ignition coil resistor

D Slater

Well-known Member
What is the ideal or closest to ideal OHM resistance to use on a antique tractor using points and 12 volts with a coil marked use a external resistor or a six volt coil. Using some 1.6 and some 1.8 OHM and just started wondering if something is better. Thanks
 
In static conditions, a 50 50 voltage divider would drop 6 of the 12 battery volts across the ballast resistor leaving the other 6 across a 6 volt coil THEREFORE BALLAST RESISTANCE WOULD BE SAME AS COIL RESISTANCE when using a 6 volt coil on a 12 volt tractor.

Many (not all) typical 6 volt coils have a LV primary resistance of 1.2 to maybe 1.9 ohms (1.5 ohms common) and many ballast resistors may be 1.2 to just under 2 ohms. HOWEVER different coils and different manufacturers use slightly different resistance coils and ballast resistors (I've seen them 1. 5 up to 1.9 ohms) and the coil voltage may be more like 7+ instead of 6.

SOOOOOOOOOOOOO if you use a ballast that's around the same ohms as the coil you should be fine.

NOTE as it appears you're well aware, a coil labeled use external resistor when operating at 12 volts, is in reality more like a 6 volt coil.

Maybe others have different answers, but this (around same ohms as coil for 50 50 voltage divider) makes sense to me.

John T
 
With the coil out of circuit, measure the ohms across the + and the - terminals. Subtract that number from 3. That will be the ball park number for the ballast resistor.

For a 12v system, you want about 3 ohms total resistance across the coil and resistor seriesed together.
 
Can you explain this one?? O got a Case VAC the other day. 12 volt battery in it but no charging system which I not worried about at this time.
It has a 12 volt coil that says right on it no external resister needed but it does in fact have a ballast resister hooked up to it and this thing fires up so fast is is a bit scary. So how is it producing enough spark to run so well but yet have that resister and coil set up
 
Sounds like it NOW indeed has a correct full 12 volt coil on a 12 volt tractor. Maybe in a past life it was 6 volt and they converted to 12 using the old 6 volt coil so the ballast was added then??? I'm not all that surprised it can still run okay, but the real test comes when she's placed under a heavy load. Summer hot weather starting isn't all that difficult either which is why it can still start good even though spark energy is less then if wired correct. Hey if it ant broke don't fix it lol. If it doesn't run good under a load or is hard to start next winter then maybe the ballast can be removed???

Keep cool down there Rich

John T
 
(quoted from post at 20:07:36 08/11/16) Sounds like it NOW indeed has a correct full 12 volt coil on a 12 volt tractor. Maybe in a past life it was 6 volt and they converted to 12 using the old 6 volt coil so the ballast was added then??? I'm not all that surprised it can still run okay, but the real test comes when she's placed under a heavy load. Summer hot weather starting isn't all that difficult either which is why it can still start good even though spark energy is less then if wired correct. Hey if it ant broke don't fix it lol. If it doesn't run good under a load or is hard to start next winter then maybe the ballast can be removed???

Keep cool down there Rich

John T
..and in addition, resistor value is unknown! If 0.1 Ohm, it makes no difference!
 
I use Dodge external resistor from 1970,80s maybe even 90s . EBAY Search "
Mopar 73 74 Challenger / Charger Ballast Resistor "

these have start & run resistors "Start" will be low ohms giving most voltage to coil and "RUN" will be higher ohms so coil does not over heat. About $2.00 at wrecking yard or $6.00 new. Easy to wire in or replace.
 
I had a 67 Barracuda. Wouldn't leave home with out a spare resistor in glove box.

I think it was later Chrysler uses a start and run resistor. If my old memory is correct, my 67 cuda only used one resistor and during start, full power bypassed the resistor and was sent directly to the coil.

I wired my Jubilee so the power would bypass the ballast during start.

Agree with JT, 50/50 split 6v on ballast and 6v on coil. However there two other things to consider. The 12v battery voltage is closer to 14+ v when alternator is charging and less than 12v when cranking.

So learn form how it was done on cars back in my day. Bypass the ballast during cranking. Use the largest ballast resistor that will work. More resistance, less current, the longer points will last.

Another consideration. Wire wound resistor get hot. As temperature increases so does it resistance. So if you want to measure the ballast voltage drop, do it fast.

My Jubilee follows the 50/50 split, works great, doesn't burn the points.

The ohmmeter is the least accurate of all meters, so who can precisely measure ohms to .1 ohms? Even the old simpson 260 aren't that accurate. Unless you have an old school analog, who has a digital that measures 0 ohms when the leads are shorted together? If you do, please post pic of your meter doing that.
geo
 
Get yourself an ohmmeter (Part of a Digital Multi-meter if you don't have one) and measure the ohms (power off of course) between the voltage source to the ignition circuit and the distributor input wire. My MF 35 service manual (1960's vintage tractor) says that 5 amperes is the maximum recommended current for reliable ignition points operation. Nothing unique about MF points: What's good for the Gander is............

Measure your "running" tractor's voltage at that input point and divide it by 5. Answer is how many ohms you need in the line and it matters not how you get them, AKA external or internal resistor.
 
I haven't check that since it starts and runs so well and that is one of those if it isn't broken don't fix it thing.
 
That reminds of one time I stopped to help some people with a Dodge car. It would start right up but as soon as you left the key go back to the run position it would die. I pulled the wires off the ballast resister and hook them to each other and it kept running. Asked them how far they where going and they said this or that so I said drive around to the local auto parts store and buy a new one of these and I pointed it out to them. They said okay and I went my way they went there but they sure where happy to be going again
 
Old, that was the test for a bad ballast, start right up then die as soon as you turned the key to the run position. Proof they bypassed the ballast giving the coil the cranking voltage.

If you don't bypass ballast resistor, you may have starting issues. Then you will use a smaller ballast and fry the points faster.

The old school wiring still applies when converting from 6v to 12v and using a ballast. Bypassing ballast seems to be overlooked on YT. Not all starter solenoids have a place to do it the old school way. Not to mention my Jubilee uses a manual starter connecting negative to solenoid and no bypass terminal on the solenoid.
 
Yep when I get one that is hard to start and has a ballast resister on it I jump across it and if it starts easy then I know where to look for the problems
 
Old:

Is the Ballast Resistor wired up as a "Start" resistor, or as a "Run" resistor? If it's wired up as a "Run" resistor, it may be used to keep the Coil from cooking while the Alternator is charging the Battery.
 
It is a run set up. It has NO charging system on it at this time just runs off the battery. Ha the old 6 volt generator on it but does not have any wires hooked to it
 
My solution, run a wire from starter to coil and install a diode with cathode end on coil and anode end looking at 12v starter. Easy solution, hotter spark, old school fix.

Didn't some fords come from factory wired like the old Chryslers?
 
yep, Ford had a 4 pin solenoid, heavy lug in from batt / heavy lug out to starter, source "S" key switch start, and "I" to the coil for cranking, then the famous resistor wire powered the ign while running.
 
Don't know about the Fords I have always been a Dodge ?GM person and learned to hate Found On the Road Dead
 
O: With a 12 volt coil and if the starter has 12 volts to it, you are not robbing enough current when starting to affect the coil operation. Also the condition of the engine, compression, etc. may be good.
 
What I think is odd is that the coil it self say right on it. 12 volt coil no external resister needed but yet it has one on it. But I have since by passed it just because it should not need to have it. Changed the plugs yesterday and they looked pretty bad so figure maybe having the resister out of the circuit the spark will be hotter so that way the plugs might do better
 

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