The following is an excerpt from:
Lucas Electrical Systems
Part 9 – Ignition circuits (point-style distributors)
By John Twist
The ignition circuit is probably the most critical circuit of the entire electrical system. Without its proper operation, the engine is unable to run, or unable to run well. It is often the source of great frustration when attempting to locate or diagnose a fault. Yet, the ignition circuit is best organized in two, distinct, circuits – the low tension circuit and the high tension circuit. The distributor and the ignition coil are common to both, but the low tension circuit uses the normal 12 volt current, while the high tension circuit packs a mean 15,000 volts.
The three items necessary to make and break the ground of the coil are the points, condenser and LT lead. The LT lead (low tension) is simply a connector between the spade terminal on the outside of the distributor body to the points and condenser within. The condenser and points are wired in parallel, so that the condenser acts as a ‘sponge’ to absorb the several hundred volts induced in the low tension circuit just as the points open. Without the condenser the points are not able to make a clean ‘break’ of the grounding circuit, and the high voltage arcs between the points as they begin to open.
FAULTS: Both the cam rider and points themselves wear on the point assemblies. Eventually, the points no longer open as far as they should, and the coil does not have time to fully de-magnetize prior to the points closing, resulting in a weak spark. The time that the points are closed is the DWELL, and on most four cylinder Lucas distributers this is 60 degrees. As the dwell decreases, the timing retards. One degree of dwell gain equals about one degree of retarded timing.
Editor’s Note: the article in its entirety can be found on the CMGC website. In the Technical Tips section..