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Testing a coil with DMM

Posted by sfhellwig 6a SE Kansas (My Page) on
Wed, Jun 9, 10 at 12:33

I kept saying I wanted to learn to work on small engines, someone must have been listening. So my Craftsman 6.5h Eager-1 wouldn't start. Had been VERY reliable thus far. Neighbor helped me pull the plug and and check for spark. Nothing with two different plugs. More research and it's not the flywheel brake or the spark arrester. Since this is a very expensive coil I wanted to test it and was told to check for continuity between the ground connection and the spark plug terminal. Is this correct? If so I have a dead coil that cost about twice what one normally costs. Just need to make sure before I shell out that money.


Follow-Up Postings:

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RE: Testing a coil with DMM

As a general rule a digital multimeter is not going help diagnose a coil. Make sure the wiring to the on/off switch is working as it should. Without a coil tester about all you can do is eliminate everything but the coil.


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RE: Testing a coil with DMM

  • Posted by baymee LehighValleyPA (My Page) on
    Wed, Jun 9, 10 at 20:11

The Eager1 machines were very well built, so you might want to buy a coil before junking it.


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RE: Testing a coil with DMM

Before you junk the mower do this 1st, Remove the coil clean it the best you can with a cloth and a bit of paint thinner and then bake the coil in the oven at 250 degrees for 25 minutes re-install and the should do it.


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RE: Testing a coil with DMM

I borrowed a friend's mower and decided that for the money I WILL repair this one. I too believe it to be well built even though I have been told that Tecumseh motors suck. This one took over 10 years to have any issues. I also came across a copy of the owner's manual with the magical part number showing me that the coil can be purchased for about $30 instead of nearly $60 that the local shop had told me.

I was really only hoping to use the DMM to check for a broken/open coil. Not knowing what the coils should test at I can't tell if it's shorted or not but it seems to be pretty accepted that you can check for open. However I need to make sure I am doing this right because both the Craftsman and my spare parts mower both seem to have open coils. I have also found documentation for testing the coil with a set of points and a battery. I would like my neighbor to help try this as he has done it before. Also virtually no chance of finding a coil tester around here. If I did I would pay as much to test as to replace.

As for baking it, I'll give it a try if the battery test fails. Care to explain how that's supposed to help? Since my cost cut in half I'm sure I'll just order it. But I'd rather KNOW that's it so I don't get it in and still fail to start.


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RE: Testing a coil with DMM

  • Posted by baymee LehighValleyPA (My Page) on
    Thu, Jun 10, 10 at 21:38

Coils are more than the old step-up coils of years gone by. Now they have ignition modules in them too.

Years ago you could ohms check the windings for a short and if your motor used points, you probably still could.


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RE: Testing a coil with DMM

The only way I know to test and electronic ignition coil is with a Mercotronic or Graham-Lee dynamic tester. Cost about $300.

Old breaker points and condenser system is easy to test with a DMM or VOM, in fact analog is even better for testing consensers.


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RE: Testing a coil with DMM

I guess I was trying to learn too much too fast. I was trying to apply principles that work for other systems. I have ordered a coil for a decent price. I guess in the future I know that as mentioned above, eliminate everything but the coil.


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RE: Testing a coil with DMM

"bake the coil in the oven at 250 degrees for 25 minutes"
"Care to explain how that's supposed to help?"

I don't know why or how this works, but I have seen it recommended numerous times, so it must work sometimes for some people, although it is reported to be unly a temporary fix.

At least one could give it a try and if the machine starts when it wouldn't before, that would give a pretty good indication that the coil is defective.


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RE: Testing a coil with DMM

i'm a printer technician. there were recommendation, and i've tried it myself and worked, of baking some faulty circuit boards for some HP laser printers. the problem was caused by faulty solder joints and the purpose of baking is to re-flow the solder and hopefully giving you a good connection again.

i'm guessing it's the same with the coil unit since it has ignition circuitry in it too.


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RE: Testing a coil with DMM

  • Posted by baymee LehighValleyPA (My Page) on
    Mon, Jun 14, 10 at 20:33

Solder flows at what? 600 degrees? I think the circuit board would look like the peanuts I tried to roast the other night at 450 degrees; black eyed peas.

