Modern coil wiring

[Petrah Phyre]

On a modern 6v coil, is there a need to wire the primary with a resistor?

[Tom VanMeeteren]

Modern coils sometimes come with built in resistance. Check with your parts house and they should be able to tell you how many ohms of resistance are built into the coil.

[John Codman]

Primary side ballast resistors were installed to lower the voltage across the breaker points. When ignition systems switched to transistors instead of mechanical points, the need for resistors went away.

[TXGOAT2]

My 1981 Dodge has a factory breakerless distributor and it has a factory ignition resistor with a solenoid control. If the resistor fails, the engine will start and die. If the solenoid fails, the engine becomes hard to start, but runs just fine once started. I have a '72 FordF 250 with a Mallory breakerless distributor, and it still uses the factory ignition resistor arrangement.

[TXGOAT2]

Older, breaker type systems used a 4 or 5 volt coil on 6 volt systems and an 8 to 10 volt coil on 12 volt systems. Whenever the starter was engaged, the resistor was switched out of the ignition circuit. That put the full available system voltage to the coil, and that voltage would typically be in the 4 to 5 volt range on a 6 volt system with the starter engaged and around 10 volts with the starter engaged on a 12 volt system. As soon as the starter disengaged, the resistor was switched back into the circuit to maintain the coil primary voltage at around 4-5 volts for a 6 volt system and around 10 volts for a 12 volt system. The idea was to provide reliable starting, especially in cold weather or with a big V8, and it worked pretty well. The switching function was usually handled by an extra set of contacts in the starter solenoid.

[TRDxB2]

From https://www.jalopyjournal.com/forum/thr ... em.685471/
No most coils are designed to work at about 6 volts input. 12V cars use resisted inputs or internally resisted coils (usually clearly labeled as such) to keep the coil from over heating. Some starter solenoids, like ford 12v have an unresisted terminal that feeds the coil 12 v while the starter is spinning. theory is to give a hotter spark while starting the engine. Then when the key is released, the circuit goes through the ballast resistor or the resistor wire.
So a 6v car will run on just about any coil you mount that is not labels as internally resisted. The resistor in the 12v system usually operate the coil at 7 or 8 volts.
The important thing is to assure the coil is wired properly for the grounding system of the vehicle. Most vintage/old 6V were positive ground. so from ign the wire should go to the negative coil terminal and the positive terminal to the points, where the circuit gets grounded to positive when the points open. Opposite for negative grounded circuits.
As a point of reference in regard to resistance, it is the most usually reason why 6 V systems do not operate as designed. Any resistance in a circuit kills the electrical flow (amps) which 6 V system depend upon for proper operation. The most usually place is a corrupted ground after the load or loose or corrupted connections in front of the load.
The other most usual suspect is that the #1 gauge or thicker main battery cables have been replaced with the wimpy stuff 12V likes , and the ground strap from the body to the frame or engine is missing. One reasons that manufacturers switched to 12V is that they could half the gauges of wires used in the vehicles circuits. Saving weight but more important saving money.
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Pertronix 45011 Flame-Thrower II Ignition Coil, 6 Volt, 0.6 Ohm
Info
The hotter the spark, the easier your engine will start, and this 45,000 volt coil is just the ticket to a quick start! Has 0.6 ohms of resistance. Black case for a stock look.
6 volt coils can be used with 6 volt point systems. If used with 12 volt point systems, it should have a 1.00 OHM resistor added.
Item Details 0.6 OHM Resistance
Installation Tips
!Caution: Flame-Thrower II 0.6 ohm coils should NOT be used with the (original) Ignitor or point type ignition system. Flame-Thrower II coils are compatible with Ignitor II ignition systems, and most capacitive discharge (CD) systems that control the dwell period, or limit the current.
Just an example...

[Art M]

Using the full 12 volts on a 6 volt coil will usually burn up the points before the coil fails.
Been there and did it in the 1950s when the conversion to 12 volts took place.
Rumer has it that this phenomenon was discovered a hundred years ago. Mr Kettering probably discovered it when he developed the points/coil system.
Art Mirtes

[John Codman]

My 1981 Dodge has a factory breakerless distributor and it has a factory ignition resistor with a solenoid control. If the resistor fails, the engine will start and die. If the solenoid fails, the engine becomes hard to start, but runs just fine once started. I have a '72 FordF 250 with a Mallory breakerless distributor, and it still uses the factory ignition resistor arrangement.

Yup, those Mopar voltage dropping resistors were notorious. We carried them in the glove compartment of our tow truck. If a Mopar ran fine, was shut off, and then wouldn't start, we would replace the resistor on the spot. It always worked, and the customer would love us for it.

[TXGOAT2]

I carry a spare resistor and a spare module. You never know when you'll need one or the other, but you WILL need them! The starter solenoid is not reliable, either. It's a great truck, but the electrical stuff is of almost English quality.