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  Topic Review (Newest First)
05-30-2019 03:30 AM
1project2many Reducing voltage through the primary winding below the expected level typically results in weak or no spark. Without enough current the coil never reaches saturation. A current limiting module would never switch to limiting mode while a time based switch such as points or a simple electronic ignition will follow the same "on-off" pattern regardless of current. This same situation occurs when a "12V coil" is installed on a 6V system with a ballast resistor or when saturated injectors are used with a peak-and-hold injector driver.

FWIW when I look up an MSD distributor for the 4.2 the only one listed is part no 8516 which is designed to be used with a complete MSD ignition and blaster coil 8228.

The MSD coil compatibility list doesn't list the 8228 coil but notes that "A ballast resistor is not necessary with any MSD Series Ignition but can be left in-line if originally equipped." This suggests that the MSD uses a current limiting switch and that failing ignitions are not likely to be caused by a missing ballast resistor.

If an actual diagnosis is requested I think more complete information is required.

Edit: Wanted to say I agree fully with this:
just replace it next time it fails with a CRT HEI distributor
There is a reason MSD ignitions have earned the name Must Soon Die in some circles. Run a 7 series or trailer it home. lol.
05-29-2019 08:25 PM
Originally Posted by Que89YJ View Post
Actually if the ballast resistor is still installed going to the MSD it would source voltage too low causing high current and burn the coil out.

Sent from my SM-G955U using Tapatalk
Good point!
05-29-2019 06:52 PM
Que89YJ Actually if the ballast resistor is still installed going to the MSD it would source voltage too low causing high current and burn the coil out.

Sent from my SM-G955U using Tapatalk
05-29-2019 10:10 AM
1project2many Coil for use with ballast resistor is different from coil for use without. Most electronic ignitions limit current so resistor not necessary but that's not a definite rule. We usually check the resistance of coil with ohmmeter to tell. If you need a resistor there are many apps to get one from. 1991 XJ is probably an easy one stocked at many stores.
05-29-2019 10:00 AM
bpounds You need to know whether the MSD distributor you have is supposed to have a ballast resistor, or if it is supposed to get a full 12v source. If it was designed to get the 7-9v like the original, blasting it with 12v will burn them out.

Your original distributor used the lower voltage, and the resistor was in the form of a resistor wire. Not a resistor mounted somewhere like was commonly used on other vehicles at the time. The resistor wire was just a yellow wire, same color as all the other ignition power wires that run all over the place, and it was really easy to confuse it or not realize that it was something special. The length of that wire was important. Every inch cut off of it would result in higher volts to the distributor (not a good thing). Usually you will find it was looped back and forth on itself, inside the firewall loom, so that it would be much longer than was really needed to reach the distributor. Another common mistake is to cut off all the extra wire, in an effort to make it nice and tidy, resulting in higher than designed voltage.

I have no idea what your MSD needs, but I think you are right to consider it as the problem. MSD makes a ton of different stuff, even plain old replacement coils that are nothing but OEM equivalent. So time to figure out what you've got, or maybe the path of least resistance (pun) is to just replace it next time it fails with a CRT HEI distributor, which definitely needs a full 12v source.
05-29-2019 08:29 AM
Siva283 Your ignition gets power from the battery. It only pulls what it needs. I think your issue is elsewhere. I however dont know your set up so I cant offer any real help. So heres a bump for you to keep it up in the list
05-29-2019 07:45 AM
YJ Ignition conversion to MSD

90 YJ 4.2 bypassed computer installed MSD distributor years ago, Then last year she needed a new alternator and yep I probably picked the one with most amp out. So the charge is going to the ignition module and apparently that is what is burning ignition module since i have replace it over 6 times and it would crank and run after. So anyone know what resister to put in front of it?

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