Ignition Coil Troubleshooting: (No) Fire in the Hole
The ignition coil 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 cannot take place within the engine. Simply put, the bottom line is without spark the engine doesn't run. So that's why when my 1963 Corvette 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 early Corvettes are black cylindrical devices; starting in 1975 the coil per se as a discrete cylindrical device was abandoned when GM made the switch to HEI breakerless ignition systems. The information I'm providing here, however, applies to all GM vehicles of the period and broadly applies to virtually any 12-volt vehicle with a discrete ignition coil.
Checking for spark is pretty 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 (do not hold onto the spark plug 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.
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 multimeter 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, you'll want to measure the resistance of the secondary coil, and 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 under 11.00, then chances are pretty good that this is the reason you're not getting any spark or a very weak one.
Replace the coil with a good quality new one, make sure you attach and tighten the leads correctly, plug in the top cable that goes to the distributor, turn the key and you should be good to go!