•1972 IH 140 coil UPDATE 2 w/Data


I posted the last 2 days about a recurring hot coil issue on a '72 IH 140. It is 12 V neg ground with 10SI alternator 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. I switch the key off and after things cool for around 30 min it cranks right back up, all seems normal, and things repeat.

I checked voltages at several points while running yesterday (not mowing) and everything seemed normal (alt 14.6 V to frame, coil neg terminal to frame 11.3 V) except the coil got too hot to hold a hand on after 60 min. Afterwards I swapped that ic14SB coil (3.4 ohm) for a used pertronix coil (3.1 ohm).

Today I installed a 1.6 OHM ceramic encased ballast resistor in the wire from the ignition switch to the + coil terminal.
This dropped the voltage to the coil from 11.5 V to 8.5 V (measuring from the + coil terminal to the frame). The current must be 3/1.6 = 1.9 amps.
The tractor started right up but I didn't run it very long. I'll try mowing with it later this week. The resistor got pretty hot pretty fast.
Hopefully this will help increase run time.

The plug wires are a solid core high performance set from Brillman. The tech reps from both Brillman and Pertronix said the plug wires were not an issue. The resistance on these wires I measured on the 1 foot wire from the coil to the distributor input was 0.5 Ohm.
The Pertronix tech rep also said their flame thrower coils (which I switched to) get hot, up to 200F, and can burn you if touched. This just didn't sound right (I haven't noticed this on my other 2 tractors) so I am trying the ballast resistor as recommended by several posters. The rep said if the tractor cranks up then the coil is still good. He suggested the hot coil is just a coincidence and something else is the issue. Since the tractor runs great after the coil cools off I'm not so sure I concur. I checked the fuel lines and both strainers and everything is clean.

Any thoughts on whether the lower coil voltage will damage the ignition module, plugs, etc.?

If this doesn't fix things I might try the Ignitor II coil and ignition module. They're pricey though.
 
If it fired up, it will be fine. Winter cold might reduce the cranking voltage below Petronix low threshold. If it does, a bypass can be used. Run a 14 gauge wire from the starter side of the starter relay (solenoid) to a 5 amp diode (or used diode from an alternator) placed so current can go out of the connection, but not in. The out put of the diode goes to the coil side of the new ballast resistor.
This allows the start relay output to the starter to be fed to the ignition, giving it voltage above that of the ballast resistor. The diode prevents ignition voltage from trying to crank the starter (which smokes wires). Jim
 
Jim, can you recommend A 5 amp diode for this purpose. I know nothing about diodes.

See my post below just 3 down list "Bypass resistor on 12 volt conversion"

Thanks, Mark
 

Jani,

I have a 4 pack of Radio Shack Rectifier Diodes (P# 276-1661). These are 6 amp (60 deg C) at 35 V. Will one of these work?
I would run the wire from the starter input stud to the output of the ballast resistor (which connects to the + coil terminal). The banded end of the Diode would go toward the ballast resistor if I'm understanding you. It will be easier to connect this new wire directly to the coil + terminal, which should be OK, right?
 
Those should be fine. Diodes stripped from a salvage alternator rectifier bridge are also good, but require some sawing and attaching wire. Jim
 
However, I would not worry about it unless it becomes a problem. In a pinch you could manually bypass the resistor with a jumper wire.
 
The Delco 10SI rectifier bridge diode is most likely at least rated for 25 amps. You can get heavier rectifier bridges with 50 amp diodes for higher output alternators.
 

That is an interesting point, but I can't understand its relevance to this topic. I'm missing the relevance about the alt bridge diodes.
 
The Petronix kits are sensitive to the proper total resistance from the ignition switch to the control unit. The instructions do not generally specify whether you should use a resistor or not, but will mention a given resistance for 4 cylinder, 6 cylinder and 8 cylinder engines. The Petronix coils are available in different resistances to match the application. In 1972, your tractor was equipped with a 12 volt coil that was internally resisted. The internal resistor (typically about 3.5 Ohms) would limit the voltage to the points to 6 volts while running.

We have had several customers experience the same problem as you, and they either went back to the points, or matched the resistance to the Petronix instructions.
Agri Services
 
(quoted from post at 23:28:04 03/26/15)
That is an interesting point, but I can't understand its relevance to this topic. I'm missing the relevance about the alt bridge diodes.

They may be a cheap source of diodes that can carry a pretty good amp load.
 

Thanks for your insight Jim. It's comforting to find out I'm not the only pulling my hair out over this.

The Pertronix instructions only say the coil must have a minimum resistance of 3 ohms. I have used their coils and others that have resistance of 3.1 to 3.4 ohms with similar results (coil getting red hot). If the tractor would just keep running I wouldn't care.

I've added a 1.6 ohm ballast resistor between the ignition switch and the coil, dropping the coil input voltage from 11.5 to 8.5 V and the tractor starts up nicely. The red wire from the ignitor module I connected to the input (+) of the coil which bypasses the resistor. The instructions say to connect that red wire to the input of the ballast resistor (but this assumes a 6 V coil). I'm hoping the lower coil input voltage will reduce the operating temperature of the coil. I'm assuming a red hot coil is causing the tractor to sputter and cut out after mowing for awhile.
 
Everything you state is correct ---except--- there are no resistors inside coils. The resistance of a 12v coil is developed from smaller wire diameter and more of it. In the 1920s there was a coil made with a built in resistor and (IIRC) a third terminal. Because true 12v coils have "no external resistor required" printed on them, there is an assumption that they have a resistor inside. Nope. Not a flame, Jim
 

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