How to Test an Ignition Coil

This testing procedure is valid for just about any automotive coil. Using an ohmmeter, check the resistance between the side terminals of the coil. Do this with all of the wires to the coil disconnected. You should see 0.75 to 0.81 ohm of resistance. Then check the resistance between either side terminal and the center high tension terminal. The reading should be 10,000 to 11,000 ohms. Any significant deviation from these numbers would indicate that the coil is defective.

About and How to Test a Ballast Resistor
by Doug Lawson

There are two type of distributor ignitions, Ballast and Non-Ballast Ignitions:

There are many misconceptions about ballast ignitions and what they do. Perhaps this will dispel some confusion. The ballast ignition system includes an external resistor (or resistor wire) in series with the voltage supply to the coil’s (+) terminal. A “standard” coil will have an internal resistance close to 3 ohms on its primary windings. The coil for a ballast ignition system will have a primary winding resistance between 1 and 2 ohms. The external ballast resistor (or resistor wire) will provide an additional 1.5 ohms of resistance. Current flowing through the ballast resistor will create a voltage drop and thus lower the coil’s operating voltage to something between 6- and 9-volts. Together, the ballast resistor and the internal resistance of the coil limit the current flowing through the system. The ballast resistor on the right, commonly used with British Classic cars, will use a ceramic or low conductor to dissipate heat harmlessly.

If the ballast coil is wired into the ignition system WITHOUT the external resistor it will always operate at high voltage. The increased current flow through the system and the higher spark voltages will cause premature failure of the points. It is imperative therefore that the proper coil be installed in a car to maximize its ignition system. When troubleshooting a ballasted ignition system the external resistor and its associated wiring must be added to the components to examine.

Determining Your Coil Type (Ballast vs. Standard)

It is important when buying a replacement coil that the correct type is selected for the vehicle’s ignition system to achieve acceptable life and performance. Lucas ignition systems have often been modified over the years and determining what is required may not be as simple as referring to the owner’s or service manual.

A sight check should be performed first. Some ballasted Lucas ignitions use a ballast resistor wire between the ignition switch and the coil (+) terminal (frequently pink in color). It is also possible that an external ballast resistor may be present instead of this wire. The ballast resistor will typically appear as a small ceramic brick (with wires) mounted in the vicinity of the coil itself and connected between the ignition switch and coil (+). Ballast ignition systems also have a wire between the coil (+) terminal and the starter solenoid. If the car’s electrical system is totally unmolested these visual clues may be enough to identify the ignition coil type installed/needed. 

To confirm your needed coil type it is best to make electrical measurements with a volt/ohm meter. Start by performing the test shown in the top coil images measuring the resistance across the coil’s low-tension terminals with all the wires disconnected. Note this resistance value but do NOT presume it correctly identifies the coil needed. Perform the following additional test.

Re-attach the low-tension wires removed to perform the test. Connect the volt meter between the coil’s (+) terminal and chassis ground. Temporarily fit a jumper wire between coil (-) and chassis ground. The jumper wire will insure that current is flowing through the coil and any ballast components during the test. It is necessary for current to be flowing to correctly measure the coil’s operating voltage. With the meter and jumper wire connected, switch on the ignition and observe the meter. If the meter shows battery voltage (nominally 12V), the system is non-ballasted and needs a standard ignition coil. If the meter displays anything between 6V and 9V, a ballast-type ignition coil is required regardless of what type of coil is currently on the vehicle. Standard ignition coils will have a primary resistance close to 3 ohms while ballast ignition coils are typically between 1 ohms and 2 ohms.

Using a standard coil on a ballasted ignition system will result in low spark voltages potentially leading to running problems. Using a ballast-type coil on a standard ignition system (without a ballast resistor) will result in excessive current flowing through the ignition system. This will cause premature wear of the points and potentially lead to reduced coil life.

To test your ballast resistor you need an ohm meter or multimeter set to ohms. Remove the connectors from both sides of the resistor. The ohms should read between 1.8 and 5 ohms. You should be getting 6-9 volts to the positive side of the coil. Also check the negative side I believe it should be 6-9 volts. If there is voltage to all those spots then you have eliminated the ballast and coil as problems.