Jaguar XJ-S. Service manual — part 105

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TIRE PRESSURES: John Goodman says, “Most of us here (UK) have long junked our 15" wheels and run 16" or 17"
rims, but the less competitive drivers, or those who do a lot of city streets always keep the tyre pressures down quite a
bit. We run them up to 8psi lower than the handbook stated figures. My handbook says 35psi front and 32psi rear, so I
run at 30 front and 27 rear. On 245/50 or 225/55 x 16" this gives only very slight loss of handling at extremes of
cornering, but a very noticeable improvement in ride comfort. Has no effect on tyre wear either. This was a local
Jaguar dealer recommendation!!!” Note: perhaps a workable plan for those who value ride quality, but high continuous
speeds should be avoided without full pressure. Also note that lower pressure will provide less protection for your
rims, so don’t drive over curbs at speed.

Goodman’s car is an XJR-S, perhaps explaining those high tire pressure recommendations; the earlier plain-vanilla
XJ-S’s recommended rather low tire pressures (on narrower tires to boot!) and then recommended you go to higher
pressures if driving fast.

Regardless of the peculiarities of one XJ-S vs. another, one thing must be pointed out: the nearly religious devotion
many automobile owners pay to the “recommended tire pressures” is completely uncalled for. As long as you don’t
exceed the pressure limit written on the side of the tire itself, tire pressures can be varied considerably to provide the
owner with the desired performance and ride. If the handling balance doesn’t seem right to you, you can also vary the
front/rear pressure distribution to alter it.

For optimum tire wear, an excellent plan is to keep an eye on the wear at the edges of the tires vs. the center. If they’re
wearing faster in the center, lower the pressure; if they’re wearing faster at the edges, raise the pressure. This helps
configure the car for the owner’s driving style, since a driver that corners hard will scrub the corners of the tire more
and should be using higher pressure. It even helps compensate for drivers that dive into turns as opposed to drivers that
accelerate through turns, since the different styles will cause different tire wear patterns and correspondingly altered
pressures. Of course, bad alignment or other complicating factors can easily screw up the data.

Rob Ward tried to follow that guidance and ended up with tire wear in the center of the tread of his Goodyear Eagle
NCT 235/60ZR-15 rear tires despite low pressures. In response, someone posted this excerpt from a discussion on a
Porsche list: “My info, which is slightly counter-intuitive, is that fatter tires need slightly higher inflation pressures to
stiffen the tire carcass and prevent the centerline bulge that causes premature wear in the middle of the tire. Apparently,
the theory is that no matter what you do, fat tires will tend to higher diameters at speed in the middle of the tread, due to
centrifugal force when you twist it up. Increasing the tire pressure tends to stiffen the tire, preventing the center bulge,
rather than accentuating it as one might expect. It's actually enough more complicated than that that you need empirical
data to come to any kind of useful conclusion. I put 42-44 psig in my 245/45x16 rears and they have always seemed to
wear evenly.”

If you can’t tell, the XJ-S is nose heavy, and one would expect that the front tires will always need more pressure than
the rears -- unless you’re carrying sandbags around in the trunk.

Paul Bachman says, “The strangest thing happened yesterday. I was checking the tire pressure on the XJS and
discovered that they were quite low at about 28 PSI. After bringing them up to proper pressure (32 PSI), the minor (but
irritating) shudder in the brake system completely disappeared! Coincidence? I don't think so.”

WINTER TIRES: In some US states, you are required by law to have either snow tires or “all season” tires on your car
during the winter months. According to Bill Weismann, “For a tire to be labeled "All-Season" it must be labeled (By
federal law...) "M+S" on the sidewall.

“It's a lot more than tread design that qualifies a tire as an all-season; the main problem with High Performance tires is
that they have to be at somewhat high temperatures in order for them to "stick". An all-season tire usually offers two
features for it to work in the snow.

1) A tread compound that will remain pliable in the cold (below 40° F or so..)

2) A tread design that will work in snow.”

Europeans laugh at the idea of all-season tires. For people in places like Norway, an all-season tire means it sucks in all
seasons. If you get only a couple of weeks of inclement weather per year a set of all-season tires is a good option, but if

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you get a serious winter where you live you should opt for a separate set of wheels and tires for winter. Some tire
stores and garages offer to store your alternate set of wheels for you.

As long as you’re going with a separate set of snow tires for the winter, you should also note that, in snow, the narrower
the tire the better. For the XJ-S you’d want to go back to the 215/70R-15’s and possibly even to a 195R-15. You also
will want to find some really junk wheels since snow, sand, and salt make both steel and alloy wheels look really
crummy very quickly. Stainless steel wheels would be ideal, but apparently the wheel manufacturers haven’t figured
that out yet.

Carefully consider your options. Winter driving often involves snow, ice, mud -- as well as a far portion of nice clean
pavement, since many road departments are really good at keeping roads clear. One highly-rated type of snow tire is
called a Blizzak which has a porous compound that grabs snow and holds on until it looks like your tires are made of
snow; they work really well on dry snow, but are reportedly not much good in mud, wet snow, ice, or dry pavement,
and they wear quite rapidly on dry pavement.

