Lucas OPUS ignition on the V12

It was fitted from launch

Lucas OPUS Mk II transistorised ignition, with distributor 36DE12, was named as production specification in the April 1971 technical press written from Jaguar's own briefing, confirmed in a January 1972 road test, and Lucas published a service document headed for the E-Type Series III specifically. The archive has found no evidence of a contact breaker distributor on any production Series 3.

The belief that early cars had points and OPUS came later is not supported. What did change through production was the location of the ignition amplifier, the ballast resistor variant, and distributor and vacuum unit part numbers. The parts catalogues break at engine numbers 7S7559 and 7S7560 for the ballast unit, and again at 7S9678 and 7S9679 for the distributor, coil and amplifier group.

The service figures

These come from Lucas's own document for this application, which is the strongest ignition data the archive has located.

• Distributor: Lucas 36DE12, 12 volt negative earth
• Dwell angle: 22 to 27 degrees
• Pick up to timing rotor gap: 0.020in to 0.022in, being 0.50mm to 0.55mm
• Coil primary resistance: 0.8 to 1.0 ohms at 20 degrees C. Lucas states explicitly that other ballast coils are unsuitable
• Coil primary voltage when running: 4 to 6 volts at the coil positive terminal
• Stall current: 5.0 to 6.5 amps
• Pick up module: primary winding 2.5 ohms nominal at 20 degrees C, secondary 0.9 ohms nominal
• Ballast unit: drive resistor 7.6 to 9.2 ohms, with two ballast ignition resistors of 0.72 to 0.80 ohms and 0.9 to 1.0 ohms, one of which is bypassed by the solenoid while cranking
• HT brush: about 0.156in protrusion when new, renew at 0.093in

The centrifugal advance for distributor 41321A-B, in distributor degrees, runs: none at 350 distributor rpm, 1.0 to 3.5 degrees at 550, 5.5 to 7.5 at 750, 6.0 to 8.0 at 950, 8.0 to 10.0 at 1,750, 11.0 to 13.0 at 2,900 and 11.5 to 13.5 at 3,500. Double these for crankshaft degrees.

The vacuum unit is a retard unit, not an advance unit. It gives 7 to 9 distributor degrees of retard, coming in between 7 and 13 inches of mercury, with no retard below 1 inch. This catches people out, and so does the fact that Lucas quotes advance in distributor degrees rather than crankshaft degrees.

How OPUS works

A 600 kilohertz oscillator feeds an E shaped transformer pick up. Ferrite rods carried in a plastic timing rotor pass the pick up and unbalance the transformer, producing an output that is rectified and used to switch the power transistor, collapsing the coil primary. It is an elegant idea, and its weakness is that it is an analogue radio frequency circuit sitting in the hottest part of a hot engine.

How it fails

• Amplifier heat soak. The amplifier is a finned aluminium block mounted down in the vee between the camshaft covers. It cooks, and it cooks worst after the engine is switched off and heat rises from the block. The characteristic symptom is intermittent failure with the tachometer dropping to zero while the engine is still turning.
• Component drift. The original circuit has no temperature compensation and no voltage regulation, so as the oscillator frequency or the supply voltage drifts with age the pick up voltage falls below threshold. The spark becomes weak and irregular, and it is worst when hot.
• Fragile timing rotor. The nylon rotor with its ferrite rods, and the pick up assembly, proved fragile. Lucas states that a dwell reading above 27 degrees indicates a faulty timing rotor.
• Centrifugal advance seizure. The plastic disc seizes, sometimes so severely that attempts to remove it break it into pieces. A seized advance is also a documented cause of overheating, because the engine runs on retarded timing.
• Distributor shaft seal failure. The seal between shaft and housing deteriorates into chunks and lets crankcase vapours into the distributor.
• Broken trigger wires. An intermittent break in one of the three trigger wires where they leave the front of the distributor in a moulded grommet. One owner cured it by replacing the three pin connector with individual spade connectors.
• Cap tracking. Lucas describes the sign as a thin greyish white line, or charring, inside the moulding. The cover must be renewed, not cleaned.

Lucas's own warnings, which still apply

• The oscillator adjusting screw in the amplifier must never be disturbed.
• The printed circuit board fixing screws must not be disturbed.
• The cable between distributor and amplifier must not be lengthened or shortened, and the individual conductors cannot be separated or replaced.
• Amplifiers cannot be economically repaired and must be renewed complete.
• When diagnosing, test the amplifier before substituting a pick up module, because an undetected amplifier fault will destroy the replacement.

The instruction not to alter the cable length is the one most often broken, because relocating the amplifier is exactly what owners and specialists do. Anyone relocating it should understand that they are lengthening a 600 kilohertz signal path, which invites crosstalk. One recommendation is to use ribbon cable with unused conductors left between the signal wires as spacers.

Relocation and replacement

Jaguar itself extended the wiring harness during production to move the amplifier from the vee to the bulkhead, after repeated failures. A great many surviving cars have had this done, either by the factory scheme, by an owner, or by a specialist. Moving it to the front engine frame is described as a partial improvement that did not fully resolve the problem.

Replacements exist. There is a drop in replacement circuit board that fits inside the original amplifier housing using the existing wiring, adding voltage regulation, a temperature compensated oscillator and hybrid coil driving transistors. Several specialists sell conversion kits, and in American practice a substituted General Motors module is a documented low cost fix. The archive does not recommend or rank suppliers.

Plugs and leads

Champion N9Y was the specified plug, 14mm. The parts catalogue lists Champion N9YC and NGK BPR6ES as current equivalents, both with the correct 19mm reach, quantity twelve. Reach is worth checking rather than assuming: one owner found plugs previously fitted that reached only halfway down the threaded portion of the head.

The archive has not established the factory spark plug gap for this engine. It is not in the Lucas document and not in the period technical reporting, and the figure commonly quoted is drawn from XJ-S material for a related but different car. Nor has the archive established the factory HT lead routing or the distributor cap lead order. Both are in the E165/2 workshop manual.

What is established is the firing order, which is 1A, 6B, 5A, 2B, 3A, 4B, 6A, 1B, 2A, 5B, 4A, 3B. Note that Jaguar numbers the cylinders by bank, 1A to 6A and 1B to 6B, not one to twelve. Any firing order written as a sequence of numbers from one to twelve is using a different convention, and at least one widely copied version of it contradicts the period record.

Common questions