I'm changing my point type distributor to electronic ignition like many have done in the past. The conversion kit states to replace the factory ballast resistor with the one in the kit. My question is, WHY? The ECU is powered by a 12 volt source, and not through the ballast resister which is used to protect the coil (scroll down instructions to see wiring diagram). Can I just use my factory ballast resistor (which interestingly has a different ohm reading than the one in the kit) or do I need to use the one in the kit which looks awful compared to the factory piece.

Ballast resistor has nothing to do with the ECU but it need to match the coil's requirement and that may vary between a points and electronic setup.

Unless you have the old school 4 prong resistor in which case the second set of prongs is for the biasing resistor used by the power transistor. But that would not apply here.

My guess is that whomever wrote those instructions wasn't an electrical engineer and didn't notice that both the ballast and the coil need to be compatible.

So to clarify Sniper, I can use the factory ballast resistor with the factory coil and not worry that the resistor in the kit which differs from the factory resistor?

I know about some guys would tell the ballast has also to do with the ECU… but I’m not the guy who will debate this LOL.

I will debate it. The ECU can matter. Points is an on off switch. Very little voltage drop across the points if the ground is good and the points are healthy. So the ballast is set to the coil based on that factor.

It is a little funny to think an electrical engineer wasn't involved with the ECU ballast selection. The ECU also act as a switch, but it is a transistor that has a diode properties, and diodes have voltage drop. Depending on the power switching transistor they used it can cause a voltage drop between .5 to 1.5 volts or more. Additionally, if can have a current limiting resistor built into the circuit. Finally, the dwell on points is a fixed value partially set by points gap. ECU has a dwell set electronically, and it could be made to vary based on rpm. I don't know if it is, but more advanced ECU can do that. The pulse from the distributor coil varies in amplitude based on RPM, and that can be measured.

Regardless, the maker of the ECU, knows your swapping into a MOPAR points set up, they know the manufacturer coil. So, I would use the recommended ballast.

How much does this all matter, probably not much as long as values are close.

In the end the coil is designed to provide more KV for discharge then a normal engine needs. It is over sized to account for deterioration of the ignition system (wires, battery, spark plugs) and poor fuel, fuel mixture and environmental.

i've used anywhere from a ,25 ohm to a 1.8 ohm resistor without issue. real deal is how much voltage you want to the coil and will the coil handle the voltage you want. i use a .8 ohm on both cars now with a pertronix coil. i believe most factory point resistors were .5 ohm; not a big difference. the mopar kits have a 1.25 ohm resistor and will get you about 6 volts to the coil depending on the charging system; very safe.

The first couple times I swapped Mopar electronic ignitions into my points vehicles, I used the supplied double ballast resistor.
Several times since, I've left the original ballast resistor in place. I ran a stock or stock-replacement coil.
All of those cars - and I have 2 currently - ran fine, and I'd drive a few of them for 8 hours or so taking trips.
This is going back into the early '80s.

My stock original equipment ballast resister delivers about 6.6 volts to the coil so I'm thinking good enough. Not a race or modified street car, just a stock, fun driver.

DS, good info but doesn't the ECU skip the ballast resister all together and is fed by the 12 volt circuit from the ignition 1 side of the ballast?

No not really. It is a series circuit. So the 12V feed thru ballast to the + of coil and then into the ECU through the power transistor which as the other end of the transistor on chassis ground. If the transistor stays open, you measure 12V on the coil positive and negative with no current flow. Once the transistor conducts to ground you know have series circuit with 12V through .5ohm ballast, 1.9 ohm coil, .7V drop across transistor, any unknown inductor, capacitor, or resistor in the path to ground. That will then determine current flowing in circuit. So raise the ballast resistance, or increase the transistor drop and that reduces current to coil which means longer to charge based on RPM and dwell.

With a points you have 12V through .5 ohms ballast, 1.9 ohms coil, and we assume 0 voltage drop across points. In reality you can have voltage drop due to contact resistance at the ignition switch, bulk head connector, points, and any other connection terminal due to corrosion, or tarnish, etc...

the ECU's on both my cars get power off the start side of the ballast. should be run voltage.

Your description of current flow thru the coil circuit is correct. Your conclusions as to the effect of the ballast resistor are not.

the ballast resistor it a variable resistance that increases it's resistance as it's temperature increases. It's sole function is to prevent the coil from overheating. It has nothing to to do with charge time, that is the function of the points or the ECU.

True it is variable, but current flow has everything to do with charging the coil. The higher the current the faster it charges. Go stick that 5ohm ballast in place and see what the current flow drops too. See the effect on the spark intensity. Charging or energy absorption of the coil is current and time dependent.

The variable nature of the ballast was designed to prevent over heating of the coil at low rpm when dwell time is the greatest, hence higher resistance and less current. At high rpm you want more current flow because you have less time between firing events to charge the coil.

I was keeping it simple to explain the ECU. It is much more involved than a simple on off switch.

Regardless, change any value of a resistance, or voltage drop in a simple DC series circuit, and you change the current throughout the circuit. The ballast can be in front or after the coil, and it would have the same effect.

Bosch has a good tutorial on the coil and charging on the web.