Testing an Ignition Coil

The Ignition Coil is the unit that generates the sparks to be sent to the plugs. They are quite robust units and rarely cause trouble, but if consistent rough running presents itself, and all other diagnostic tests have revealed nothing, the coil should be suspect. The following test will determine whether or not it is working correctly.

Before carrying out any work on the ignition coil, it is essential that the car battery be disconnected and the coil unit removed from the car to be tested separately.

1
Using a multimeter, first test the primary circuit by placing the meter across the two terminals of the primary coil. A reading of between 0.75 and 0.81 ohms (Ω) should be given.

2
Next test the secondary circuit by placing the meter between the positive terminal of the primary circuit and the central terminal of the secondary circuit. A reading of between 10,000 and 11,000 ohms (10 - 11k) should be given.

Should either of the readings fail to fall within these tolerances, the coil is defective and should be replaced with a new unit. Consult the vehicle owners handbook to ensure that the correct type of coil is ordered. A component removed from a car is not necessarily a reflection of the component that should have been fitted originally.

Diagnostics
Beyond ascertaining the working order of the coil, there is very little in the way of diagnostic testing that can reveal the cause of its failure.
If it is obvious that the coil is many years old (or even an original component), the main culprit in its failure is clearly age. Provided the low-tension circuit is thoroughly tested (or ideally replaced), the classic owner can be reasonably confident that the root cause of the problem has been cured in the replacement of the coil.

If age is not the issue, the entire ignition system should be checked thoroughly. Begin by checking the details of the coil against the factory recommendations: ignition coils are commonly available in both 6v and 12v forms, with or without the need for a ballast resistor. If the details do not match those of the handbook, replace with the correct type.
If the correct coil is fitted, check that the connections on the primary coil are the right way around and tightly secured. Check the security of the spade connector on the distributor's low tension block, and proceed to carry out a full continuity test of the ignition system.
The most common cause of coil failure is overheating caused by either an absent or defective ballast resistor (where fitted), or intermittent low-tension circuit. In the case of the latter, this is most commonly caused by a defective condenser or sections of high resistance caused by broken or dirty electrical contacts at any point between the distributor, coil and earth.