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REPAIRING LUCAS ALTERNATORS - by John Martin

John Martin’s Tech-Tip originally published in GV June 2000

The Lucas alternator is usually a very reliable component, consisting of one stationary part the Stator, and one rotating part the Rotor, so in theory there is nothing much that can go wrong. The Stator consists of a number of thin steel circular laminates stacked together with six “fingers” that protrude to the inside diameter. The outer circular part has holes through it, which are used to bolt the item to mounting studs that are usually around the crankshaft.

The laminations make the Alternator more efficient than if it was made of a soil core of iron by reducing circulating currents in the core itself, called Eddy Currents. Around these fingers are wound thick wire coils that are interconnected with each other in diagonal pairs. In early 6 Volt Alternators with 3 output wires, these pairs or coils were switched in and out of the charging circuit by the lighting switch as the demand load required. 

These early Alternators had open coils and were a lot easier to repair if need, the later 12 Volt 2 wire Alternators were encapsulated in resin to make them more robust and therefore a little more difficult to repair.  The Rotor consists of magnets embedded into a circular piece of aluminum that is fixed around a steel mounting-sleeve that is secured to the end of the crankshaft. The later Rotors are of welded construction, and are identified by a stamped W. They are more reliable than the earlier ones, which had a problem of the magnets coming loose on the mounting sleeve.

Basic electrical principals state, that, if a magnet is moved within a wire coil, an electrical current is produced, a very simple machine indeed. Because the magnet is rotating, an AC current is produced, which has to be rectified to DC by a Rectifier, in order for the battery to be charged.

There must be a running clearance of about 8 thousandths of an inch between the Rotor and the Stator so that the two don’t weld themselves together in normal use. This is the biggest problem that can effect the assembly, if this happens, you are looking at renewing the whole lot!

The other problem is that Lucas used regular wire to take the power from the coils to the bullet connectors. They should have used heat insulated wire, as the normal wire becomes brittle with the heat that is generated in the primary chain case can snap off at the base of the Stator, leaving you with a flat battery.  At this point people usually buy a new Stator, but they can be repaired. 

The encapsulating resin is quite soft and it can easily be chipped away with a sharp knife, if you are careful. Lucas actually fed these wires through an extra hole in the core to the back of the Stator, where they are soldered onto the thicker coil wires.

Once the resin has been chipped away and the welds uncovered, they can be disconnected and a new pair of wires can be easily soldered on. It is then just a matter of mixing some epoxy cement and filling in the resin that you have just removed, and leaving it to set.

I have fixed a number of alternators this way and they have performed faultlessly since. It is a cheap and easy repair that you should consider before purchasing an expensive new one.

John Martin