Testing an HEI coil is a fundamental skill for any mechanic or DIY enthusiast working on classic GM vehicles. The HEI (High Energy Ignition) distributor is the heart of the ignition system, and a faulty coil can lead to frustrating misfires, hard starts, or a complete lack of spark. Before replacing parts, a systematic diagnostic approach saves time, money, and guesswork.
Understanding the HEI System and Coil Function
The HEI system relies on a self-contained ignition coil that steps up the battery’s 12 volts to the 20,000+ volts required to create a spark across the spark plug gap. This coil is a type of ignition transformer with primary and secondary windings. When the ignition switch is on and the points or electronic module complete the primary circuit, energy stores in the coil’s magnetic field. When the circuit opens, this stored energy collapses, inducing a high-voltage current in the secondary winding that is sent to the distributor cap.
Visual Inspection: The First Critical Step
Before grabbing a multimeter, conduct a thorough visual examination. A damaged coil is often immediately obvious. Look for the following signs:
Cracks in the plastic casing or primary and secondary wiring insulation.
Burn marks, blistering, or evidence of arcing around the terminals.
Leaking oil, as HEI coils are filled with oil to aid in heat dissipation and insulation.
Corrosion on the spade connectors at the base of the coil.
If any of these physical defects are present, the coil should be replaced immediately, as testing may be unnecessary and potentially unsafe. Testing with a Multimeter: Resistance Method The most common way to test an HEI coil is by measuring its electrical resistance. This checks for open windings, shorted turns, or excessive wear. Set your digital multimeter to the Ohms (resistance) setting. You will need to identify the correct terminals: the primary connections are typically on the side of the coil (marked + and -), and the secondary is the high-output terminal that goes to the distributor.
Testing with a Multimeter: Resistance Method
Primary Resistance Test
Place one probe on the positive (+) primary terminal and the other on the negative (-) primary terminal. A healthy HEI coil will typically show a resistance between 0.5 and 1.5 ohms. A reading of infinity (OL) indicates an open primary circuit (burnt wire), while a reading close to zero suggests a short.
Secondary Resistance Test
Place one probe on the positive (+) primary terminal and the other on the high-output secondary terminal. The resistance here should be significantly higher, usually between 9,000 and 11,000 ohms (9k to 11k). A reading of infinity indicates a break in the secondary winding, while a very low reading suggests a short-to-ground.
Live Voltage Test: Confirming Spark Signal
Resistance tests can confirm a coil is dead, but they cannot always predict a failure under load. A live voltage test confirms the coil is receiving and firing correctly. You will need a test light or a spark tester tool. With the ignition off, locate the coil output wire (the one that goes to the distributor) and the ground wire. Connect your test light between the output wire and a good ground.
Turn the ignition to the "Run" position. The test light should illuminate brightly. Crank the engine; the light should flash rapidly. If the light does not come on at all, check the power supply to the coil (fuse and ignition switch). If the light stays on steadily without flashing, the coil is likely not triggering the points or module, or the points are stuck closed.
