Lamborghini V12 Single Distributor

The following observations and deductions are based on the setup and wiring of the single distributor used in my 1974 Series 3 Espada. I believe there is a different rotor and cap design used in earlier single distributor V12’s, of which I have no information.

The distributor is essentially 2 six-cylinder distributors packed in a single body, one six-cylinder distributor supplying spark for an individual bank with the other bank being supplied by the second six-cylinder distributor.

Externally, the distributor has 12 spark plug connection points grouped in a single ring, with 2 connection points for the 2 coils. Also located on the distributor are 2 condensers.

Internally, there is a double-ended rotor. Each end of the rotor is at a different height, to match the 2 different length posts arranged alternately, in a single ring within the distributor cap. The H.T. from the coil connections on the top of the distributor are transferred to the rotor arms by 2 different methods. The central coil connection is transferred by a standard type button to the centre of the rotor, and then to the lower arm of the rotor. The second coil is connected to a post located to one side of centre, and is carried to the upper rotor arm via a slip ring located in the distributor cap and picked up by a small wiper located on the upper section of the shorter arm, and visible in the picture above.

Below the rotor arm and the upper cam bearing support bracket are 4 sets of points, and it is here that the real difficulty in understanding the distributor’s operation begin.

The 4 sets of points are located at 90 degrees around the cam, and are connected in parallel, so that points 1 and 3, and points 2 and 4 are linked.

The cam has 3 lobes. The lobes are long and flat and the set up is something like the diagram over the page.

The diagram is attempting to show the approximate cam shape and the locations of the wipers attached to each set of points.

The wiring of opposite sets of points in parallel configuration is shown in the next diagram, and with reference to both diagrams the operation of the distributor can be understood.

Initially, it is best to just follow half of the distributor – 2 sets of points connected in parallel, 1 coil, and one end of the rotor.

In any coil, to generate a spark, initially there must be current flow in the primary or Low Voltage (12V) coil, sufficient to allow the coil to reach saturation. When the current flow is interrupted, the break down of the magnetic field induces a high voltage in the High Tension side of the coil, and a spark is generated.

Referring to the above diagram, if either of the points connected in parallel are closed then a current will flow, and it requires BOTH of the points to be open to break the current flow and generate the spark.

The geometry of the cam within the distributor is such that as a single point has just opened, the opposite point will be nearly shut and this is the firing point for the coil. This situation with one of the pair just open and the other about to close occurs in 6 positions during each rotation of the cam. It also provides for a considerable dwell angle because both points are only open for a few degrees of distributor rotation.

FAULTS

There are 2 possible faults with points – constantly open or constantly closed.

Referring to the circuit diagram,

If a single set of points fail in the closed position, there will never be a time when BOTH sets of points are open, so that particular half of the distributor and coil will be dead and the engine will try to run on one bank only – 6 cylinders

If a single set of points fail in the open position, there will only be 3 positions when BOTH sets of points are open in each rotation, so that particular bank will loose 3 cylinders and the engine will run on 9 cylinders.

There has been some discussion where it was thought that each set of points fires for 3 cylinders only. This is true, but it requires the opposite parallel pair to be working correctly for full engine operation

Likewise the other thought has been that the 2 sets are used only to increase dwell angle and while this is still true, both sets of points in each pair in parallel are used for the firing trigger, not just the second in the group, which it would be if they were being used for dwell angle extension only.

John Allott

August 2004