farmall M has no spark

i have a 1950 M with a 12volt conversion, i've never had a problem with it starting until recently. i replaced the coil (tried both a 12volt and 6volt coil), i replaced points, condenser, rotor, cap, plugs, wires, and resistor.i had the alternator tested. it is wired exactly to the diagram that is on this site with the exception of the warning light but still no spark. any ideas? this tractor has always, no matter of the weather, fired off first shot. thanks in advance for any help.

First of all, as its now 12 volts you need EITHER a full true 12 volt coil (has around 2.5 to 3.5 ohms primary resistance) OR ELSE if you use a 6 volt coil you need the series voltage dropping (12 to 6) external Ballast Resistor after the ignition switch before the coil... If its a full true 12 volt coil DO NOT use any ballast...........

Since your question is so wide open I will post my Troubleshooting Procedure and if you work down through it the cause of non spark should become apparent. Its often an ignition points problem but run the tests below and try easy simple things first like for voltage getting to the coil via the ignition switch....Even if allll else is good a bad ignition switch (no power to coil when ON) can cause non firing (see Para 1 tests below)


TROUBLESHOOTING A BATTERY POWERED EXTERNAL COIL TYPE IGNITION SYSTEM:

PRELIMINARY CHECKS:

(A) To see if it happens to be a cap n rotor problem and to see if at least the coil is firing, remove the coil wire from the distributor (leave coil end intact) and place its bare end to within 1/8 inch from tractor iron, turn her on n crank her over, and see if she jumps that gap with a good visible blue spark?????? If so but the plug wire ends (from wire end to 1/8 inch to frame) or the plugs themselves don’t fire, its a cap n rotor or plug wire problem. If the coil wire isnt even sparking, see below.

(B) Next open the cap and see that the points are gapped correct and indeed opening and closing as the engine is cranked and the distributor shaft rotates and MAKE SURE THEY ARE NOT BURNED OR PITTED OR CARBONED UP BADLY !!!!!!!!!!!! If so, running a point file between them to clean them up might make her run again HOWEVER that’s only a temporary cure, so if that cleaning makes her spark, INSTALL N GAP NEW POINTS. In the event they appear good but only gray oxide coated, non abrasively clean/buff/polish them using say a dollar bill or shop cloth etc. and see what happens.


MORE TROUBLESHOOTING IF ALL THE ABOVE STILL FAILS TO MAKE HER SPARK


1) THE VERY FIRST THING YOU GOTTA HAVE is voltage to be present on the coils high supply (NOT to distributor) terminal when you turn the Ignition switch ON. If not she cant ever fire, but in the event the ignition switch or circuit/wire down to the coil or any Ballast Resistor is bad or open, you can HOT WIRE it by jumping a hot ungrounded battery voltage source to the coils high input supply (NOT to distributor) side n see if she runs then???? If she fires hot wired, you could have a bad ignition switch ((That can happen, when Ignition is on, the switches IGN terminal must turn hot)),,,,,,,or an open Ballast (if it has one) or a bad/open wire from switch to coil.

If the switch is good, if you turn the ignition switch on and place a test lamp on the coils high (NOT to distributor) terminal SHE MUST LIGHT UP. If not again, look for an open Ballast Resistor (if it has one, it should read around 1.25 to 2 ohms across its terminals) or bad/open wires from the switches IGN output down to the Ballast (if it has one) and distributor.


2a) When the Ignition switch is turned on, voltage should appear on the coils high input side. That would be 6 volts on a straight 6 volt system or 12 volts on a 12 volt non external ballasted system, or around 6 volts on a 12 volt system that used a 6 volt coil plus an external Ballast Resistor and the coil is good and the points are closed and they and ALL wiring is good.

2b) To insure the coils low voltage primary winding is not bad/open, use an ohmmeter and measure its DC resistance between its lil + and -terminals. If its an open circuit (no continuity) its bad/open and will NOT work. It should measure around 1.25 to 2 ohms or so if its a 6 volt coil and maybe 2.5 to 3.5 if its a 12 volt internally ballasted coil. NOTE CAUTION have all leads and any voltage source DISCONNECTED FROM the coil for this simple primary winding continuity test.

