Ignition coil failure on old 14hp Kohler

[Scott Willing]

Last time I tried posting here, the msg never showed up, so this is something of a test as well as a genuine request.

I bought an old Case garden tractor last year. My neighbor, a farmer/logger/machinist had rebuilt the engine. This old unit has battery ignition, i.e. coil and points, with a generator/starter.

It used to die on occasion after a stretch of especially hard work. Let it cool off for a few hours and it would go again. But this year one day it died for good.

After finding no spark, I measured the coil with a meter and discovered the primary winding was open. Got a new coil (along with points, condensor and plug) and tried the measurement with the new one just for future reference just to make sure I knew what I was doing. Measuring the "points" terminal to the +12 terminal I saw the primary coil resistance, and between "points" and HT terminal, there was the secondary coil resistance. (I believe this coil may also include a resistor in the primary, but I don't know that for certain.)

I got it all back together, used it several times and it was running like a dream, never better. Then the other day, again after a stretch of particularly hard work (but certainly not the first on the new parts) wound up close to max rpm, it died.

Lo and behold, the primary winding has gone open on the brand new coil(!) Just bad luck to get a bum coil, coincidence that the old one failed? The guy who supplied it is going to exchange it for me but I gotta wonder if there's a coil-eating problem in this machine.

There's not much else I can think of to suspect except the old mechanical voltage regulator (and no, unfortunately, I never checked voltage while running but I surely will). But two experienced repair guys assure me that there is no way, even if the coil was getting full unregulated voltage, that it should fail. I'm not so sure.

Other opinions? It's a two-hour drive to pick up a replacement coil, and I'm not keen to repeat this exercise this year if I can help it.

Would you replace the old mechanical voltage reg just on spec? Should I be able to find a ready solid-state replacement? Should I care?

Oh, and why is the sky blue?

End of test.

-=s

[RBM]

I can tell you why the sky is blue, but your coil problem, that's a different matter. If your Kohler engine is a model K, I believe they make a nice pointless ignition kit for it, however that only eliminates the points. My guess is that you've got a bad regulator and I've gotta believe sending high voltage into the coil will cook it. When you get the new one, run the engine with a voltage meter across the coil and see

[calhoun]

I know what the problem is if it was a car. Missing or shorted ballast resistor. I don't know if kohler uses them though. For those young'ins that don't know about old ignition systems a ballast resistor limits the coils voltage to about 9volts. The coils are designed to operate at this voltage. The only time the coil sees 12v is when the starter is engaged. This allows for a hotter spark (overloads the coil) for starting. If there is no resistor than coil will see 12v while running and it's life will be greatly shortened. If this is the case than don't replace coil with a stock unit. Get one that operates on constant 12v.

[Duane Bozarth]

Terry wrote: .....

That's a bad/non-functioning regulator more likely than the battery itself...

[ds549]

that kohler coil for that uses a built in resistor so id get the right one , they tend to burn up points if a non resistor coil is used. high battery voltage would be hard on the coil. a normal charging system on a k series engine is about 14v .

[Terry]

Agree there may, or should be, a resistor in series with the 12 volts to the coil? See Note. If that resistor (which in some cases may be built into the coil or may be a separate item) is missing or short circuited the full 12 volts is applied continuously to the coil, too much current flows, which eventually overheats and burns out the coil. Resistors are more likely to go 'open' than shorted though? maybe someone removed an original one? Or the original could 'take' a full 12 volts, continuously? Another, less likely reason might be a bad battery? A really bad battery might allow the voltage to climb when engine running to say 16 or 18 volts, again too much current and the coil might burn out. However if battery capable of starting the engine it is unlikely that this would happen. Recall old, hand-crank start vehicles with batteries so bad that the lights would brighten up as engine revved and die right down dim when engine slowed down. So the voltage was varying widely! Note. On some more sophisticated engines I seem to recall that the resistor would be out of the circuit while starting, in order to get a stronger spark while starting? Once engine was running the resistor was back in the circuit to avoid coil burn out? This was either done automatically as part of the electric starter circuit or manually with or as part of the starter switch? Good luck with the tractor.

[Scott Willing]

That's the plan...

I have a service manual for the K series (downloadable from Kohler's website) but this funky belt-drive starter/generator arrangement isn't covered. Maybe it was unique to Case, I dunno.

And my buddy in the support dept at Kohler's off for a week.

-=s

[Scott Willing]

[original post about frying coils snipped]

Some folks are convinced that there are Kohler coils with integral ballast resistors. I'm drawing blanks from the only sources I've been able to contact thus far.

I'm just trying to restore this thing to the original configuration, whatever that is. My Kohler manuals don't show my config (belt-drive starter/generator) and none of the illustrated systems show a ballast resistor, either outside or inside the coil. (One particularly nice diagram shows the inside.)

I have Case-specific manuals hung up in the mail, and my support guy at Kohler is off this week so I'm at the mercy of the webbernet...

Thanks for your thoughts.

