'72 IH 140 hot coil issue UPDATE 4 w/ DATA

Fifth and final thread.
I started 4 posts about a recurring hot coil issue on a '72 IH 140. It is 12 V neg ground with 10SI alternator, 12 V (3.1 Ohm) coil, and Pertronix electronic ignition. Tractor runs great for 30 min to one hour then spits, sputters, and cuts off. It won't crank back up. I notice the coil is red hot when this happens and the ammeter shows a large current drain. The tractor will not restart. I switch the key off and after things cool for around 30 min it cranks up, all seems normal, and things repeat. I’ve installed new coils several times included the Pertronix recommended coils and an ic14SB coil from Napa and a new alternator.

Tested all the voltages and resistance and the alternator, 12 V coil, etc. all seemed normal. Most recently I installed a 1.6 ohm ballast resistor in the ign switch lead to the coil as posters suggested which dropped the coil input voltage from 11.5 to 8.5 V. Tractor mowed fine for 70 min (a new record) and the coil was only warm (I could hold my hand on it). That was encouraging.

At posters suggestion I measured dwell on the tractor yesterday and it stayed 58 deg through an 800 - 1400 rpm range. I don't think the lower input voltage would affect dwell but I could be wrong. I believe this is 57 - 65% more dwell than a 4 cyl engine with points which other posters claim is 35 - 37 deg. Neighbor and I then measured the dwell on another IH 140 and a '77 Cub that have similar EI's and Pertronix coils. Dwell on the other 140 was 54 deg and the cub was 58 deg. It doesn't look to me like higher dwell is the cause of the hot coils since I don't have this issue with the other 2 tractors. I thought it might be helpful to post this follow-up data. I hope it keeps running but I can't say what caused the issue in the first place.

That is all. - Gator out

Got my dwell meter out yesterday and will do some checks on different tractors. We never used dwell meters on tractors as their slow rpms never really showed a need for it and no specs were given from IH. I used to on the trucks, especially the V-8's.

Pete23,

Your approach to helping someone out is why I like this site. That and I get to learn quite a bit as well.

D.

First off, your coil is not getting "red hot"... it would be interesting to check it with an infrared heat gun and see just how hot it's getting. As JMOR has told you they can get quite hot under even normal circumstances.

I can't remember... have you ever changed out the Pertronix module... if NOT, it my be "thermal" and causing the HOT stall/no start rather that the coil you have been focusing you attention on.

Thirdly, it would be interesting to watch the ignition waveform/check dwell with an ignition oscilloscope. I would suspect that the readings being made with simple (cheap) dwell meters comparing point ignition dwell and EI dwell are not accurate/reliable because of the difference in waveforms. Actual dwell can be checked against the graticle of an ignition scope. I have attached a 'scope tracing I photographed several years ago on an H or M tractor with points, with same annotation explaining what we are looking at. EI dwell can be checked in the same manner. Unfortunately, I don't have a 'scope tracing of a Pertronix EI handy to post. That is something I will do "down the road" when I get time.

Yes you're right. Red hot in this post means I can't hold my hand on it for 3 sec.

It doesn't get nearly as hot with the lower coil voltage while mowing and it never cut out as usual. Coincidence?

No, it wouldn't just suddenly "heal." Reducing the coil voltage definitely had a positive effect on the issue. Whether it's just masking the real issue or actually fixed the problem...

Did you measure dwell with or without the ballast resistor? It looks like you measured WITH.