Hot coil

[Charlie M]

My 300U runs great for about 30 minutes, then starts to loose power and even starts to skip. After it sits for a few hours it runs normal again. It started last fall when I was brush hogging. When it runs bad the coil is so hot you can't tough it so that is where I thought the problem was. Last fall it was a 6 volt coil with a resistor (12 volt system) so I put on a 12 volt coil that doesn't need a resistor. It is doing the same thing, getting to hot to touch after a bit. Yesterday I learned what the resistance should be on a good 6 and 12 volt coil and the old 6 volt and the current 12 volt check out OK so seems like something besides a bad coil is making it get hot. Its the same cap, wires, plugs, etc that has been on the tractor for a few years. Nothing was changed when the problem started last fall. So my question is what else could be causing it. Its a 12 volt generator as it has been from the beginning and its charging in the right range. Can a condenser cause something like this?

 

[DeltaRed]

Possible. It's a cheap 'fix'. Try it.Condencers can go bad,especially new ones.

 

[Bob]

It would be GREAT if you could measure the primary current draw with the engine not running.

Conventional wisdom says it takes 3 Amps or a bit more for a good, HOT spark, if current exceeds 4 Amps or a little more, there will be more coil heating and quicker breaker point burning. (The primary current is determined by the actual primary resistance of your coil.)

I trust NOTHING ignition/electrical that's being sold today to be of good quality or made correctly "to spec's..

The coil DEFINITELY sounds "suspect".

Wouldn't hurt to check the charging system voltage, though, 13.5 to 14.5 Volt range.

Also, point gap has an effect on coil heating, if point gap is too small DWELL increases making for more coil heating.

 

[JMOR]

(quoted from post at 14:55:06 04/04/20) My 300U runs great for about 30 minutes, then starts to loose power and even starts to skip. After it sits for a few hours it runs normal again. It started last fall when I was brush hogging. When it runs bad the coil is so hot you can't tough it so that is where I thought the problem was. Last fall it was a 6 volt coil with a resistor (12 volt system) so I put on a 12 volt coil that doesn't need a resistor. It is doing the same thing, getting to hot to touch after a bit. Yesterday I learned what the resistance should be on a good 6 and 12 volt coil and the old 6 volt and the current 12 volt check out OK so seems like something besides a bad coil is making it get hot. Its the same cap, wires, plugs, etc that has been on the tractor for a few years. Nothing was changed when the problem started last fall. So my question is what else could be causing it. Its a 12 volt generator as it has been from the beginning and its charging in the right range. Can a condenser cause something like this?

on' know your tractor, but on nearly all engines that I have seen, the coil is mounted on the engine & will be almost as hot as the engine. You probably never felt of the coil before this problem and if you had, I'll bet it would have been hot then too. I would be looking at some of the things that others have pointed out.

 

[John T]

Charlie, while a coil normally runs "warm" it shouldn't run so hot you cant keep your hand on it PLUS if its drawing too much current to the point it overheats so bad it shuts down THERES A PROBLEM. Here are some thoughts:

1) A full true 12 Volt (NO ballast required) coil typically measures from say at least 2.5 up to near 4 ohms (many around 3) LV primary resistance as measured between its two small + and - terminals WHAT DOES YOURS MEASURE ????????????????

2) NOTE a coil labeled "12 Volts for use with ballast resistor" is in reality more like a 6 volt coil and absent an external series voltage dropping (12 down to 6) ballast resistor on a 12 volt tractor it will overheat and fail !!!! That's not yours I hope ????????????????

You need to know your coils resistance PLUS the current its actually drawing

3) If you actually have a coil that has around 3 to 4 Ohms of LV primary winding resistance at 12 volts the coil and points current would be around 12/3 = 4 Amps or 12/4 = 3 Amps and a full true 12 Volt coil should NOT overheat. NOTE Charging voltage at fast RPM using a good generator may be 13 to near 14 or so, that changes these numbers a bit...

4) Even if you were overcharging at lets use say 14 Volts, 14/3 or 14/4 still should NOT overheat a full true 12 Volt coil so bad it shuts down HOWEVER A CONSTANT SAY 14+ TO NEAR 15 VOLTS ON A COIL COULD POSSIBLY CAUSE IT TO AFTER A LONG TIME OVERHEAT...?.

You need to check your charging voltage

5) You ask can a condenser cause something like that??

Well if a condenser were shorted the coil would constantly draw current (no points switching NO duty cycle) HOWEVER if so it wouldn't run. NOTE a faulty condenser can cause a weak or even no spark and its another device that can fail AFTER ITS HOT

I suspect the wrong voltage coil (insufficient LV primary resistance) coil or its failing,,,,,,,,,,,,,,possibly you're over charging,,,,,,,,,,,sure a condenser can cause problems once warmed up,,,,,,,,,,,could be a points related problem like its eventually shorting to some extent and drawing constant no duty cycle excess current causing spark failure plus a hot coil ???

Check or suspect points, coil, condenser, charging voltage, excess current draw.

John T

 

[Janicholson]

Adding to John T's info. A coil can have thermal runaway. as it warms normally from internal resistance of the windings, and engine heat associated with where it is, the internal windings can change size as they heat, and cause an internal short that makes the resistance go lower and lower (more and more amps) this causes even more heat and that causes more short circuit. The end product is a breakdown in the high voltage windings. then it begins to have erratic weak spark. Changing the coil with a known one of the same resistance, will tell the story. Jim

 

[rc_cola85]

Like a couple others have said, I would check your charging voltage with the engine running at operating range. It sounds like you have an overcharging situation.