The Ford 8N tractor was the successor to Ford's 2N tractor and was manufactured between 1947 until 1952. Testing the ignition coil can be accomplished using a combination of two methods. On all vehicles, ignition coils amplify the spark so that it can make the jump at the spark plug gap; Ford 8N tractors are no exception. An ignition coil either works or it doesn't, although a weak spark can indicate a coil that is beginning to fail. You can do a primary test using simple tools; however, if you notice a weak spark, a multimeter is necessary.
Remove one of the spark plugs using a socket wrench and a spark plug socket: the forward-most plug is the easiest. Place the spark plug end back into the wire. Put on a pair of work gloves at this time.
Place the end of the spark plug against a ground such as the engine block. Have someone crank the engine while you hold the plug against the block. If the coil is working, there will be a spark. If the spark seems weak, test with a multimeter. At this point, replace the removed spark plug.
Locate the ignition coil. The coil wire attaches to the centre of the distributor cap. Follow this wire to the coil. Original Ford 8N coils are six volts and square, while most aftermarket coils are cylindrical and twelve volts.
Set the multimeter to the ohms setting. Place the black prong to the negative coil terminal and the red prong to the positive coil terminal. This will give a resistance, or ohm, reading, which, according to the My Ford Tractor site, should be approximately 2.2 ohms for the six-volt system and 4 ohms for the 12-volt system. The battery must be connected to perform this test.
Replace the Ford 8N coil if it does not pass this test.