My poorly Pawla – XJ-SC V12
My own recent challenge has been trying to persuade my 1986 XJ-SC Cabriolet V12, affectionately known as Pawla, to run properly again.
The original problem eventually led me into the Lucas distributor, where I discovered a broken wire connected to the ignition pickup. That seemed sufficiently promising, so the damaged pickup and wiring were replaced and everything carefully put back together.
Then when I couldn’t get the car started at all I discovered the supplied lead from the distributor to the Amplifier was wired the wrong way round (I swapped the wires at the Amplifier plug and the car fired up).
There has been progress. Pawla now starts and runs, which is considerably better than not starting at all. Unfortunately, she misfires badly and is still extremely reluctant to rev with any enthusiasm.
That creates a slightly awkward situation. The car really needs someone with more diagnostic experience to look at it, but getting it to a specialist is difficult when it does not run well enough to drive and may not even cooperate sufficiently to place it neatly onto a trailer.
I am gradually working through the ignition system, checking the distributor setup, replacing suspect components and trying not to introduce two new faults while looking for the original one.
I’ve removed and checked a couple of the spark plugs, and though they don’t look the best, I don’t think they are the main cause of the rough running (feels like it’s almost half not firing).
These are the trials of XJS ownership: one small broken wire can lead to weeks of investigation, a growing collection of spare parts and a much deeper understanding of a Lucas ignition system than you ever expected to acquire.
Still, every successful start feels like progress, and I remain determined that Pawla will return to the road.
Laurence
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Laurence, a comment on your ignition problem with your XJ-SC.
Now you can start and run it, a number of things to check that can cause it to misfire or run unevenly.Check that your front coil is still working.
The best solution if you haven’t done it already, is remove both coils as if the front one is faulty it will interfere with the other coil, and replace with new single coil #DAC 6093, Ducelier coil – 0.62 ohms primary, that was fitted to the later V12’s.The black throttle pot on the Jaguar V12 with Lucas Digital P EFI (1981-88 or so) is made by Bourns and is notoriously unreliable. The wiper wears through the resistive coating inside the pot and it starts “breaking up”. Fortunately, this doesn’t stop the car; in fact, many owners continue to drive for years. The car hesitates a bit on acceleration, but they put that down to normal aging or some such nonsense.
The pot is easy to check with a voltage meter; just make sure it provides a smooth resistance change from idle (0.32V) to full throttle (4.6V). If not, replace it.
Better replace it anyway, as the black pot will break down.The later XJ-S’s came with a much better red throttle pot made by Lucas (!) that costs much less too!
A little bit of fiddling to fit, nothing too onerous.
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