1. Overall condition - cracks in the body, perished rubber boots, corrosion on the terminals etc.
2. Electrical resistance.
Carry out checks for (1) with the Mk.1 eyeball.
Carry out checks for (2) with a multimeter.
There are two parts to a coil, the low tension side (LT) and the high tension side (HT). The LT side connects the 12v coming from the ignition unit to the primary coil (the black and green terminals) and the HT side connects the secondary coil to the spark plug (HT lead and plug cap).
Still awake?
The coil is simply a multiplier, turning the 12v into 20-30,000 volts needed to jump the gap on the plug and create a spark. It does this through a transformer, where the primary and secondary coils act as the multiplying circuit - the secondary coil has a lot more windings than the primary.
This drawing is taken from the MC18 wiring diagram over on the main site, with lables added for clarity:
It works like this:
1. When a voltage is passed through the primary windings a magnetic field (flux) is generated around the windings.
2. If the voltage is interrupted (switched off by the PGM) the magnetic field collapses, which generates a voltage in the secondary windings causing the spark.
Simple!
To electrically test the coil, all you have to do is check that the resistance of the primary and secondary coils are correct.
Using a multimeter set to Ohms, check that the resistance across the two termials is less than an ohm:
Primary showing 0.3 Ohms here.
Now put one probe of the multimeter in the end of the HT lead. Put the other in the negative terminal and you should have about 20K Ohms (20,000 ohms). You may need to change the range on your multimeter to get a good reading.
Secondary showing 18.9 K Ohms (Scale is set to 20K, so multiply reading by 10)
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