Is a resistor required??

Do I need to put a resistor between my coil and my points? If so what type is best to install?

Too much resistance will cause a weak spark, possibly hard starting or poor running.

Not using a resistor when needed is asking for coil trouble, and burnt breaker points.

ALL OEM systems using a ballast resistor, (NO exceptions that I know of), have the ballast resistor between the ignition switch and the coil.

How do you know if a resistor is needed?

Well, if you have an accurate Ohmmeter, you can measure the primary resistance of the coil.

You want to see at least 3 to 3.5 Ohms, up to 4 Ohms, otherwise add a ballast resistor, so the combined resistance of the ballast resistor AND the coil primary is up in the 3+, up to nearly 4 Ohms range.

If you run a low-resistance coil, on a 12-volt system, without a ballast resistor, the coil will run HOT, probably eventually fail, and breaker point life will be SHORT.

Are you familiar with ignition waveforms, as seen on an ignition oscilloscope?

When the breaker points open, and spark occurs, the coil and condensor form a sort of simple resonant tank circuit, and several ocillations occur AFTER the initial spark, due to the actions of the coil's inductance, and the condensor's capacitance.

The coil oscillations are seen at the left of center, as the waveform that tapers off, after the "spark line".

It is undesirable to have resistance between the coil and condensor, because it would dampen these oscillations, and make the spark weaker.

When you test an ignition condensor on a tester, one of the tests is for lead resistance, and the capacitor will fail the test if an internal poor connection causes excess resistance.

Adding the ballast resistor between the coil and distributor would have the same effect as a capacitor that would fail the lead resistance test.

Believe this, or not, as you choose, but I feel there is valid reason NOT to put the resistance between the coil and the distributor, and every single factory system I've ever seen that uses a ballast resistor places it between the ignition switch and the coil, NEVER after the coil, so I have to assume the factory engineers think there was a valid reason to set it up that way, too!

Go ahead, and set your's up as you choose... there's always a contrarian in every bunch.

Most likely, it will work just fine, and you will say "AHA"... my way worked, and I agree with you that it will, but there is the advantage of a slightly "hotter" spark doing it the conventional way, and, likely, breaker point life will be a little better, as the condensor will be able to better do it's job.

Remember, energy "bounced" back into the coil by the condensor equals a "hotter" spark, while energy dissipated as heat by an arc across the breaker points is wasted as heat at the points, and anything that affects the designed relationship between the coil and the condensor is likely to cause MORE arcing across the points, as they open.

Good luck!

PS. I'm don't think I'm a "contrarian". I just had an honest question about your statement.

PS. I'm don't think I'm a "contrarian". I just had an honest question about your statement.

If you are having ignition problems look to the resistor first. Over volt can cause the coil to overheat.

Try it without and if you have problems try one.

jt

real 12v coils are cheap.. 16$.. etc..

Soundguy

Actually I did my conversions on 2 WD 45 Allis' and a Silver King.

Didn't even switch the coil and works ok.

One of those is a Mower tractor that gets a couple of hours run time every week this time of year IF WE GET RAIN:<(

jt

Soundguy

Of course if I need a new coil in the future it will be replaced with a 12V.

You are also correct that the points and condensor are being taxed a little harder with my set up but it works.

The other side of that engine is also modified. Underslung exhaust on a tractor that never had it available to my knowledge.

jt