Farmall H4 Magneto

For the last several months my "49 Farmall H would absolutely refuse to restart after being shut down hot. Let it sit a couple of hours and it"d start right back up and run good. Plenty of fuel and wet spark plugs but no spark. All the usual tune up items done including 4 new 3116 Autolite spark plugs, new copper core spark plug wires, new points and condenser, new rotor and cap, even a brand new coil. (The old one was date stamped 3/49) Still no spark out of the coil wire when hot. I removed the coil cover and checked continutity between the little spring tab that contacts the coil output and sends it out to the center hole of the distributor cap. Zero ohms so I figured it was good. Installed it back on the mag and it started just fine. After about 45 minutes run time I shut it off and it wouldn"t restart. Once again I pulled the coil cover off and it checked good again with the ohm meter. In desperation I had my wife hit the starter button while I held the end of the coil wire directly on to the coil output with the coil cover removed. The engine started instantly. The only thing that I can figure out is in the 5 minutes or so it took to remove the coil cover and take it into the shop to check it out it cooled off enough to remake continutity and after running for long enough to get hot it would open up again. Anyway, with a new coil cover installed I ran it for 6 hours yesterday with several shutdowns and hot restarts with absolutely no problems. Sorry for the long post but I thought it might help someone else out someday.

Glad you found the problem. Buddy of mine has had problems with a restored WW II jeep not running right over 35mph. for the last two months. Turned out to be a weak spring on the points, so you just have to check it all.

What do you mean by coil cover? This is one of the places I have yet to go on my Farmalls.

Magnetos have a built in coil with a removable cover. This cover has some electrical parts incorporated which complete the path for the electric current from the coil to the cable that goes to the center of the distibutor cap.
Apparently, continuity of this path was sensitive to heat. A good reminder; I would never have thought of this failure mode.

Well, let's be careful here. The H4 coil is absolutely replaceble and a better way of putting it is that the coil is enclosed. The coil cover does have a contact tab on it for the output. If the contact tab is not making appropriate contact with the high side of the coil, then yes, it could have intermittent spark. That contact tab also has to be clean and free of corrosion.

I can see where if the contact tab was barely touching the coil, upon being heated, it could have enough air-gap to not work properly.

In any case, good for you that you located the trouble.

Tom, my thoughts were also that there would be some corrosion on the contact tab but it was clean and seemed to be making good contact with the coil. I shined it up anyway with a piece of fine sandpaper and it made no difference. I did do a little experment last night with wifey's hairdryer. With one ohm meter lead connected to one end of the spring tab and the other meter lead to the well where the coil wire plugs in I was able to lose continutity after heating the coil cover for about 2 minutes. After cooling down for about 30 seconds there was once again continuity. So it is definately heat related. That coil cover might have been on there since my grandfather bought the tractor new in 49. At least I don't remember it ever being changed in my 62 years.

Mark, you definately need a new coil cover cap. My post was more about the fact that the coil is not "built in", but rather enclosed. Not taking a shot at anyone here, just clarifying that the coil can be replaced if necessary. Coils that have an intermittant open when warm will behave the same way.

Tom, I understood what you were saying. My first thought was
also the origional coil breaking down when it got hot. However
replacing it made no difference. I do have a new coil cover
installed and have had no problems with hot starts. Thanks for
your comments.