Author	[Modern View]
RTR 12-30-2012 19:33:01 98.83.136.199 Report to Moderator	I have a bin full of old coils from Farmall Tractors. Most of them are not marked 12 or 6 volt....if they were at one time, it's rubbed off by now. Also, they aren't marked as to wether the ballast resistor is required or not. For the ones I can't identify, is there a way to do this. I needed one earlier and it didnt' have any markings except for "Made in USA". I went ahead and put it on 12 volt tractor. How can I know if its a 6 or 12 volt coil, and how can I know if the ballast resistor is required or not?? [Reply]   [No Email]
John T 12-31-2012 06:54:35 216.249.82.117 Report to Moderator	Re: Old Coils - Any Way to Identify?? in reply to RTR, 12-30-2012 19:33:01   Id check the LV primary winding between the small + and - terminals which may well (Cant guarantee perfect or for every coil ever made mind you) indicate if its a 6 or 12 volt coil, see below: If youre talking typical old circa 40's through 60's stock farm tractor ignition coils (NOT talkin after market or high voltage or performance or electronic ignition coils or many Mallroy or Accel mind you) MANY (NOT all) coils: 1) If labeled "6 volt" ORRRRRRR "12 volts for use with ballast resistor" ORRRRRRRR "12 volts requires balalst resistor" had a LV primary coil winding resistance in the neighborhood of: 1.25 to 2 ohms, many around 1.5. They were designed to operate at 6 volts "nominal" and could still produce a spark if starting dropped voltage down to 4.5 and if at 7+ volts while charging still spark and not overheat..... NOTE if on a 12 volt tractor you can use EITHER a 6 volt coil PLUS external series voltage dropping (12 to 6) ballast resistor ORRRRRR a "12 volt" coil no ballast required. That way many (NOT all) old tractors, 6 or 12 volt, still used the same 6 volt coil, its just that if on a 12 volt tractor they used the voltage dropping (12 to 6) ballast resistor between/after Ign switch and before coil 2) If labeled "12 volts" ORRRRRRR "12 volts NOT for use with ballast resistor" ORRRRR "12 volts NO ballast required" had a LV primary coil winding resistance in the neighborhood of 2.5 to under 4 or so ohms, many around 3 ohms. They were designed to operate at 12 volts "nominal" and could still produce a spark if starting dropped voltage down to 9 and if at 14 volts while charging still spark and not overheat..... SO GET OUT YOUR OHM METER AND GO TO WORK MAY IDENTIFY IF ITS A 6 OR 12 VOLT COIL NOTE FYI HERES SOME EXTRA CREDT INFORMATION Contrary to what many lay persons believe who call them coils that have an "internal ResisTOR" YOU WILL NOTTTTTTTTTTTT FIND A DISCRETE STAND ALONE RESIS"TOR" TUCKED AWAY SOMEWHERE INSIDE THE CAN. The way a "12 volt" coil achieves its necessary 3 or so ohms of LV primary resistANCE is by enough wire coil length (more wire or more coil turns) orrrrrrrrr using wire with certain sufficient resistANCE per unit length so the resistANCE end to end (+ to -) ends up in the range of 2.5 to 4 or so ohms so the points dont burn up and the coil doesnt overheat and handles the current when 12 volts is applied. SORRY CHARLIE, DONT X RAY OR DISECT ONE AND EXPECT TO FIND A STAND ALONE "RESISTOR" HIDDEN INSIDE SOMEWHERE!!!!!!!!!!!!!!!!! NOTE they actually did make some very early automotive coils that DID HAVE a stand alone discrete ResisTOR in a seperate part of the can!! Many had like a ring where the internal can portions were seperated, coil in one end resisTOR in other end all by itslef!! HOWEVER in alllllll my years as a used tractor dealer (older tractors) I never saw a coil with such an internal resisTOR inside the can!!!!!!!! NOTE: For those who arent familiar with electronics terminology and definitions: Wire and the coils and turns of wire used in the coil, not being a "perfect" conductor, sure, has some (but very very low) inherent ResistANCE, BUT WIRE IS NOT CALLED A RESISTOR, ITS CALLED WIRE which of course contains some small degree of ResistANCE. If you go to Radio Shack and ask for "wire" they show you wire NOT ResisTORS. lIKEWISE, if you ask for a ResisTOR, they will show you resisTORS NOTTTTTTTTTT wire lol Sooooooo when I say theres no stand alone discrete "ResisTOR" hidden away somewhere inside the coil can, you know what I mean and what standard definitions of WIRE and RESISTORS are, theres WIRE inside there but NO stand alone discrete RESISTOR Hope this helps, God Bless yall John T [Reply]  [No Email]
