Jaguar V12 Lucas Digital P EFI Throttle Pots

The 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. If not, replace it. It is possible to rebuild it, if you're the kind to rip into such things. 

If replacing, good news: later XJ-S's came with a much better pot made by Lucas (!) that costs much less too! The only problem: it requires buying a couple of other parts to be able to use. The total cost is still less than the original Bourns pot, that's not the problem; the problem is that Jaguar won't sell it to you unless you ask for it! If you merely ask for a replacement pot for your car and tell them the year, they will happily sell you the overpriced unreliable Bourns pot without even telling you there is a better option. 

The following illustration of the part differences:

The Bourns pot is the black one on the left, bolted directly to the bracket. The better Lucas pot is the red one on the right, installed on the same type bracket with an adapter mount. You don't really need to know the part numbers; just go to your local Jaguar dealer and insist upon the "red pot" and the necessary hardware to mount it. Their fancy computers will tell you the parts you need. 

I only bought the new red potentiometer, reused the existing drive shaft and made an adapter to suit mating the new potentiometer. 

QTY PART NUMBER    DESCRIPTION
1       C44477/1               SETSCREW 
1       C2261                   GUIDE PIN CIRCLIP
1       EAC9634              THROTTLE POTENTIOMETER
2       SN104101J           SCREW
1       EAC9567              POTENTIOMETER ADAPTOR 
1       PA103161J           SPRING PIN
1       EBC3566              DRIVESHAFT
1       EBC3568              COUPLING THR

HE Coil Replacement

The original parallel coil arrangement was "conceived when coils of sufficiently low resistance were not available." 

Although the V12 constant energy ignition module is fairly tolerant (it runs OK with the blanked second coil removed - albeit with a loss of spark energy) the ignition system will behave oddly if the coil is not the correct load match.

Initially I had a problem with the main oil filled coil located adjacent to the distributor, attached to the throttle pedestal. I originally replaced this coil with a non oil filled Bosch MEC 723, which coupled with the secondary front coil worked well without trouble for over 5 years and 70,000 klms. 

Jaguar now recommends replacing both coils with a single "solid" (not oil filled) coil (#DAC 6093, Ducelier coil - 0.62 ohms primary) that fits in place of the main coil. If the resistance is any more than that it will not be able to build up enough coil energy to fire a spark at the higher end of the rev range when the coil "on time" is very short (about 1.4 milliseconds at 6000 revs). It could also struggle around the peak torque point.

I purchased from XJ Autos Spares the DAC 6093 (a little bit more expensive at $200 than 2 separate coils) but it does the job and works well in place of the earlier twin coil set up. 

The aux coil and wiring is removed, which eliminates the problem of the auxiliary coil failing from water damage through constant exposure being located behind the grille, and tidies up the top of the engine from the wiring running exposed.

WOLF EMS Computer

Fitted a crank angle sensor and activating plate, which is bolted between the front removable crankshaft pulley and dampener, providing an accurate pick-up point for the computer.

All advance is now controlled by the Wolf, going to 40 deg advance instead of the 22 deg with normal mechanical advance mechanism.

Crank angle trigger plate

Crank Pulley

Crank angle trigger plate 

fitted to back of pulley

Ford ignition pick-up

Ignition pick-up

The car then went Alex's on the dyno for set up and tuning the WOLF. End result 215KW (285HP) & 7480 N traction effort at the rear wheels. Accelerated like a scalded cat on the road. Will post more detail and acceleration times when I do them.

My son Byron & I enjoyed trying the XJ-SC out at the Bathurst Speedweek in Dec 05, the Supersprint down Conrod Straight, and the Hillclimb up through the Cutting and around McPhillamy Park - what a buzz and great workout for the XJ-SC.                           

Removed the distributor, coil and associated wiring. Installed 2 Jaguar triple pack coils linked to 2 M&W 9A Triple Channel Ignition Modules, controlled from the Wolf computer. Installed Denso IT22 Iridium spark plugs with gap set to 0.060" (1.5mm).

Apart from control over the direct firing of the coils to each spark plug, cleaned the valley up and made very neat and tidy layout.