The only thing I can think of is that it drives out any moisture that affects the electronics in a compromised coil sealed shell.


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RE: Testing a coil with DMM

FYI, the Tecumseh motors that Sears used in their Eager-1 series were far from sucking. I'd take a 15 year-old Tecumseh over any new Briggs on the Sears showroom floor right here and now.

I've had several and never had any problems with them that couldn't be fixed cheaply and easily. I was a little sad when Tecumseh went out of business, their products were generally top-notch.


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RE: Testing a coil with DMM

"Solder flows at what? 600 degrees?"

umm....no, sorry.

solder used in electronics melts below 200 deg celsius. typically about 170 deg c.


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RE: Testing a coil with DMM

63-37 (Eutectic) solder melts approx 365F and also solidifies at same point 60-40 and 50-50 become plastic at slightly high temp, but don't fully melt until more than 400F is case of 50-50.

However, Baymee is tight about moisture. Although Tecumseh and Briggs coils are pretty robust, Lawnboy and Suzuki had coil problems which were related to encapsulation problems. Sometimes baking the coil at 300F to 350F would salvage the coil, until new moisture wicked into the windings. However, you weren't reflowing the connections.

As far as baking a PCB, I won't contradict the previous post, but since thru-hole parts were first wave soldered with higher temp solder, and then the SMD chips were reflowed with eutectic solder, any heating to liquidus temperature would cause the SMD parts to fall off like dead fleas from a dog.

When I was a tech, we would look for questionable joints or connections visually and then reflow with an iron using RMA flux.


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RE: Testing a coil with DMM

When you use a lawnmower engine for ten years before you have any kind of problem, don't let anyone tell you that engine sucks.


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RE: Testing a coil with DMM

Ten Yrs of Service without incident , pretty stout engine in-dead !


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RE: Testing a coil with DMM

Well the new coil is on. I have spark with the ground disconnected but none with it on. Unfortunately the kill contact is hidden up under the flywheel. Being so greasy I tried to clean it with cotton swabs but couldn't really do a good job. If it's just a contact I've got to be able to clean it but... can I use a typical puller to pull the flywheel? I read else where that was a no-no and should use an appropriate plate and three bolts that would thread into the wheel. That is obviously not how this one was meant to go. I just want to make sure a pulley puller was right before I shatter the flywheel and really junk it.

Thanks guys for the conversation so far. I never expected to learn so much on my first time out. This is a much more active forum than the other plant forums I frequent here.


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RE: Testing a coil with DMM

Yes, you can pop the flywheel w/o the 3 bolt puller- caution must be used. Remove the spark plug wire. Strap down the zone bail on the handlebar to release the brake. remove the flywheel nut (3/4"), washer and cup- then reinstalling the nut flush w/ the end of the crank. A sturdy pry bar between a more solid part of the engine and a more solid part of the flywheel. IE the thicker part where the magnet goes or opposite end and maybe a shroud boss w/ the bolt reinstalled. Pry firmly and pop the end of the crank squarely w/ a hammer. Unsure if you can pull it off- then get someone w/ a puller to pop it. I've seen where pulley pullers crack them.


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RE: Testing a coil with DMM

Just did mine last night without a puller (timing issue). Small pry bar , brass hammer . Rotate the flywheel 90 degrees pry a little rotate another 90 degrees tap end of crank , rotate additional 90 degrees pry , another 90 degrees pry and strike the crank and off comes the flywheel . Another method I have used numerous times on clutchs or flywheels when a puller was not available is to make two small (hardwood) tapered wedges place the wedges 180 degress apart and tap them in place while gently striking the crank end witha brass hammer . Normally one 180 rotation of the wedges will suffice , if not continue the process while watching the crank end play movement . Penetrating oil sometimes applied to the shaft over night will be required . In either instance do not over be impatient or apply undue force on either hammer or pry bar .


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RE: Testing a coil with DMM

  • Posted by baymee LehighValleyPA (My Page) on
    Wed, Jun 16, 10 at 6:11

As stated above, be sure to tie down the bail, as you can ruin the brake by removing or installing the flywheel when the brake pad is against it.

