Ballast Resistors, Resistance and Coils

What is a Ballast Resistor and what does it do and why?

A Ballast Resistor is an electrical device that reduces voltage to the coil so that more energy is available for the spark plugs during starting.

Why would a Ballast Resistor be fitted in a car?

Traditionally, ignition systems didn’t use a ballast, meaning that the ignition system used a 12v coil with a 12v feed from the battery via the ignition switch.

This system is fine when the engine is running, however can be problematic when starting. Because the starter motor draws a huge amount of current from the battery, the spark plugs are left with less energy to create the spark. This is worsened in colder starting temperatures or when the starter motor is worn, when even more energy is drawn by the starter motor leaving even less available for the spark plugs.

To counteract this distribution of energy, ignition systems were changed to use a 9v coil instead of a 12v coil. A 9v coil gives the same output as a 12v coil when provided with a 12v feed, resulting in a better spark for starting with less energy draw on the starter motor.

Once the engine is running, the 12v feed is cut and the coil continues to run on the 9v feed.

How can I tell if I have a standard or ballast coil system?

Ballast ignition systems were introduced in around 1970. To find out for sure, use a multimeter to check the voltage on the positive wire of the coil with the ignition on.

You can test the resistance of the coil using a voltmeter and measure the Ohms (resistance) across the +’ve and –’ve terminals.

A standard coil should read around 3 Ohms or around 12 volts

A ballast coil should read around 1.5 Ohms or around 6 volts

(maximum 3.4 Ohms is ideal for our systems)

Which coil should I use if my car has a ballast resistor?

Using a non-ballasted coil would mean that you are running a 12v coil on a 9v feed, resulting in a weak spark. You can however use such coils provided you remove the ballast resister.

Never try to run a ballasted coil when a ballast resistor is not present in the ignition system!

Can I fit a Powerspark Electronic Ignition Kit if I have a ballast resistor?

If you have a ballast coil installed with a ballast resistive wire or a ceramic block then you can keep the coil as it is, there is no need to change it BUT if you change the coil you do need to match it to a ballast if one is present and not bypassed.

Some customers report that bypassing the ballast resistor by disconnecting the ballast feed wire ( from the starter solenoid or built into the loom) and providing a direct 12 volt feed from the starter solenoid/relay or fuse box to the ignition coil has proved to be a success and has given good results.

Testing for a ballast resistor

Checking for a Ballast resister/ This test checks for the presence of a ballast
resister or resister wire before and in series with the ignition coil.
This is vital in choosing the correct ignition coil for the vehicle.

1. Test procedure:

1.1 Switch off ignition.
1.2 Disconnect all wires from coil(-).
1.3 With the ignition switched on, measure the voltage at coil(+) with red
probe to coil(+) and black probe to chassis/earth. Note the voltage, which
should be approx 12.5v on a fully charged 12v battery.
1.4 temporarily earth coil(-) while repeating 1.3 above.
1.5 Reconnect the wires disconnected from coil(-) in 1.2 above.

2. Interpreting the results of the test:

2.1 If no significant voltage drop is detected in 1.4, the vehicle does not
have a ballast resister.
2.1.1 Use a 3Ω coil for points ignition or a ‘standardʼ electronic
distributor or ignition kit.
2.1.2 Or use a 0.8Ω ‘high energyʼ coil with a ‘high energyʼ distributor
or ignition kit.
2.2. If the voltage drops to approx 5 volts or 6 volts, there is a ballast
resister or resister wire.
2.2.1 Use a 1.5Ω ballast coil.
2.3 If the voltage drops to approx 8v to 9v, there is a ballast resister and
possibly a non-ballast 3Ω coil.
2.3.1 This is not a good combination and will lead to a weak spark and
poor running.
2.3.2 Test the primary resistance of the coil and replace with a ballast
2.4 If the voltage drops to below 4 volts, there is a ballast resister and a
low resistance / ‘high energyʼ coil.
2.4.1 This is not a good combination and will result in no spark and
possibly overheated ballast.
2.4.2 If you intend to use a ‘high energyʼ ignition system, the ballast
will need to be removed or bypassed.
3. Recommended coils
3.1 Non-ballast:
3.1.1 Viper VCS (push fit HT connector).
3.1.2 Viper VC12VST (screw fit HT connector).
3.2 Ballast: Viper VC110.
3.3 High Energy: Viper VCE.