time.

Well, with a brandy new coil, I can't get spark. So... what's the procedure for testing these things?

As it sits, it's a sealed unit with two spark plug wires and one kill switch wire. With the kill wire not connected to anything, and the gap set to .010" and the engine turning over, I don't get even the weakest of spark.

I cleaned the flywheel magnet with light sandpaper, I cleaned up the contacts where the ignition coil bolts to the body of the engine, and I'm testing it with the sparkplug grounded to the engine head.

Also, holding the bare plug wire ends in my hand while I'm grounded, I feel nothing.

Tried with two new coils and one "I know this worked when pulled" unit- I have a hard time thinking they're all bad. What else is there to try...?

Replacement ignition coil for Briggs & Stratton 394891: Amazon.com: Grocery & Gourmet Food
 
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  #2  
Old 10-02-15, 09:42 AM
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Update:
Being frustrated, I tried the used but used to work coil again- and got a spark. A weak one, but a spark. Going to see if it runs like that.

Baffled- are there more than one kind of coil like this, maybe? This older one looks like it might have a capacitor or something in there?

https://dl.dropboxusercontent.com/u/...2012.39.31.jpg

Here you can see the coil that came off- looks the same as the one I cooked.
You can also see the engine in the background- very common 18hp B&S twin
 
  #3  
Old 10-02-15, 09:57 AM
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Are you disconnecting the kill wire @ the coil?
 
  #4  
Old 10-02-15, 12:46 PM
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A twin would have the diode in the kill wire harness. You probably blew that with the 12V charge you sent down the kill wire.
 
  #5  
Old 10-02-15, 01:20 PM
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I definately cooked the origonal coil- the used one is in and seems to be working (although the spark seems awful weak, to me)

the two new ones (amazon link) won't spark at all, with the kill wire disconnected from everything. They do NOT have the capacitor/diode/whatever it is, though.
 
  #6  
Old 10-02-15, 02:07 PM
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The diode would be in the wiring harness for the common kill wire hookup. If your magnets and coils are good and the coil air gap is close to .010, you will get a spark at the plug wire with the kill wires disconnected at the coil.

What you're describing is what you get with a bad diode.
 
  #7  
Old 10-02-15, 02:14 PM
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I'm not sure where the diode lives in this case, but the tractor is up and running with the coil assembly off the parts engine now.
I could not get a spark with either of the brand new amazon-linked coils with the cutoff wire completely removed.

What are the chances they're both no good? Is there a way to test them with a multimeter? I don't mind keeping parts like that around for when they're needed (Family now has three of these engines in service) but if they're junk I should probably send them back.
 
  #8  
Old 10-02-15, 03:53 PM
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In looking at your picture I see you have one coil with two leads = no diode. When you mentioned the ones from Amazon I thought you had a pair you had replaced at the same time.

I can't say why those two would cause problems and a coil with a weak spark would perform well.
 
  #9  
Old 10-02-15, 08:34 PM
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Be sure you are installing the coil right side up or it won;t work. Also, it only took a couple of tries using aftermarket coils before I decided never to do it again. I've had so many of them to be bad right out of the box or shortly after.
 
  #10  
Old 10-03-15, 09:08 AM
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I couldn't find an indication of direction, so I tried each of the new coils in both directions without luck.

I also suspect I'm done with aftermarket coils after this experience.
 
  #11  
Old 10-03-15, 10:15 AM
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The side with the connector for the kill wire goes down towards the cylinder. Usually they are stamped with something like "This side down" or "Cyl. Side".
 

Last edited by cheese; 10-03-15 at 03:06 PM.
  #12  
Old 10-03-15, 01:27 PM
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You might try cleaning the area where the mounting bolts go in for a good clean contact.
 
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