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© 2000 Brian F. Schreurs
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Not so with the Jaguar V12. The distributor is located in the center of the block, between the intake manifolds and cylinder heads, and under all manner of wires, tubes, hoses, and accessories. It lives at the heart of a 262-hp sauna. Which wouldn't be a problem except that the diaphragm inside the vacuum advance is made of rubber. If you don't think this is a problem, throw a rubber band into your oven at 300 degrees for an hour and see what's left of it.

Of course, the diaphragm is made of better stuff than the rubber band. On the other hand, it has to withstand ovenlike temperatures for years -- perhaps even decades if you or the previous owner are not attentive about such things. No matter what the outside of the advance looks like, odds are, yours is baked. If you don't know when it was last replaced, then it's time to replace it.

Note that some skinflint Jaguar owners report that it is possible to disassemble the original vacuum advance, replace the innards with over-the-counter Chevy advance parts, and reassemble. Since the Jaguar advance (Jaguar #JLM519 or Lucas #54408148) is readily available for around $45, we decided our Saturday afternoons were better spent elsewhere and just bought a replacement. You can decide for yourself what your time is worth to you, but we can't help you with the cannibalization route because we haven't done it. We replaced the entire vacuum advance on the Paradise Garage 1985 Jaguar XJ-S H.E. V12.

Getting to the vacuum advance is no picnic, since it resides on the distributor. To be more specific, it resides on the right-rear corner of the distributor housing, pointing rearward. To get at it, you'll have to remove the cruise control bellows (for cap access), the primary ignition coil (again for access), and the throttle pedestal (to give you more working area). We documented the details of these operations in Accessing the Vee.

Original vacuum advance is to the rear; new Lucas replacement is foreground.

When you remove the coil so you can get a good look at the advance, look at the end of it. If the end has a nut on it, your vacuum advance is original equipment and surely as baked as your well-meaning but forgetful grandma's cookies. But if it has a solid, flat end, it's been replaced. It still might be dead but at least you know someone cared enough to swap it out once already.

Once you've got enough space to work on the distributor, the cap has to come off. That means something will have to be done about the wires -- either unplug them at the spark plugs (and possibly end up having to move the A/C compressor) or unplug them at the cap (keeping track of where they go). You should only have to unplug half of them (the rear six plugs); just move the cap forward once the rears are removed. The cap also has two vent hoses connected to it; one on the top and one on the side pointing toward the rear-left.You'll need to unplug them as well.

The cap is held in place with three flathead screws. Loosen them (they need not be removed) and pull the cap.

Lo, the internal workings of the Lucas distributor. Now is a good time to check it for damage. Inspect the cap and rotor for cracks, burns, and gouges. Check the centrifugal advance's operation. This is done by manually turning the rotor counterclockwise. It should move 11 degrees CCW. It should move freely without hesitation (although it is spring loaded so you may feel a little spring tension) and it should snap back to position as soon as you let go, with authority and a metal clink. If it does not move, or moves sluggishly, or doesn't snap back, your centrifugal advance is shot. A common failure; don't feel bad about it. Luckily ours passed the test. If yours does not, consult Experience In A Book by Kirby Palm for guidance. In fact, if you're not in a hurry, you might want to rebuild the thing anyway.

Testing done, it's time to proceed. The rotor pulls off. It pulls off carefully. Do not yank it. Gently wiggle it loose. You do not want to damage the shaft it mounts on.

The distributor with the cap, rotor, and flash shield removed.

The anti-flash shield, the big plastic thing, comes off next. It's held in place with four small flathead screws with washers. Losing these will not make you happy. While loosening a screw with one hand, hold a telescoping magnet in the other right next to it. As soon as that screw is loose it'll jump straight for the magnet. Once all four are removed, the anti-flash shield practically falls into your hand.

Next out is the odd-shaped grey wheel-lookin thing. It's the inductor rotor. It's held down with a snap ring on the shaft. If you don't have snap ring pliers, go get some. There's no other way to safely get this thing off. Under the snap ring is a wavy round black spring washer. It should pull off easily.

The inductor rotor is a tight fit so be gentle. Like the other rotor, it should be removed with kindness, not impatience. A small amount of penetrating oil may help but don't douse the thing. Ours came off with simple gentle pulling. The key is to not pull on the shaft.

Finally, the only thing in the way of removing the vacuum advance is the pickup assembly. It is held to the distributor with two screws. There are not Phillips; these are Pozidriv. Using Phillips will cause you no end of grief. Can't find Pozidriv? You live in an area where no one has even heard of Pozidriv? Try asking for GM Headlight Adjuster Screwdrivers. The one you need is Pozidriv #2, but if you can get a set of them, go for it.

It's not actually necessary to remove both screws. Simply remove the one toward the front of the engine and there will be enough slop in the pickup assembly to allow a slight tilt; just enough to slip the vacuum advance rod out from the pin.

Looking at the outside of the vacuum advance, it would appear to be held in place by magic. However, it in fact has a locating pin. Unplug the vacuum line and move it out of the way. Do your best to clean the advance's mounting pad. You may discover under all that grime a very small locating pin. It is there whether you discover it or not so keep looking. It must be punched out, and guess what -- your new vacuum advance doesn't come with a new pin!

Fear not: you won't have to fabricate a new one. Put a paper towel underneath the vacuum advance, and lay your trusty telescoping magnet on the towel. Put the head of the magnet very near where you expect the pin to fall once you knock it out. The pin is iron and will fly straight to the magnet as soon as it is freed from the distributor!

Now take a 1/16" punch and gently tap the locating pin out. The vacuum advance should pull off once this pin is free. Thank the advance for its years of dedicated service and retire it.

Before installing the new vacuum advance, dip the rod end in heavy synthetic oil. This will keep its pivot on the pickup assembly nice and lubed.

Slip the new advance through its mount and press the rod onto the pickup assembly. Replace the pickup assembly's screw. This will help to position the advance.

The new advance gasket will, of course, not be as compressed as the original. Get the locating pin started by tapping it in just a little, then perform the finger gymnastics necessary to press the advance into position (to align the holes) and tap the pin into place. Sooner or later it will go in.

Reattach the vacuum line. Gently press the inductor rotor back on. Place the spring washer on the shaft. Reinstall the snap ring. Make sure the ring is seated in its groove; the spring washer will fight this and can leave the impression that the ring is seated when in fact it is not. Push down on the snap ring until you see it click into place.

Reattach the rotor, the distributor cap, and the wires. Reassemble the parts of the vee that you chose to remove. Go for a drive! You're done.

Odds are you won't be able to tell much of a difference, if any at all. But rest assured your Jaguar thanks you. Kitties have more delicate senses than their owners so they'll be able to tell an improvement where you can't. Plus, you now have a better understanding of the inner workings of your V12!