?Not sure. The coil is a step up transformer.. It takes the 12-13 V on start up and the 12-13V/ 1.35 Ohm for voltage on Engine RUN. The coil steps up the voltage and applies the higher voltage 20k-45,000 Volts to the spark plug gap.
The coils do get banged up and get internal shorts. Where there is a voltage charge and not GOOD ground to spark to the coil will short out internal. The Voltage jump internal to the coil will put small shorts across the magnet wire by melting the formvar insulation (like shelack coating). The more shorts the less windings and the less the voltage will be put out.
Poor Grounds cause shorts in coil.... and is the reason the spark plug boot should not just be pulled off to see is engine changes. That will cause small shorts in coil. We have all done this but not a good idea.... we have a 30 yr old truck... I am sure the spark plug boot has been pulled off a few times. If a spark plug boot is pulled give it a good ground to engine prior to stating or running.
There is a RESISTANCE reading on the coil that can be done with a multimeter. The fixed winding will have a resistance range.... if lower than the range it has shorts and net effect of having less windings/coil winds or it will have a high resistance and means the coil wire broke and has a short now. Multiple short in the coil is common, esp when you consider some of us have 30 yr old coils.
The resistance range to test your old coil is below. The test procedure can be found in the Jeep Field Service Manual, FSM in the Ignition Section. Also found in the aftermarket manuals.
1.... Primary Resistance Test..
remove coil connector. Measure ohms from the neg to pos terminal on teh coil. Should read 1.13 - 1.23 ohms at 75F degrees
2... Secondary Resistance TEst.....
remove ignition wire from center terminal. Measure ohms from center terminal (coil spark plug wire) and either terminal on the coil. REsistance should measure 7700 - 9300 ohms at 75F
POOR GROUNDS kill the COIL and the spark needs good gounds for the DC Spark to get to the spark plug gap and onto the battery. DC make a complete loop from the battery to the auto and back to battery.
Upgrades I learned from JeepHammer for spark energy & Control:
1..Use premium spark plug wires. I use the MSD premiums or the NApa Drk Navy Beldins.
2..Use copper antizeise on the spark plug threads. Helps with the grounding and many heads are alumium and the plug has steel threads. Aluminum oxidation happens in air, not to mention a 220F engine, not to mention its no in contact with steel. Steel and Alumium a no no with out antizeise. Even for your small engines too.
3..Ground your heads. The spark engergy needs to jump the spark plug gap to light the fire. The strap is a ground from the threads and engine head. The head is often alumium, sits on a steel block, uses a thick gasket and has oil varnish, chemicals, and 30 yrs of corrosion and use. These do not have the best ground. It helps a great deal to ground your head or both heads with a 14 or 12 awg wire.
4..Ground your IGNITION Modual. The ignition modual gets its ground from the Distributor. The distributor is cast alumium, sits on a steel block (issue galvonic corrosion & oil varnsih) and is the GROUND for the Ford DuraSpark Igntion used Jeep in the late 70's and 80's. Poor Ignition Ground will yield poor performance and the weak ground can kill your Jeep DuraSpark Ignition Modual, it will get hot, melt the black potting compound, and you will see black goop oosing out of the Igntion Modual on the drivers fender.
Can ground the Ignition two ways.
A... Ground the Distributor
B... Ground the black wire going to the DuraSpark Ignition Modual. THe modual on drivers fender has two connectors. One has a black wire, that is gound. I cut this wire on the HARNESS SIDE NOT THE IGNITION MODUAL SIDE. We cut the harness side so the mod stays with the jeep wire harness and if the Igntiion Modual needs to be swaped out the aux ground will always be there on the harness side. So cut the Black Gound Wire on the Harness side about 8 in from the connector. Slide on some heat shrink down the wire and solder in a 12-14 awg wire Black and long engough to reach the Neg Battery Terminal or the fire wall for the aux ground. Add some dielectric grease and shrink down the heatshrink to protect the new aux ground for igntion generation.
5.Do the ignition grounds above and you may get more pep, easier starts, better idle.
This would also be done with the TEAMRUSH larger cap & rotar during a tune up . New Spark plug wires and the autolite copper plugs. Do all this and I would say you will be 20% better pep, more rpms, better mileage, better starts, better idle. ON my 78 it was adding a 7th cylinder to my
AMC 258 i6 / 4.2l six banger that had all new parts and a complete rebuild...
Big a affect on the
AMC 258 i6 / 4.2l and even a bigger affect on the 8 cylinders since they have more rpm range.
Not mentioned in your post... If you moved the distributor or tightened the hold down foot. This might have given you a better igntion ground. FYI.