I use a heavy screwdriver to pry up with force below the flywheel and hit with a hammer from above. I use the tools that protect the crankshaft from the hammer. You want to be careful where you put the screwdriver because there are some soft spots in the engine case.

Torque to about 30 ft. lbs. These flywheels can crack easily if too much torque is applied.


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RE: Testing a coil with DMM

Tonight I will get my biggest screwdriver and a hammer. Hopefully the neighbor has some prybars and maybe a brass hammer. I would really like to use a puller but renting one will be last resort as they sound like the less safe route. I will just have to see what combination works and keep an eye on all parts.

Baymee, you said " I use the tools that protect the crankshaft from the hammer." Could you please elaborate. The nut on the end of the crank helps but too much force will jack the threads still.


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RE: Testing a coil with DMM

  • Posted by baymee LehighValleyPA (My Page) on
    Wed, Jun 16, 10 at 11:27

The proper tools fit over the top of the crankshaft or screw onto the threads. Even using a brass or bronze hammer can damage the end of the crank if you are aggressive.

I don't know if you can see a Stens catalog online, but they sell them in the tools section.


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RE: Testing a coil with DMM

I have seen more Light Flywheels damaged by Novices with the improper type of puller do more damage than anything ! Gently tapping on the top of the crank will not do any damage threads or bearing included . A Proper Puller at times requires that you tap on the Hex head threaded rod other wise you will strip out the internal threads on these light flywheels , thus the alternate removal methods suggested . Anyhow seems were getting away from the Jest of the Thread "Coil Testing via DDM" I would suggest that if you have not done routine flywheel removal previously you had better not attempt such work , since Tomplum and I have probably removed more types of flywheels than you have owned engines.


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RE: Testing a coil with DMM

Yes, this thread has run it's course. I will be starting another to address the flywheel removal. I understand that the pulley puller is not the right option but not doing it is NOT an option either. And all 2 of the mower-repair shops around here are pretty useless. One has never answered the phone and the other stopped me with "I don't work on Tecumsehs" so it's all me or none.


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RE: Testing a coil with DMM

sfhellwig: No Problem Dude ! I can see that you are apparently getting the run around by your local Repair Facilities and are feeling the frustration. Since you are the Originator of this Thread no foul in expanding the Coil Trouble Shooting Criteria lol .
I hate when Threads are Hijacked . If you follow what Baymee has advised for Safety (Bail & Torquing Specs) You should not damage the Bonded Brake Liner or Damage the Flywheel by over-tightening . If you do not have a Torque wrench use a normal 3/4" drive ratchet with 3/4" till snug unless you are a Gorilla no foul . If you feel a Small brass hammer is to evasive utilize a Fibre (Plastic) face mallet . Follow my or Tomplums advice and you will have the Flywheel of asap without incident ! I have removed two lawnmower and 3 Chainsaw Flywheels in the last month and only used 3 finger puller on the one chainsaw which was 40 yrs old it req'd the penetrating oil overnight soak since it was galled on pretty bad . Patience and gentle tapping & prying allowed removal without any thread damage . Hey don't fret $30-$60 I have seen some Coil / ignition modules go for $130+ . Sad that your Repair Agent is such a deadbeat , I have picked up used dealer units for a third of original price which outlasted the Original Mowers lol . Anyhow all the Best keep us posted and Welcome to the Site .


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RE: Testing a coil with DMM

Just to let everyone know who contributed to the thread, the mower is running. After putting on a new coil it was still shorted out. I ordered a new switch only to find out the old one was fine. It was the bracket. How it got bent that far I will never know but last night after alot of bending and test fitting, I was able to start it properly several times. This has been an interesting venture, nothing like crash learning. Thanks for for all the help. For less than $40 in parts and shipping I get to keep using the one I like instead of spending alot more for junk.


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RE: Testing a coil with DMM

Well Strange things do happen ! Look at it this way you have gained some experience , have some spare parts should you eventually need them and you did not damage the Flywheel or Burn Down the Garage lol . All in all a Win / Win Situation !


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