actual need for ballast resistor with electronic ignition?

str12-340 said
I have three LA small block engines in street driven cars. All 1970 Darts with factory wiring.

All use Pertronix Flame Thrower coils. The one that has an MSD system and the one that uses a Mopar electronic ignition distributor with and FBO brand ECU have both had the Ballast Resistor bypassed as directed by MSD and FBO.

The last car has a Mopar electronic ignition with a Mopar orange ECU that has the ballast resistor functioning in the system. I realize that the purpose of the ballast resistor is to reduce the voltage to the coil between Run and Start (although I can never remember which mode has the reduced voltage). I understood that this was originally done in points ignition systems to extend the life of the points. Why is it needed with electronic ignition? What are the potential issues if the resistor is, in effect, bypassed?

In a Chrysler wiring diagram which is the Start and Run circuit (in the FSM they are labelled ignition 1 & ignition 2). One is a brown wire to the ballast resistor and one is Blue (and then blue with a tracer between the bulkhead connector and the welded connection which also feeds the blue alternator field wire and the blue voltage regulator wire). Which is which?

Thanks for your help!
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The single ballast resistor, or the ballast resistor half of a dual resistor, performs a similar function as in the contact points system with Chrysler Electronic Control Units (ECUs) whether a four-pin ECU or five-pin ECU. That is, it limits current through the series connected coil and switching power transistor in the ECU system, as with the series connected coil and points in the contact points system. The resistance increases as the current increases, which hears the resistor, in general due to engine speed increase. The increased resistance limits the current further protecting the coil and transistor or points. Both systems bypass the resistor during engine cranking. Voltage at the coil varies in relation to the current and resistance.

The Chrysler ECU employs timing and driver circuitry internally to switch a power transistor with “turns on and off”, similar to contact points, to switch the coil primary on and off, producing the secondary spark. The electrical current primary ignition path is from the battery through the bulkhead connections, through the ignition switch contacts, through the ballast resistor, through the switching transistor, then to ground. The power transistor mounted on the outside of the ECU case with a heat sink can sink more current than contact points. The dwell time of points that is adjusted by setting the point gap is controlled electronically within the ECU timing circuitry. The ECU mounts solidly to a body ground, both mechanically and electrically, and uses a molded five-pin connector to wire into the vehicle electrical system, ignition coil, and distributor.

When the engine is started, during “Ignition Start”, or "Ignition 2", the ballast resistor is bypassed, to provide the highest available voltage, while the key is held during cranking, ie. the brown wire. When the engine begins to run, and the ignition switch returns to "Ignition Run", or "Ignition 1", ie. the blue with tracer and blue wires. At this point, the ballast is initially cool and resistance is low. As the engine runs at low speeds with longer induction build and collapse times in the coil, the ballast heats up, resistance increases, and current through the coil and voltage applied drops, preventing coil overheating and reducing point arcing. At higher engine speeds, with lessened induction build and collapse times in the coil, the ballast cools, resistance lowers again, and more current and applied voltage for the coil is available for higher speed secondary circuit spark plug firing.

The primary ignition circuit wiring of the old point system was modified and expanded to include the ECU when Chrysler converted to electronic ignition. Although there are some wire color variations in production cars and trucks, the general color coding of the changed wiring, and that used in the Chrysler wiring harness kit, Chrysler P3690152, or Standard Motor Products S516, is as follows:
A single, two terminal used with production ECUs since 1980, part number 4106340, 5206436, et.al., is 1.25 ohms ( 1.12-1.38 ohms). This resistor is a closed back, ceramic power resistor with different thermal properties than earlier single ballast resistors. The Chrysler ignition ballast resistor used in the 1960s through 1972 for point systems, part numbers 2095501, 2196316, or 2275590, is also an open back, ceramic power resistor. This ballast resistor measures 0.5 - 0.6 ohms af 70-80 degrees Fahrenheit and has specific, expected thermal design properties. The Chrysler P4120505 orange ECU is specified to use a 0.5-0.7 ohm ballast resistor with a production or similar coil, 1.3-1.8 ohms, good to 5500 RPM.

The primary, or compensating, side of the dual resistor initially used with the ECU is the same type as the earlier point systems with an open back, ceramic housed, wire wound nominal 0.55 ohm resistor in part number 3656199. This value was changed during 1975 production to 1.25 ohms with part number 3874767. The resistor performs the same thermal adjusted coil current/voltage stabilization as the single ballast resistor. The resistor also limits the current through the power transistor of the ECU, protecting it, similar to the same action with points.

The other side of the ceramic case dual ballast resistor houses an enclosed "non-thermal" auxiliary resistor which measures 4.75 - 5.75 ohms af 70-80 degrees Fahrenheit. This nominal, five ohm resistor is connected to the "fifth pin" of the ECU. Internal to the ECU, the auxiliary resistor connects to the collector lead of the driver transistor for the main power transistor and part of its biasing. It limits overall current from the 12 volt supply. The other four pins of the ECU connect to the dual lead of the distributor pickup coil, the 12 volt Ignition Run primary power, and the lead to the negative side of the coil for the coil switching action, similar to the connecting lead from the distributor points in the old system.

The same type of coil, such as part number 2495531, et.al. as used with the points system is also used in production with the ECU. The Chrysler 1960s - 1979 production coil resistance at 70-80 degrees Fahrenheit measures 1.6 - 1.79 ohms for the Prestolite 2444242 type coil and 1.34 - 1.55 ohms for the Essex 2444241 type coil. The total dynamic action with an ignition coil involves its total impedance, but the direct current (DC) limiting effect is based on its resistance. The combined total resistance of the coil, ballast resistor, and the marginal wiring, connections, and effective transistor resistance still add to about two ohms as it did with the point system. The ~two-three ohm total, which varies as the ballast resistor varies, limits the transistor and coil primary current to about four-six amps peak, although the transistor can actually sink more current. The Darlington pair power transistor mounted on the outside of the ECU case with a heat sink can potentially sink up to 10 amps.

Bypassing the ballast resistor can lead to over driving the ECU's switching transistor,and potentially the coil itself depending on the coil resistance and construction. The Pertronix Flame Thrower part number, 40011, specifies a primary resistance of 1.5 ohms, and that it can run without a ballast resistor regarding the coil itself. Running without a ballast resistor and assuming ~12 volts across the coil and the ECU switching transistor yields a current of ~8 amps which is at the upper end of the P4120505 transistor specifications. It might handle it okay for a while, but with extended use and heat, the ECU could fail.

NOTE: The foregoing discusses ratings and expectations with the original P4120505 ECU, and subsequent units built the same. Units made in recent years likely will not perform the same. OEM production Chrysler ECUs will perform similarly with regard to current and resistance ratings, and the original higher performance Direct Connection and Mopar Performance units, 3438850R, P3690011, P3690256, P3690256A, P3690256B, P4007298, P4120505, P4120534, P4120600, will perform at and above the level of P4120505. Aftermarket ECUs might vary in what current they can handle compared to original Chrysler units.