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Resistor question

grandpa Love

grandpa Love

Well-known Member
Is this what I need to add in line from ignition switch to coil on Oliver 77? 12 volt battery, 6 volt system.
cvphoto17470.jpg

Correct ohm to keep coil from damage?
 
That one may be special to the MSD coil, a cheap 1966 Chevy 6 cylinder Bellaire ballast is fine. One ohm may be a static reading when the resister is cold, but when they get hot they may be in the 1.5 to 2.5 ohm range. All of them will smoke at first, burning off the factory residual oil. Jim
 
How pricey is it? I never used one. The one's I've converted,I just bought a different coil.
 
What's his price on a new coil? How old is the one that's on it? If it goes bad,you'll end up buying one anyway. You'll probably buy the right one then and have that resistor just laying around. It's like the argument between using one of those little alternators that'll fit without much alteration vs spending a day and a half cobbling one in there that's too big. If you don't have one and have to buy one anyway,why not go with what works right in the first place?
 
No. It is .8 ohms, for Oliver 6 volt (1 1/2 ohm primary coil) you need 1 1/2 ohm resistor

Looking at the web page for this resistor it appears made to match their coils, and one of them showed only .7 ohm primary

For tractor 12 volt system a "12 volt, no external resistor required" would eliminate need for resistor. NAPA IC14SB or equivalent
 
The goal is to have about 3 ohms resistance across the coil with the resistor in series.

First, measure the resistance across the + and - terminals of the coil, out of circuit.

A 6v coil will have about 1.5 ohm resistance.

So, you will need a resistor with 1.5 ohm resistance.

Using that resistor, a .8, would be 1.5+.8=2.3, so the resistance would be a little low, resulting in a hotter spark, but short lived points and a warm running coil. It might work, but better to use a true 12v coil, or a 1.5 ohm resistor.
 
NO the 0.8 ohms is too low. Ideally to drop 6 volts across a 6 volt coil on a 12 volt tractor you need a 50/50 voltage divider meaning the ballast resistor is about the same ohms as the coil IE many (not all) old tractor ballast ignition resistors are somewhere around 1.4 to 1.8 ohms NOT 0.8. Many (not all) old tractor 6 volt coils are around 1.2 to 2 ohms (1.5 is a typical) GET A HIGHER RESISTANCE (1.2 to say around 1.8 ohm) BALLAST

John T
 
One more thing, if you do buy a 12v coil, take your ohm meter with you, check it before you buy.

Old style coils are dinosaurs. They are commonly reboxed, resold between retailers, and poorly labeled.

Same thing with the resistor, it may have been reboxed.

Check it first, what you get may not be what you need.
 
I see no one mentioned the reason for having a ballast resistor and a coil. During starting an extra wire from the "I" terminal on the starter solenoid to the coil side of the resistor applies 12 volts to the coil for a hotter spark while starting. This makes a considerable difference in how the engine will start when it is very cold. If there is no provision for connecting this wire to the solenoid, it can be run though an extra push button switch from 12 volts to perform the same function. There are other ways to supply 12 volts to the coil during starting using a diode in the wire with the cathode of the diode pointing toward the coil.
 
X2, Ballast voltage same as coil voltage, 6v/6v. However back in the day when cars used ballast resistors, they bypassed the ballast during starting to get a hotter spark because the batteries cranking voltage is less than 12v. If cranking volts drops to 10v, then coil gets only half, 5v.

There are many ways to wire around the ballast during starting which depends upon your ignition switch or starter solenoid. I had neither option on my Jubilee so I used a diode, took the starter voltage and applied it directly to the coil.

No seems to mention the old school way of bypassing the resistor.

Another thing you need to have about 4 amps going to coil. If you want points to last longer, Use the largest resistor and get the least amount current to make it run OK. bBcause when alternator is charging you may have battery voltage of 14v. 7v ballast and 7v to coil.
 
If you want to use your old 6v coil, you need a ballast resistor.
Or you could buy a 12v coil that doesn't require a ballast.

Read my post below for reasons ballast resistors were used, cranking voltage..
 
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