The Series Land Rover Ignition System
Negative earth ignition system
In the diagram above, when the ignition switch is turned on, low energy electrons (NOT "current") pass from earth through the contact points (if closed). They then continue through the primary circuit of the ignition coil and onwards through the ignition switch to the positive side of the battery. When the contact points are suddenly opened, by the distributor cam rotating, a high voltage stream of electrons is induced to move in the secondary circuit of the the coil and pass to earth through one of the spark plugs, creating a spark in the process.
Often, for simple electric circuits, the direction in which an electric current passes
does not matter e.g. for basic light bulbs and simple electric heaters. However, it DOES matter which way the coil is connected. Early Land Rover coils worked on positive earth systems and their coils were designed to give maximum performance when the electrons entered the coil via the contact points and left towards the ignition switch. These coils had terminals marked SW(ignition switch) and CB(contact breaker) so that the correct connections could be made. However, later Series IIA Land Rovers had negative earth systems and the coils were designed accordingly BUT the same terminal markings were initially used, so it was necessary to have the correct coil design(negative earth or positive earth) and this could not be confirmed by appearance alone. So it was possible to fit a positive earth coil to a negative earth system, with a resulting drop in performance and possible misfiring. Modern coils have their terminals marked "+" and "-". The coil terminal connected to the contact points on these coils should be the same as the earth sign of the vehicle.
Correct coil connection means that electrons jump FROM the centre electrode of the spark plugs TO the bent electrode. This can be checked by using a sharpened soft graphite pencil. Remove a plug lead, turn back the rubber sleeve to expose the metal connector, place it temporarily where it won't spark to earth and then start the engine. Hold the plug lead by its insulation with insulated pliers such that it sparks across to the cap of the spark plug it was removed from. Now place the sharpened pencil tip within the sparking arc. Look carefully which way the graphite particles move from the pencil tip. It should be from the pencil to the spark plug. If it is from the pencil to the plug lead then reverse the connections on the coil. This is the case whether the vehicle is postive or negative earth.