Testing the resistance across several terminals on your ignition coil is the only safe and accurate way to tell if the coils is damaged or not. Measuring the primary and secondary resistance requires an ohmmeter, and a digital readout is best if you have access to one. Most ignition coils will fall into similar ranges when reading the resistance, but there can be some variation by manufacturer and coil type. If you are not sure what yours should be, check with the dealer service department.
- Skill level:
- Moderately Easy
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Things you need
- Wrench set
- Digital ohmmeter
Remove the negative battery cable from the battery with a wrench. Isolate the battery cable so that it can not contact the battery.
Locate the coil on your Ford engine. The coil looks like a black cylinder with a centre wire similar to a spark plug wire and two side posts with nuts and wires attached.
Remove the large wire from the centre of the coil by pulling it straight out of the coil. Remove the retaining nuts and all the wires attached to the side terminals on the coil as well.
Place the test leads from the ohmmeter on the side terminals, one on each side. It does not matter which lead goes where in this case. Note the reading on the meter. This is the primary resistance, and it should read between .4 and 2 ohms on your Ford coil.
Move the test lead from the negative post on the coil to the centre terminal where the large wire was removed. Keep the other lead on the positive post of the coil. Note the resistance reading on the ohmmeter. Your coil should have a reading of between 6,000 and 15,000 ohms for your Ford.
Replace the coil with a new one if the readings are outside either of these ranges, as that would indicate a defective coil. If the readings are within range, reinstall the wires and retaining nuts on the side posts, being sure that they go back on in the same position they were in before you removed them.
Tighten the nuts with a wrench and reinstall the large coil wire on the centre terminal of the coil. It just pushes in place, but make sure it is all the way in or it may come loose when the engine is running.
Reinstall the negative battery cable on the negative post of the battery. Tighten the retaining bolt with a wrench to secure the cable end on the battery.
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