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Ignition Ballast Resistor

(4.3 Ohm Coil with 1.5 Ohm Ballast Resistor)


Just what is a Ballast Resistor and why is one needed? A Ballast Resistor is an extra electrical "load" which is installed in the electric circuit providing power to the ignition coil for the purpose of reducing the voltage that the coil receives. This may seem somewhat counter-intuitive as one would think that the higher the voltage to the coil, the better the spark to the plugs and the better the engine will run. That is true up to a certain point. After that point, the extra voltage simply causes the coil to run at a higher temperature than it needs to. For years, the Indigo Electronic Ignition Kit has been offered and installed without a Ballast Resistor with essentially no feedback that there was any issue with the coil running hot. It has been known that the coil can be found to operate near 200 F but that was to be expected and has not presented any problem. However, a very lively and informative discussion was held on the Moyer Marine Forum in 2011 regarding coil heating, failures and possible solutions. At that time it came to light that a few A4 owners, at least those who participate in the Forum, had indeed experienced multiple coil failures. The concensus was that the coils were operating at too high a temperature and that was leading to premature failure. It was also realized that there are a number of variables that can have a significant affect on coil temperature, including the coil internal resistance, alternator output voltage, type of alternator regulator, A4 operating speed, A4 operating temperature and engine compartment temperature, and A4 duty cycle (long periods of operation versus short periods).

We took this concern to heart and decided it was time to conduct our own testing. Three different Ballast Resistors (1.0, 1.5, and 2.0 Ohms) were tried initially. Picking the proper resistor is not as straightforward as one might think. While the coil has a certain internal resistance (as measured across the two brass studs on it), a coil does not behave in the same manner as does a plain resistor when installed in an electrical circuit. Inductance and capacitance come into play such that the current flowing through the coil is less that the value obtained by simply dividing the Voltage across the coil by the coil resistance (I=V/R). As testing progressed, it was quickly realized that the 1.5 Ohm version would be the optimum choice for several reasons. First, the 1.5 Ohm resistor provides about a 2 Volt drop. This will give a range at the coil of from 10-12 Volts with the system Voltage being 12-14 Volts. That range insures that you have sufficient spark to keep the engine running yet protects against high system Voltage. Second, the 1.5 Ohm Resistor reduces the Wattage (and thus the heating and operating temperature)in the coil about 35%. The type of Ballast Resistor chosen provides reasonable cooling surfaces and a miminal full load rating of 50W. However, the resistor selected has to be de-rated from the 50 Watt rating as no additional heat sink is installed and also for an ambient temperature of 150F (typical engine compartment). Nevertheless, the resistor is still rated for continuous operation at 6 Watts which is just about two times the worst case operating condition for the A4 (See Table Below).

One very important consideration given to any ignition system is that is must be capable of starting the engine when the starter motor is running. It is at this time that the system voltage is at its lowest as the starter motor draws a very significant amount of current. It is not unusual to see the voltage drop as low as 10 volts when starting. For this reason, a jumper wire is utilized during starting to remove the Ballast Resistor from the circuit and provide full available voltage to the coil. On all late Model A4 starter motor solenoids, there is an #8-32 stud which attaches to an internal auxiliary contact. This stud is normally not energized. However, when the starter motor is engaged and running, full system voltage is present at this stud. A jumper wire can be run from this stud to the (+) terminal on the coil thus providing full system voltage to the coil for starting. Once the starter disengages, the stud is no longer energized. That is OK as the system voltage has returned to its normal level as the starter motor is no longer energized and system voltage is supplied via the normal path.

Test data was obtained utilizing a bench mounted system which is routinely used to verify Electronic Ignition Kit components prior to packaging and shipping. The shutter is rotated at a constant speed of 360 RPM which is equal to an engine idle speed of 720 RPM. The slower the engine runs, the greater the heating of the coil so this shutter speed represents a worst case condition. Voltage applied to this system is maintained with a 12 VDC battery and a Smart Charger. A voltage of 13.5 VDC was chosen to simulate an alternator which charges at a voltage level appropriate for proper charging. Voltage and Amperage were both measured with a Fluke Digital Volt-Ohmeter and temperatures measured with a non contact Infrared Laser Thermometer and verified with a contact probe used with the Fluke meter.

Indigo Ignition System Performance - With and W/O a Ballast Resistor


Voltage from Ignition Switch VDC

Measured Current Amps

Coil Temperature / Watts

Ballast Resistor Temperature / Watts

No Ballast Resistor



195F / 25.7

1.5 Ohm Ballast Resistor



165F / 16.2

178F / 3.0

A Complete Retrofit Package including:

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Indigo Electronics, Inc.

105 Pipe Kiln Court
Williamsburg, VA 23185

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1-800-428-8569 toll free
or 757-593-8569

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