Testing a HEI coil is a fundamental skill for any DIY automotive enthusiast or professional mechanic working with classic GM vehicles. The High Energy Ignition system, while robust, relies on a specific component—the ignition coil—to generate the high voltage necessary for spark creation. A faulty coil is often the root cause of misfires, poor fuel economy, and difficult starts, making it essential to verify its function accurately before condemning other parts of the ignition system.
Understanding the HEI Coil and Its Function
The HEI coil is not a simple transformer; it is an inductive energy storage device. It stores electrical energy in a magnetic field when current flows through its primary winding and releases it as a high-voltage spike in the secondary winding when the circuit is interrupted. This rapid collapse of the magnetic field is what generates the thousands of volts required to jump the spark plug gap. Testing methods must therefore verify both the integrity of the windings and the coil's ability to store and release energy efficiently.
Preparing for the Test
Before touching a multimeter, proper preparation ensures safety and accuracy. The vehicle battery must be disconnected to prevent any electrical surges and to protect the tester. Locate the ignition coil, which is typically mounted on or near the distributor cap in older HEI systems. Visually inspect the coil for any obvious signs of damage, such as cracks in the plastic casing, oil saturation, or corrosion on the terminals. Clean any corrosion with electrical contact cleaner before proceeding to eliminate false readings caused by surface contamination.
Testing Coil Resistance with a Multimeter
The most common initial test measures the resistance of the coil's windings. This test identifies open circuits or shorted turns. Set your digital multimeter to the Ohms (resistance) setting and touch the probes to the primary terminals, usually marked as the positive (+) and negative (-) or S and E. Refer to the service manual for the manufacturer's specified range, which is typically between 0.5 and 3 ohms for the primary winding. Next, measure the secondary resistance by placing one probe on the positive terminal and the other on the high tension terminal (the wire that goes to the distributor cap). Secondary resistance usually falls between 6,000 and 12,000 ohms. Significant deviations from these values indicate a faulty coil.
