Here is a problem that is is pretty common in 1960’s Jaguars and other British cars with the standard Lucas points and rotor distributor. You are driving along and the engine simply stops or you decide to go for a Sunday and drive and the engine simply won’t start. Sometimes this problem can be intermittent, but often it is a sudden failure. You can hear the fuel pump ticking, see gas in the glass filter bowl and know fuel is not the issue. You then test for a spark on one of the plug leads and there is nothing. The coil and distributor leads are firm, the coil tests ok and the distributor rotor and cap look fine. What is going on?
It is very likely, particularly on Jags of this age that the low tension lead inside the distributor has broken. This lead is a
special highly flexible wire that connects the condenser and the coil negative terminal to the points. With time this wire becomes brittle and will eventually break – severing the connection between the ground terminal on the coil and the points. No connection equals no spark. The good thing is that the low tension lead is easy to replace and cheap – these can be bought for $5 to $7. The same replacement low tension lead serves everything from Aston Martin DB6’s and Jaguar E-types to humble Austins and the Triumph GT6. Although a regular piece of thin wire can be used as a very temporary patch if you stall on the road – regular wire will break relatively quickly as it flexes.
You can check if the wire and points are functioning by connecting a test light between the low voltage positive lead coming into the coil and the low voltage negative lead coming into the coil from the distributor and then crank the engine over. First test that you have power in the low voltage positive line by attaching one line from test light to it and turning on ignition and grounding test light to the engine. If it does not light you have no power coming to coil – check upstream in ignition system. If it does light, then connect test light to the ground lead from distributor to coil, crank the engine and if the test light does not goes on and off as the distributor cam rotates than you can assume that the problem is in the points or the low tension lead.
As originally equipped the Lucas distributors had a plastic collar between the cap and the body and the lead was attached to this. Replacement leads typically just clip to the side of the distributor. The low tension lead is also connected to the condenser – so this is a good time to replace that also – of course you may want to look at your points, rotor and cap. The replacement low tension lead and condenser being installed in a distributor is shown below and should provide many more miles of motoring at a reasonable price. This is the kind of replacement that should be done as a matter of course if you do not know the history of the car.