Farmall Cub

any ideas why my ignition switch would suddenly short out ? smoked and burnt the insulation of the wires. Just replaced my coil because it was over heating, now I am wondering if a bad ignition switch would cause the coil to heat up? I'm guessing the switch is grounding itself? 1970 Cub 12 volt.

I should add this is a stock key switch/ kill switch and not the starter button.

A + (hot) wire might have gotten grounded, leading up to your switch letting its smoke out!

OK So since the ignition switch feeds the coil, if the ignition switch was shorting, would that cause the coil to overheat? I'm guessing that it would.

Hi,
TM Tractor has a new switch, this should be the switch for a 1970 Cub. It says it is a Case IH part. It gives the tractor serial numbers in the listing.

http://www.tmtractor.com/new/el/816fp.htm

yellercub wrote:OK So since the ignition switch feeds the coil, if the ignition switch was shorting, would that cause the coil to overheat? I'm guessing that it would.

Maybe if it failed so it stayed "on" when in the off position. It would be like leaving the switch on. But it seems like kind of a long shot.

is the wiring grounded out damaging the switch ?

yellercub wrote:OK So since the ignition switch feeds the coil, if the ignition switch was shorting, would that cause the coil to overheat? I'm guessing that it would.

More like something got grounded somewhere and smoked your ignition switch!

That is my bet!

I hope you can find the root cause before you replace any parts and possibly have the same thing happen again!

Replaced the ignition switch and all seems well. Coil is now not overheating and mo loss of power. first thing I noticed was ammeter showed good charge after switch was replaced.

Is that the same key switch as on the ‘77 and ‘74? If so, the tabs on the back are concerningly close together. I splay mine apart a bit, just to get a bit more clearance between the spade connectors. Easy to see how a bit of detritus plus humidity might cause at least a partial short.
The switch on my ‘77 came apart internally, ceasing to functionary have any detents or limits on key rotation. There are springs and ball bearings and brass tabs in there. If something gets out of place, sure, it could short internally as well.
Not sure what the situation was with the ‘74 when it came to me. The first time I turned the key, the back popped off the switch and it puked its guts out behind the dashboard.
Can’t say I’m a huge fan of that switch design. Doesn’t seem terribly robust. On the other hand, both switches were nearing 50 years old, so I probably shouldn’t complain.

Not sure if it is the same switch as the 74 - 77, but is just an on/off switch, you still need to push the starter button. Yes the tabs are only maybe 5/8 ths of an inch apart.

Crimson Tim wrote:Is that the same key switch as on the ‘77 and ‘74?

Hi,
IH changed to the key start switch at Cub serial number 239837, which was during 1972.
Before that serial number they used an on, off key switch, and a push button starting switch.
The link in my post above shows the earlier switch and the serial numbers when it was used.

Below is the newer switch at TM Tractor, it has 3 wire connections. The connections are closer together on the newer switch in the pic, than on the older switch in the link in my other post above.

http://www.tmtractor.com/new/el/907fp.htm

yellercub wrote:OK So since the ignition switch feeds the coil, if the ignition switch was shorting, would that cause the coil to overheat? I'm guessing that it would.

No. If the ignition switch was shorting out, the coil would not be getting any power, so it would not heat up at all.

A coil would only overheat if it was internally shorted itself, or if the voltage it was receiving was too high (i.e. bypassing the ballast resistor), or if the ignition switch were left on with the tractor not running and the points closed.