Bizarre coil spark issue

iamjohnny45

New User
My Massey Ferguson 165 has a bizarre problem with ignition spark. Typically the tractor will start and run perfectly when cold, but after about 2-5 minutes starts to misfire and the problem gets worse until it's backfiring and running so rough it eventually stalls.

When cold, the spark tester shows decent spark (up to 20Kv, is that normal?) but after she stalls, Spark tester shows no spark from the coil at all, even with the tester set to the minimum gap.

I've already replaced the entire ignition system; plugs, wires, rotor, distributor cap, points, condenser, and coil and yet the problem persists. I even rewired all the ignition related wiring going straight from the battery to the + side of the coil and completely disconnected all the rest of the electrical (thinking it might be some kind of short someplace). Also brand new wire from the - side to the points.

When I take off the distributor cap and hook up the spark tester I can see it's sparking at the points really badly which you can see in the below video.

https://www.youtube.com/watch?v=iNcTa_hVO3Q

Everything seems to be pointing to a bad coil or condenser but like I said they are brand new.

I'm completely out of ideas and short of replacing the entire distributor I have nothing left to replace.

Anyone have any ideas?
 
Put the old condenser back in and I bet the problem will go away. If it does not go away hot wire it and see if that fixes it. If being hot wired and it works like it should then the problem is from the coil back to the battery and you have bad wire or switch
 
Often an ignition failure AFTER shes warmed up may be due to a COIL OR A CONDESOR problem. Since you already replaced the coil the next suspect may be the condenser, and that's true even with a new one as many are bad even new out of the box.

Are the new points clean and free of any oil or residue that may have been placed at the factory?? Check and clean them.

Are they gapped correct??

That tractor doesn't have any sort of a ballast resistor does it??? On ones that do I have seen them heat up and fail.

A loose/resistive connection in the primary ignition circuit (Bat to Switch to Coil) can heat up and develop more resistance. Put a voltmeter on the coil with ign on and points closed when she's warm to insure full voltage there. If its low due to a voltage drop (switch or connections) that can lead to ignition problems.

While the points will normally produce a spark, if its excessive that sounds like a short, maybe the condenser ??? or the points spring assembly is shorting out?? Ive seen that happen but that would be hot or cold

I will put a link to my Ignition Troubleshooting Procedure but if yours is fine until she's hot I'm thinking a condenser or coil problem or else a bad connection.

Only other thing is are you using a full true 12 volt coil on a 12 volt system?? If a 6 volt coil was used with no ballast on a 12 volt tractor it will overheat and the points burn and carbon soon.

John T
John Ts Ignition Troubleshooting
 
Take the coil out of circuit, check the resistance across the primary terminals.

If it's around 3 ohms, it's a 12v coil.

If it's around 1.5 ohms. it's a 6v coil.

Regardless what the box said, or what is written on the can, check it. They are often miss boxed or miss labeled.

A coil that is operating at the proper voltage will be about the same temperature as its surrounding environment. If it is heating to the point it's uncomfortable to hold, there is a problem.

Also check the distributor shaft for side play. It must be tight for the points to stay set. A worn distributor will do strange things, the point gap will change at different RPM. It may check good not running, then change when started. Out of adjustment points will arc and burn, overheat the coil, give varying spark quality.
 
I tore apart my shop and manged to find the old condenser which I luckily didn't throw out. I cleaned up the points which were blackened from the arcing and I then Re-installed and re-gaped the points (.020). Then I made sure the points, capacitor etc were fully grounded by testing with a meter just to be sure.

Fired her up and this time she ran perfect for the 20 minutes or so that I tested so far. Did a few runs up and down the farm yard, lots of power and almost no misfire.

Previously it start misfiring in less than 5 minutes so all indications so far are this is solved!

Thanks everyone for your help.

Regarding the comment about testing the condenser; I'd love to do that but as far as I know there isn't any good way to do it unless you have a meter with that function (which I don't have). I know in theory you can use the ohms setting to watch it put a charge on the condenser then flip it to volts to see it discharge but that all seems pretty arbitrary to me.

I take the advice from another post I read about testing a coil with a meter: "you can prove it's bad, but you can never prove it's good."

Which in this case, given it runs until it heats up I would say is likely true.
 
johnny, Congratulations and thanks for the feedback

FYI What you read on another post is correct.

An ohm meter (if used properly) can tell you if a condenser is definitely BAD However it cant prove that's its GOOD. IE even if it passes the low voltage ohm meter checks with flying colors DOES NOT MEAN it could still not fail under actual high voltage and temperature running conditions. For yearsssssssssss Ive used my trusty old Analog Simpson 260 meter to test for BAD condensors but it takes an actual true condenser tester to provide more accurate information.

I had a friend who rebuilt mags and ignitions and he said almost 50% of the new bulk condensors he bought were bad or out of spec grrrrrrrrrrrrrrrr

PS if those points ended up badly pitted or burned you may want to replace them just to be safe.

Keep safe, God Bless America Keep her safe strong and great in these trying times.

John T
 

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