Corvette Ignition Coils - No Fire In The Hole

Use a nut driver or a small... 

   

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Use a nut driver or a small wrench to loosen the securing nuts on the coil terminals and remove the leads; the positive side of the coil has a condenser lead attached in addition to a positive lead. Loosen the bottom clamp screw, pull out the center cable that goes to the distributor, and remove the coil. Alternatively, if you remove all three leads first, you can check the coil right in the car with your multimeter.

The ignition coil in C1, C2, and early C3 Corvettes is truly an amazing little device. An ignition coil is a type of pulse transformer that multiplies the low voltage received from the battery or alternator to many thousands of volts when the breaker points open and close. The coil contains a primary winding, a slot iron core, and a secondary winding. The high-voltage output of the coil is directed to the appropriate spark plug by the distributor. Hence, without this high voltage, there is no spark and, consequently, internal combustion can't take place within the engine.

The bottom line is, simply put, without spark the engine doesn't run. So that's why when my midyear kept cranking but wouldn't start, after making sure there was fuel entering the engine, the next thing to check was that there was spark to ignite the fuel. The coils on C1, C2, and early C3 Corvettes is a black cylindrical device; starting in 1975 the coil per se as a discrete cylindrical device was abandoned when GM made the switch to HEI breakerless ignition systems.

Checking for spark is easy to do. You simply pull a spark plug from the engine (or use a spare plug if you have one in your parts box, snap one of the plug wires onto it, and make sure the shank of the plug is grounded against the engine block (don't hold onto the spark with your bare hands unless you want to get a really nasty jolt-spark-plug boot pulling pliers or other insulated-handle pliers are a good thing to use for this purpose). Have someone crank the engine and observe the center electrode of the plug. If there's sufficient voltage reaching the plug, you should see a bright-blue spark jump across the gap of the electrode every time that cylinder fires. If you see no spark, the coil is a good thing to suspect next.

This is the aftermarket coil... 

   

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This is the aftermarket coil that was in my '63. Normal reading on the primary windings is 1.6, but 1.5 is still within the acceptable range, so this by itself doesn't necessarily indicate a problem.

You'll need a digital multimeter to check the resistance levels of your coil. The side terminals of the coil are marked positive (+) and negative (-), and these are where you can measure the resistance of the primary windings. Set the multi-meter to the 200-ohm setting and attach the meter's leads corresponding to the terminal markings, red being positive and black being negative. The normal primary reading for 12V (1956 and later) Corvettes on the primary side is 1.6, although a range of 1.5 to 1.7 is acceptable.

Next, measure the resistance of the secondary coil, as this is the real business-end of the spark-producer. Switch the meter's resistance range to the 20K-ohm setting, and attach the negative (black) meter lead to the center terminal of the coil. The reading here should be 11.00 or better, with 13.49 being about normal. If your coil reads below 11.00, chances are pretty good this is the reason you're not getting any spark or a very weak one.

Replace the coil with a good-quality new one (e.g., GM Delco), making sure you attach and tighten the leads correctly. Plug in the top cable that goes to the distributor, turn the key, and you're good to go.

This reading of 8.84 indicates...        read full caption This reading of 8.84 indicates that the coil's secondary windings aren't putting out as much voltage as they should; the minimum acceptable resistance reading here should be 11.00, with approximately 13.5 being normal.

Here's the primary reading...        read full caption Here's the primary reading on the replacement GM Delco coil from Mid America. The 1.6 reading is exactly what it should be but, again, the real telling is in the secondary resistance reading.

Ah, yes--13.49 is more like...        read full caption Ah, yes--13.49 is more like it. Now there should be plenty of fire in the hole. All that remains is to put it back in the car, connect the leads to the terminals, plug in the distributor cable, and start her up.

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