Testing an HEI ignition coil is a fundamental skill for any mechanic or serious DIYer working on classic GM vehicles. The High Energy Ignition system, while robust, relies on a single component—the ignition coil—to generate the spark necessary for combustion. A faulty coil is often the prime suspect when an engine cranks but refuses to start, or when it misfires under load. Understanding how to properly verify the coil’s function saves time, money, and the frustration of replacing unnecessary parts.
Understanding the HEI Ignition System
The HEI distributor is more than just a distributor; it is a complete ignition module that houses the coil and a built-in electronic igniter. This design, pioneered by GM, eliminated the need for external points and a separate ignition module, simplifying the ignition circuit. The coil itself is a step-up transformer that converts the vehicle’s standard 12-volt signal into the thousands of volts required to arc across the spark plug gap. Testing must account for both the power supply to the coil and the integrity of the coil itself.
Safety Precautions and Initial Checks
Before touching any wiring, ensure the vehicle is turned off and the key is removed from the ignition. Disconnect the negative terminal of the battery to eliminate the risk of electric shock or accidental short circuits. Visually inspect the coil wiring harness for obvious signs of damage, corrosion, or loose connections. Pay specific attention to the primary wire (usually the thick black wire) and the positive terminal, as these are common points of failure that can mimic a faulty coil.
Checking for Power Supply
The first step in diagnostics is verifying that the coil is receiving power. With the key turned to the "Run" position, use a multimeter set to DC voltage to test the positive terminal of the coil. You should see approximately 12 volts. If power is present, the issue likely lies within the coil or the ignition signal. If power is absent, the problem shifts to the ignition switch, the ballast resistor, or the wiring harness between the battery and the coil.
Testing the Ignition Coil
To test the coil itself, you must measure its resistance. This is done using the ohms setting on a digital multimeter. The process requires checking both the primary and secondary windings of the transformer. The primary winding is a low-resistance circuit, while the secondary winding is a high-resistance circuit. Comparing your readings to the manufacturer's specifications is the only way to determine if the windings are intact or shorted.
Procedure for Resistance Testing
Locate the two terminals on the side of the distributor cap that connect to the coil. These are the positive (+) and negative (-) terminals of the primary winding. Disconnect the wires and touch the multimeter probes to these terminals. A typical primary resistance reading should fall between 0.5 and 1.5 ohms. Next, test the secondary winding by placing one probe on the positive terminal of the coil and the other on the distributor cap output where the wire that goes to the distributor center terminal.
Interpreting the Results
A healthy secondary winding will show a resistance reading between approximately 10,000 and 16,000 ohms, or 10k to 16k ohms. A reading of infinity indicates an open circuit, meaning the coil is burned out and needs replacement. Conversely, a reading of zero ohms indicates a short circuit. It is important to note that environmental temperature can cause slight variations in resistance, but the values should remain within the specified range.
