Still in the 200 Ohm mode, measure between coil’s + and - terminals. Allow a few seconds for the reading to settle, until it stabilizes. Subtract the previous reading, taken with the leads crossed, to compensate for multimeter’s inherent resistance. Do not use a low-resistance coil, such as the MSD or Accel coil; they don’t have enough primary resistance for this application. Using a coil with too little primary resistance can cause the ignition module to overheat and misfire until it cools down again or fails, voiding the warranty.

Ohmmeter Calibration: When the Ohmmeter’s red and black leads are connected to each other, the reading should be 0.00 or very close to zero. If, though, for example, with both Ohmmeter leads shorted together, the reading is 0.5 Ω, you’ll need to subtract 0.5 Ω from the reading you get when measuring the coil’s primary resistance. For example, if the coil’s primary resistance reading is 3.5 Ω, but when the Ohmmeter leads are shorted together the reading is 0.5 Ω, then the coil’s primary resistance is 3.0 Ω: 3.5 Ω - 0.5 Ω = 3.0 Ω. This calibration procedure applies only to resistance measurements made in the 200 Ω mode of a digital Ohmmeter.

The inherent resistance of the leads is negligible in higher Ω modes of the Ohmmeter. If you’re using an analog Ohmmeter, touch the leads together while setting the needle on zero before measuring resistance.

Measuring Coil Secondary Resistance: For best performance, coil should have 7K Ohms or more secondary resistance. To measure secondary resistance: Label and remove all wires to coil ( + or - ). Using a common digital multimeter in the 20K Ω mode, place one Ohmmeter lead on the coil's + or - terminal and the other Ohmmeter lead in the coil's center high-tension wire socket, the socket that holds the lead from the center of the coil to the center of distributor cap.

Check the voltage reading at the coil's + terminal, engine running, at maximum RPM at the coil's positive terminal. If the voltage reading is more than 13.9 volts at any RPM level, the voltage regulator likely needs replacing and/or a 1.4 Ohm ballast resistor should be wired between the coil's + terminal and the red Hot Spark ignition wire. Too much voltage can damage the ignition module and other electronic components. The ballast resistor is usually mounted on the firewall or directly on the coil. Charging system voltage of 13.4 volts or so is plenty.