farmall m electric problem

I have a M farmall which I converted to 12 volt several years ago. It has the delco alternator, a 6 volt coil, and a ballast resistor..and has been working fine all along. Today, I started the tractor, warmed it up, and ran up the throttle til the ammeter went into charge mode. I turned on the headlights, just to see if they still worked. They did, but then i noticed the ammeter pegged all the way to the right, and the engine stopped running. I have no spark coming out of the coil. The headlights still work. I do have proper continuity throughout the circuit. The resistance across the ballast resistor is 0.00. According to the spec, it should be around 1.5 ohm. Did I perhaps fry the resistor, and or the coil? Thinking about bypassing the resistor, and installing a 12 v coil. Any advice, I would greatly appreciate.

The ballast resistor is toast. They are cheaper than coils, and do go bad. if your coil primary is less than 2 ohms, I would replace it with a 12v no ballast resistor required. However if the one wire alternator went wide open from a regulator issue, the voltage might be 20 plus volts and that was the reason the resistor went incandescent. Battery voltage running should be 14.2 or 14.8 max with a charged battery. A three wire alternator (stock auto wiring) is likely to be the ticket. Bob M wiring diagram in the search for box will find it. Jim

I killed the resister on my SMTA fairly quickly. I went to a 12 volt coil with internal resister and that has worked fine. I don't understand why turning your lights on should fry anything if wiring is correct.

Zero across the resistor say it is still good but you meter is off. If you had read infinity then it would be bad. If I where you I would try hot wiring it and if it runs then your problem is from the coil back if it does not run then from the coil forward. Could be as simple as the points slipping or some such thing

Infinite resistance is no conductivity, Zero resistance is none. Either is wrong. Jim

check your regulator, sounds like the alternator went to max charge and fried the resistor.
I don't like the alternator conversions, put a 12 volt generator on and a 12volt resistor coil, saves a lot head aches.

Thanks Jim...So I guess the first thing i should do, is get the alternator checked?

For sure. if it is a one wire system, a 3 wire will work without reving it up, and be more well regulated. Jim

(quoted from post at 20:41:41 11/05/12) I have a M farmall which I converted to 12 volt several years ago. It has the delco alternator, a 6 volt coil, and a ballast resistor..and has been working fine all along. Today, I started the tractor, warmed it up, and ran up the throttle til the ammeter went into charge mode. I turned on the headlights, just to see if they still worked. They did, but then i noticed the ammeter pegged all the way to the right, and the engine stopped running. I have no spark coming out of the coil. The headlights still work. I do have proper continuity throughout the circuit. The resistance across the ballast resistor is 0.00. According to the spec, it should be around 1.5 ohm. Did I perhaps fry the resistor, and or the coil? Thinking about bypassing the resistor, and installing a 12 v coil. Any advice, I would greatly appreciate.

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K, read all the postings here & suspect looking in all the wrong places. When you turned in the lights, the high current draw opened up some weak connection between battery and the power path to the ballast resistor & coil, .....so now, no power to coil. Forget continuity........turn on ignition put a test light on coil input, ballast resistor (both connections), etc. and see if it lights.........then close points & see if still lit. You will find it. When all that looks good, then you can mess with alternator, coils, resistor replacement if it still won't run. Or you can replace all that stuff & if you get lucky, accidentally 'fix' the bad connection in the process, or not & still have it not run with all those new parts.

Did you check that little 20 amp fuse in the fuseholder? I have no idea if that could be it, but it"d be easy to check.

A cheap low quality ohm meter may not be very accurate at low ohms reading like say 1.5 to 2 ohms which a typical ballast resistor is.

If you unhook the wires to it and measure the ballast resistor resistance, Id expect maybe 1.25 to under 2 ohms, if its open (infinite resistance no continuity) its obviously baddddddddd.

With all hooked up like normal with the ignition ON if you place a DC volt meter ACROSS the ballast and if the poitns are good and closed and all is working, Id expect around a 6 volt drop.

If (all ON and working and points good and closed) you place the meter with Neg to frame case ground and the positive to the coils input (NOT to distributor) Id expect 6 volts (cuz the ballast if good and conducting current drops 12 battery volts down to 6).

If you put the meters Pos on the coils output (TO distributor) Id expect 12 volts when points open but 0 volts when closed.

SHES NEVER GONNA PRODUCE A SPARK UNTIL YOU HAVE VOLTAGE ON THE COILS INPUT (NOT to distributor) with the ignition ON and that means a good and conductive ballast resistor after the ignition and bepore and ahead of the coils input.

I suspect an open or burned circuit somewhere in the Ignition primary (switch, ballast, coil, points) GET YOUR METER OUT N GO TO WORK

Run my Troubleshooting procedure

John T
John Ts Ignition Troubleshooting

You definitely should get the alternator checked.

The meter pegging to the right tells me that the voltage regulator in the alternator has fried. The high voltage in the alternator burned out your coil, or the ballast resistor, or the condenser.

I suspect if you just replace the coil/resistor/condenser/etc, and try again, you will be replacing ANOTHER coil/resistor/condenser very soon.

First of all, thank's to everyone who is helping me in this crisis. Nice to know there's a big RED family out there. Here's the latest update..today I installed a 12v coil, and bypassed the bal. resistor. Fortunately, she fired right up, and keeping the engine at just about idle, so the alt. wouldn't kick in, I managed to get her out of the driveway, and into the shop. Took off the alternator, and will have it tested tomorrow. Hopefully, that's where the problem is..and we can get ready to plow snow.

Good procedure. I hope for the best on the alt. The resistors do deteriorate and can fail. A over volt alternator would for sure help it into the scrap bin. Jim

Had the alternator tested today...and it checked out ok. So i bought and installed a 12 v coil, which by the way, the tec told me, the only difference between the 6 and the 12, is the 12 has the resistor built in, and the 6v doesn't. Installed that, and everything seems to be working ok. Evidently, the resistor went kaput..cutting off the current to the coil. Thanks again, everybody..till next time..and I'm pretty sure there will be!

The tech was incorrect. There are no "resistors in 12v coils, only different winding ratios for 12v operation. A common tale of misinformation, but just not true. The coil is designed to have 12 v (or charging voltage) applied without burning up. Jim

I'll have to check my H but I'm pretty sure that the words "internally ballasted" are silk screened on the coil. It was purchased from Tractor Supply.

That, to me, says that at least some 12V coils do have resistors in them. From a production standpoint, it's certainly more efficient to have the coil winder doing one thing, than having to operate/maintain two coil winders, or switch the one you have back and forth between winding 6V and 12V coils.

Most of that big white chunk on a ballast resistor is ceramic material. The actual resistor is not very large, and could be tucked into the middle of the coil quite easily.

Anybody know why they encase the resistor in ceramic? Ceramic is normally not a good heat sink, and is awfully brittle to call it protection.