Posted by Johnd
Don't be quick to dump the old ballast resistor type coil, it has a feature that you will give up by ditching the old ignition system.
All ignition coils operate internally on about 9 volts , even when you connect 12 volts to them. This is accomplished by either an external resistor, or the resistance is built inside the coil canister.
On the surface, it makes sense to dispose of the external resistor and replace the coil with one that has the resistor built in - doesn't it? One part instead of two? Maybe not! You see, when you buy a coil with the internal resistance, you give up access to the resistor connections, and they can be valuable in older vehicles and engines, mostly because of starter motors!
When you start an engine, the amperage required to spin the engine over spikes, and the by product of those high amps is a voltage drop in the entire electrical system. Most troublesome is lower voltage going to the coil. Its not good to have low voltage to the coil during starting, its the worst time to have that happen. If normal 12 volts delivers 9 volts to the coil (after the resistor) - and the coil is happy, what happens when starting current reduces the 12 volts to 9 volts? The answer is that the resistor drops the 9 volts down to six volts, and you lose your spark while cranking. No spark, no run!
The old starters from the 50's and 60's used a lot more amperage than the efficient gear reduction starters from today. Its why the external resistor was needed, because you could wire your starter switch (or foot pedal) directly to the coil positive terminal (bypassing the resistor) - but only while cranking. Thats why you always saw two wires hooked to the coil + terminal, one from the ignition "ON" switch, the other from the "START" terminal or foot pedal. This assured that you had as much voltage as was available going to the coil, even while cranking.
Many of us have had an engine that wont start while you have the ignition on, and you are cranking the starter. BUT, the very instant you release the starter, the engine begins to run!! This is because when you release the starter, and the battery voltage goes back to normal, your spark resumes while the engine is just barely still rotating from momentum of the flywheel. WOW! An engine that only starts when you STOP trying to start it!!!
Older vehicles with external resistors knew there would be low voltage going to the ignition coil when starting, so they simply bypassed the resistor by sending the full battery voltage directly to the coil, but ONLY DURING CRANKING. Now you can see what you give up when you go to the internal resistor coil - you give up the access to the output of the resistor, so you can bypass it during starting.
A lot of people have ditched the external resistor and have not seen any bad effects. The reason is that the starter didn't drag the voltage down so low that the coil stopped working. Use big heavy battery cables on these old trucks, they will help reduce the low voltage effect during starting. It happens more in cold weather, or first start up of the day than after the engine is warmed up. The reason is the starter isn't working as hard, and drawing less amperage.