Troubleshoot a coil

Is there a way to test or troubleshoot a coil on my tractor other than swapping it for another one. It is on my Hydro 70 with electronic ignition and 12 volt coil.

Yes (for 12v coil). Measure the resistance across the two small terminals (when disconnected) 4 ohms maybe 4.5 is OK. Check from coil can to terminals Should be infinite ohms (no conduction) Measure from either terminal to the high voltage tower. It should have between 8000 and 15,000 ohms. If either terminal to terminal test has no conduction, or is way out of range, The coil is bad. HOW ever the coil may be bad when subjected to real voltage with a path to ground, or be affected by heat buildup.
A 6v coil, or one with an external resistor required will measure 2 to 3 ohms in between the terminals, and same as the 12v on the secondary windings. 12v coils made today don't have resistors inside them, they are wound with more and finer wire to make the higher voltage work. Jim

You can make continuity tests of the primary and secondary, and check for a winding short-to-ground, but even one shorted "turn" (which won't show up on an Ohms test) will waste spark energy (convert it to heat) deep inside the coil, making the spark weaker.

Also, coils that test "good" when cold can act up as they (and the engine) heat up.

I have an old Army Surplus coil tester that "ring tests" them under high voltage, and also heats them up if you continue the test for a while, but even that's not 100% conclusive that a coil won't act up while in use.

If you have access to an ignition oscilloscope, you can observe the waveform of the coil under operating conditions, and observe a "before and after" if you've got a problem that shows up after the engine runs/heats up for a while..

First photo shows a normal spark pattern, note oscillations in the "spark duration" and "decay" period. These show the coil "ringing".

Even a "shorted turn" (or several) will cause a pattern more like the second pattern, and indicate a "bad" coil".

<img src = "http://oi68.tinypic.com/28it9ap.jpg">

<img src = "http://oi65.tinypic.com/2dw9f1d.jpg">

(The first pattern is of the primary, looks like the second is of the coil secondary. Primary and secondary patterns are very similar in the area we are talking about here, but vary at the "points close" area. With "good" points, primary pattern will show a sharp cutoff when the points close, secondary will show some "ringing" there.)

I meant to add, if one has several similar "points ignition" machines/vehicles, it's not a bad idea to have a generic new coil on hand to swap into the circuit and see if this corrects the problem.

If it does, the "test coil" can be put "back on the shelf" for next time, and a coil more specific to the machine in question can be purchased and installed.

An easy way to test a coil on the tractor is:

Remove the small wire on the coils output terminal (- if Neg ground) that wires to the distributor (or the elec switches input) and connect a jumper wire there (on coils small output terminal) instead.

Remove the big coil wire off top the distributor cap (leaving the coils end intact) and rig it so its steel/brass tip portion is about 1/4 inch away from the block or head etc so you can observe a spark jump.

Turn the ignition switch ON and verify there's full battery voltage present on the coils input terminal (+ if Neg ground) NOTE if the ign switch is bad or suspect simply hot wire voltage to the coils input to be safe.

Take the jumper wire and touch it to a good frame ground so it conducts coil current (mimics points closing) and remove it.

EACH TIME ITS GROUNDED THEN REMOVED THERE SHOULD BE A GOOD SPARK OFF THE COIL WIRE TO GROUND.

Of course using an ohm meter similar to Jim's tips there needs to be continuity of 1+ to 4 ohms (subject to coil) between the small + and - terminals or its bad/open. There should be in the neighborhood of say 5000 to 10,000 ohms from the HV tower output to the small terminals or its bad/open. HOWEVER it can pass both tests and still be bad due to HV breakdown especially once warmed up. The ohm and continuity test failures can ONLY show its BAD like a full open circuit, but it can pass those and still be bad !!! It may still pass these tests but fail once warmed up !!

NOTE an elec switch just replaces mechanical points opening and closing. The coils output just wires to an elec switch versus mechanical switch. The coil still needs good battery voltage on its input and you use a jumper wire on output to ground to mimic elec switch or points first closing then opening which produces HV on the coils HV output tower

Are you sure the elec switch is doing its job? Still have the old points and condenser to try ??

John T