3) Next, place your voltmeter or test lamp over on the coils other low to distributor terminal side, turn her on and crank the engine over.

4) A test lamp there should flash ON (when points are open) and OFF (when points are closed) as the engine is cranked slowly.

5a) If the lamp never comes on there, the coils primary is bad/open,,,,,,,,,,or the points are never opening,,,,,,,,,or theres a shorted/bad condensor (remove its lead to points and see if lamp comes on, if so, bad shorted condensor or its wiring),,,,,,,,or the points wire is shorted,,,,,,,,or the distributors side pass thru stud is grounded (use ohm meter to test that),,,,,,,,,or the points may have a shorted spring.

5b) If the lamp never goes off as engines cranked, the points are not closing or are bad,,,,,,,,or the wire or circuit is missing from the distributor to the points,,,,,or the distributors not well grounded to the tractor.

She cant fire the coil unless its low side is getting a conductive ground return path via closed points and then the circuit is open when the points open.

Be sure the condensor or its wiring is NOT shorted out and see if the lite comes on (when points open) with the condensor disconnected. If removing the condensor makes her spark, replace the condensor.

SUMMARY

Be sure the points are closing fully and open on high cam and ARE NOT BURNED OR PITTED OR CARBONED UP BADLY,,,,,,,theres voltage present on distributors high side at all times when ignitions on (or its a bad switch or open ballast or bad wiring to col),,,,,,,voltage on coils low side flashes on and off as distributor is cranked,,,,,,,,,condensors not bad/shorted,,,,,,,,no shorts in wires to points and no shorts in pass thru side out distributor stud,,,,,,,,coil has continuituy.

You may luck out n just need a new set of points. If the coil wire fires (see above) and the plug wire ends to 1/8 from frame but NOT the plugs, they are badddddddddddd. Check them BOTH.

Good Luck n God Bless, post back any questions and your findings and any questions.


John T Nordhoff in Indiana, retired electrical engineer who usually lurks over on the Mother Deere boards versus over here on the “dark side”.

Do you have current to the coil? If not, start with that.

If you have current coming in to the coil, do you have current going out to the points?

Not trying to insult your intelligence here...but the best way to diagnose the problem is to determine what part IS working, and trace things in a systematic and methodical way until you find what isn't working. No sense in throwing more parts at the problem if you haven't determined whether the wiring is working right. Just start at the beginning--current to the coil--and keep working downstream until the problem becomes apparent.

The alternator has nothing to do with your engine firing. Its job is to keep the battery charged.

You need to check for battery voltage at your coil. I would use the 12 volt coil. The hot wire from the ignition switch goes to the + terminal on the coil. With the points open & ignition switch on you should have battery voltage on the small terminals on the coil. Use a volt meter or test lite when checking. Rub those new points with some 400 wet or dry. If there's no voltage at the coil you need to check the ignition switch for voltage. Could be its defective or its not being fed battery voltage.

You need to check with your volt meter to see if there's voltage being fed to your ignition switch
could be a broken wire or a bad connection. You should've done this before changing parts. It's very easy to check for battery voltage. Hal

Look in the distributor where the primary voltage (12 V.) wires are connected; you may have accidentally grounded the wire inside.
Disconnect the wire on the outside of the distributor and check for voltage. Hold the points open & re-attach the wire; if there is a spark at that terminal, the inner connection is grounded.

All good advice for you here but there are indications in your post that you are, like myself, mechanically challenged. The response by John T below is included in my personal archives for my Farmall B and I have used it repeatedly. Print it out and take it with you to the tractor and I bet you will find the problem. I always start with the center coil wire. If it sparks I move toward the distributor, if not then back toward the switch.

thanks to all who got me going in the right direction. the first thing i found was my old multi meter was giving me bogus readings and leading me astray so i broke out one i had that was new in the box and things began to make sense. i've never worked on anything that had a condenser & points but i found out through the step by step troubleshoot that my condenser was grounded out in the dist. housing. the wire insulation had been pinched just slightly enough to expose enough wire to ground. couldn't see it until i pulled it out. anyway she is up and running like a sewing machine again. thanks guys.