mkirsch 12-31-2012 06:14:12 64.80.110.74 Report to Moderator	Re: Old Coils - Any Way to Identify?? in reply to RTR, 12-30-2012 19:33:01   The "external resistor required" coils will have the same resistance as a 6 Volt coil, because they *ARE* 6 Volt coils. "External resistor required" coils were made to keep the less-smart people from getting confused at the parts counter... Customer: "But, the coil is marked 6 Volts... My 1970's car has a 12 Volt battery." Parts man: "It's okay sir, your car has a ballast resistor in it." Customer: "But, it's a 12 Volt battery. Don't I need a 12 Volt coil?" Parts man: "No sir, your car has a resistor that reduces the voltage for the coil." Customer: "But, it's a 12 Volt battery. I think I need a 12 Volt coil." Parts man: "Trust me, you're fine." Customer: "I really think I need a 12 Volt coil. The car as a 12 Volt battery. This coil is marked 6 Volt. It's got to be the wrong coil." Parts man: *cold cocks himself with a pipe wrench* [Reply]  [No Email]
Dusty MI 01-03-2013 05:17:06 76.250.62.134 Report to Moderator	Re: Old Coils - Any Way to Identify?? in reply to mkirsch, 12-31-2012 06:14:12   Quoting Removed, click Modern View to see A couple years ago I was on a 2 day antique tractor carvan, and was having engine trouble. Thinking it could be the coil, this tractor has a 12 system, I went to an auto parts place,several miles away, and asked for a 12 volt coil. The gail brings out a coil in it's box. I asked if it was a 12 volt one, and she says that the bin was marked 12 volt. I went back to where we were parked for the night, and opened the box, and inside with the coil was a slip of paper stating that the coil needed a ballast resistor. Some days you just can't win. Dusty [Reply]  [No Email]
RTR 12-30-2012 21:25:26 98.83.136.199 Report to Moderator	Re: Old Coils - Any Way to Identify?? in reply to old, 12-30-2012 19:33:01   How would I know if the coil is one to be used with an external resistor or not. They make some coils that state an external resister is required. Is there a way to know? [Reply]  [No Email]
Jerry/MT 12-31-2012 09:19:52 206.183.116.145 Report to Moderator	Re: Old Coils - Any Way to Identify?? in reply to RTR, 12-30-2012 21:25:26   If it has ~1.6 ohms of resistance than you can use it as is in a 6V system. If you add a ballast resistor in series with the coil in the primary circuit, you can use it for a 12V system. [Reply]  [No Email]
RTR 01-02-2013 18:24:59 96.37.158.78 Report to Moderator	Re: Old Coils - Any Way to Identify?? in reply to Jerry/MT, 12-31-2012 09:19:52   Quoting Removed, click Modern View to see Having all that said about checking to see what type they are, can you also check to see whether they are good or bad, or would you have to hook it up and test it? [Reply]  [No Email]
Ierry/MT 12-31-2012 09:19:13 206.183.116.145 Report to Moderator	Re: Old Coils - Any Way to Identify?? in reply to RTR, 12-30-2012 21:25:26   If it has ~1.6 ohms of resistance than you can use it as is in a 6V system. If you add a ballast resistor in series with the coil in the primary circuit, you can use it for a 12V system. [Reply]  [No Email]
Jerry/MT 12-30-2012 21:22:41 206.183.116.145 Report to Moderator	Re: Old Coils - Any Way to Identify?? in reply to RTR, 12-30-2012 19:33:01   You can check the reistance on the primary windings. In general, a 6 volt will have~ 1.6 ohms of resistance and a 12V will have~3.2 ohms. 1.6 ohm coil can be used on a 12 V system if you add a ballast reistor in series in the primary circuit. That will limit the primary current to ~4 amps max. [Reply]  [No Email]