As a result of the problems I had with the spark on the MK 1 3.4 auto while headed for the display day which I never got to [see Oct 03 Nuts and Bolts] I had a serious think about the ignition system in this car. It is very simple and on a par with the FJ Holden employing a coil, points, condenser and distributor. In fact alongside the modern day electronic systems it is downright crude. I pondered is there a better way?
There are modern electronic ignition system which can be fitted to run without points and deliver a massive spark however I detected some reluctance from a couple of commercial distributors of these devices when I wanted firm guarantees that they would fit a MK1 distributor. In fact they asked what was a MK1!
Using my vast background of Datsun 120Y experience I decided that the simple way was to retain the system, as it was however I would introduce a ballast resistor and coil. As I had lots of 120 Y used components in the parts bin including ballast resistors and coils I thought that a simple no cost experiment would be worthwhile. The results were quite spectacular. This car, which was always finicky on the start roared into life immediately. From dead cold it would catch on the first revolution of the crankshaft. Encouraged by this I purchased a new coil, ballast resistor and relay and the car has not looked back. In fact tonight I had to move it around in the shed and it still stated immediately although it is at least one month since the last time it ran.
How it all works is quite simple. When you are cranking over an engine on a 12-volt system the actual battery voltage drops to about 8 volts. This means your 12-volt coil will not work very efficiently when starting but is OK when running. If you substituted an 8-volt coil it would be good when starting but would burn out with continuous running. However a resistor designed to get rid of 4 volts when continuously running will protect the 8-volt coil. That is the ballast resistor. The relay is used at start up to short out the ballast resistor. The coil of the relay is connected to the starter motor voltage supply and activates when the starter is turning. The points in the relay are connected to either side of the ballast resistor and when they come together the relay shorts out the ballast resistor and full battery voltage is available at the 8-volt coil.
As we get more sophisticated I will try to introduce circuits and diagrams but at the moment I am stuck with a pretty basic computer and you will just have to bear with me on the description. While the foregoing information is not rocket science it appears to be a worthwhile modification to older Jaguars and is relatively inexpensive. Anyone who is really stuck can phone me and I will see what I can do to mail them a circuit diagram.