-=s

[Scott Willing]

[snipped my original diatribe]

Yep. But the two coils that have failed have gone open-circuit on the primary, so if there was a built-in resistor it could have gone south.

Any idea what a typical value is for such a thing? I can't imagine it would be more than a few Ohms at most, but I don't know.

Both strong possibilities. Many hands have touched this baby.

Battery's good.

I hear ya, but no schematics I've seen in the Kohler manual (which covers points/coil systems but not my funky starter/generator thing) show a ballast resistor. I may not know for sure until my Case manuals arrive in the mail.

Thanks, I'm sure it'll work out. I'm actually having quite a lot of fun fixing this old thing. It came along at the right price at the right time, with a freshly-rebuilt motor (bored out) but with lots of other little flaky details to keep me entertained. It's like an ongoing repair workshop, and I need to acquire some skills in that department anyway.

-=s

[Scott Willing]

I have a new coil in my hand. What's a typical resistor value? I only measure 4 ohms across the primary terminals, no way for me to know whether that's just the coil or coil + resistor.

Thanks. I'd have thought a little higher, but that's the ballpark anyway. if it's 23 at max revs I'll have a pretty good clue that something's wacked. :-)

-=s

[RBM]

Scott, the generator-starter is a thing of beauty. All the old Cub Cadets used them, they never die. I have one from 1962 works like a charm.

[Chris Lewis]

According to Scott Willing :

You're right, the K series book doesn't appear to cover the starter/generator. I have a Case manual that has a few pages on the starter/generator.

[Chris Lewis]

According to Chris Lewis :

Belay that. The Cub Cadet manual I have has about half a page on it talking solely about brushes. There's _another_ manual for the starter/generator.

My manual does have, however, a few pages on the regulator, including adjustments. But doesn't seem to give expected voltages for some reason.

[Scott Willing]

[blah blah blah two coil failures on old Case garden tractor with 14hp Kohler K321, starter/generator, voltage regulator, points etc.]

Gentlemen,

I sincerely appreciate your efforts to help.

Although my regular guy (he'd cringe at that) at Kohler was off this week, another fellow at Kohler got back to me in a day or two.

Here's the 411, direct from the horse's mouth:

"No, this coil does not have an "4-8 ohms across the primary terminals 8,000-12,000 ohms from the high tension tower to either of the primary terminals"

Then he went on to caution that a mere DC resistance measurement doesn't prove that the coil is good; for that you need to have it in service or on a tester. I agree, but it's nice to know when the measurement you make indicates that the coil is bad for sure.

So... I put the thing all back together and once again it's running like a dream.

This time I stuck a meter on the charging system. It freaked out -- a cheap digital meter I use for "unfriendly environments" which was apparently picking up RF interference, reading wild random numbers on any range whether or not the leads were connected to anything. Good humour.

I fished out a good old-fashioned analog meter, and using that I measured 14V steady as a rock throughout the operating range. I was quite impressed frankly; I expected more of a stepped response. Helps that the battery is in good shape I suppose.

I'd love to analyse in detail how this particular voltage regulator works. Replacements are still available but as an electronics guy I'd be tempted to build myself a bullet-proof solid-state replacement.

It remains a mystery why I suffered two coil failures. I'm thinking of mounting a couple of gauges on this thing - even gaff-taping my old analog meter to it temporarily - to keep an eye out for some kind of intermittent problem. If I lose another coil -- well, let's just say it's a good thing I don't have any hair left to pull out.

BTW, I've been getting parts and manuals for my old Case from:

(that's >>tractorS

[Scott Willing]

Indeed it seems like a simple and elegant thing.

-=s

[Scott Willing]

I've pretty much ordered the works. There are several. However I may take you up on that.

I wondered about that, but then I don't know how comparable (aside from the basic principle) a regulator for a generator, and a regulator for an alternator are. I suppose the control mechanism is the same, i.e. control the field current and you control the output, yeah?

I have a Home Power magazine article about setting up an old-style alternator for charging large battery banks. It discusses controlling field current... I'll have another look at that. If there's an off-the-shelf solid state replacement I'd be all over it. I enjoy doing my own design / build, but there's already a long list and not much spare time.

I was frankly surprised at how little information on mechanical regulators I was able to find in a net search. I'm trained in electronics, but my only exposure to automotive / small engine electrical has been Random, Reluctant, and only as Required.

Cheers,

-=s

[Chris Lewis]

According to Scott Willing :

Yeah. A good battery will generally not vary much at all.

Are you getting the tractor maintenance manuals? If not, I could probably scan the pages from mine.

Perhaps I can figure out how it works from the manual.

I was told by a good small-engines guy: "Don't buy a replacement regulator. If it fails, just about any generic (solid state or otherwise) regulator will work fine, and cost MUCH less."

[Andy Asberry]

Keep in mind that a generator regulator has a reverse current relay.

[Scott Willing]

[giant snip]

That being... ?

-=s

[Scott Willing]

A-OK there.

Sweet! That's exactly the sort of treatise I've been looking for! Thanks so much for that.

Also interesting.

Thanks a whole swack again Chris.

Cheers,

-=s