Studs are another option that the owner might regret. Great on ice, but not particularly beneficial anywhere else. Also,
they will damage pavement, so they are frowned upon or banned outright in some areas. They also have a detrimental
effect on the rolling resistance of the car, so your fuel economy gets much worse and the car feels like the brakes are
dragging or something.

As far as speed ratings go, snow tires typically have a speed rating of Q. Don’t drive them fast. A rubber compound
that works in freezing weather will not like the heat generated by high speed.

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BRAKES

BRAKE FLUID: Use DOT 4. Period.

DOT 5 BRAKE FLUID: Many people swear by “Dot 5” silicone-based brake fluid, and others swear at it. Silicone
brake fluid does not absorb moisture, does not harm paint when spilled, and has a very high boiling point. It is also
non-toxic.

Some of the arguments are based on it not absorbing moisture. If there is no water in your brake system to begin with,
theoretically none will get in by being absorbed by the fluid in the reservoir. But if there is water in the system to begin
with, the silicone fluid will not absorb it, and it may puddle in local spots and cause corrosion.

Silicon fluid absorbs more air than conventional fluid, meaning that the brake pedal will feel spongier, even when fully
bled. Of course, this will not be apparent at first, since it takes some time to absorb air within the reservoir and for the
air to diffuse throughout the system.

It is important that the two types of fluid not be mixed. Silicon fluid is less dense than normal fluid and won’t mix with
it, so it is theoretically feasible to fill the reservoir with silicon fluid while bleeding the conventional fluid out at the
wheels. If you’ve had the calipers apart, you probably don’t believe this; maybe it’d be better to only change fluid
types in the midst of a full caliper rebuild.

The name is a deliberate misleader. DOT 3 and DOT 4 brake fluids are referred to as such because they conform to
Department Of Transportation standards. “Dot 5” silicon-based fluid, however, is a trade name, not a DOT standard.
There are even reports that DOT has now issued a standard #5, and it is not for silicone-based fluid but for something
else.

Older (pre-XJ-S, actually) Girling systems require DOT 4 fluid to keep the natural rubber seals pliable to seal properly.
Some components are reported to get hard when exposed to silicone fluid; others are reported to absorb too much
silicone and swell so much as to jam the works.

According to Jim Beckmeyer, who had a master cylinder resleeved: “White Post (page 701) said that the warranty
would be voided if I used DOT 5”. Draw your own conclusions.

WHEN TO BLEED BRAKES: Bleeding the brakes is essential after disconnecting brake lines in order to get air out of
the system and ensure a “hard” pedal. Bleeding is also a periodic maintenance action, sorta like changing the oil; brake
fluid absorbs moisture, either that was already within the system or from the air in the reservoir, and this moisture is not
good for the insides of the brake cylinders. Moisture also lowers the boiling point of the fluid. Also, the fluid
eventually becomes contaminated with tiny particles of metal from wear within the cylinders, and the particles only
serve to accelerate the wear. So it is beneficial to occasionally bleed the brakes even if there is no air in them, and to
bleed them enough that the entire system is filled with fresh fluid.

Steve S says, “In Delphi-AP Lockheed tech notes I came across the following quote:

In addition to regulations that define performance hydraulic brake fluids are required to meet international
standards with regards to their colour. The following classifications have been established:

ƒ less then 0.006 Abs

Colourless

ƒ 0.006-0.100

very pale straw

ƒ 0.101-0.197 pale

straw

ƒ 0.198-0.296 straw

ƒ 0.297-0.397

very pale amber

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ƒ 0.398-0.530 pale

amber

ƒ 0.530-0.800 amber

The figures relate to Abs (absorbance units). Delphi Lockheed ensures its colours conform by using an in-
house absorbance scale at 455nm with a 1cm cell.

End quote.”

This means that the color of the brake fluid indicates how much moisture it has in it. So, it needs to be close to
“colourless”; if it’s closer to “amber”, it’s time to bleed that stuff out and get new, dry stuff in. Of course, you can’t see
the color of the fluid anywhere except in the reservoir, but that’s where moisture typically gets into the system in the
first place.

Modern brake fluid reservoirs are translucent white plastic. This is so that you can see not only the level but also the
color of the fluid without removing the cap, since removing the cap introduces more air and moisture into the system.
Of course, with the OEM reservoir on the pre-ABS XJ-S, the reservoir is vented anyway so it will gradually gain air
and moisture even if you don’t ever remove the cap; see the idea for replacing the reservoir on page 594 if you’d like to
correct this shortcoming.

BRAKE BLEEDING: Before beginning the bleeding procedure, remove all of the old fluid from the reservoir using a
turkey baster or something. And, of course, get any dirt or crud out that you can. It simply doesn’t make sense to try to
pump that crap through the system to get it out; get it out the easy way, and fill the reservoir with fresh, clean fluid
before starting.

Bleeding the old-fashioned way requires two people. One person sits in the car and “pumps” the pedal to get the pedal
up where it belongs, and then holds pressure on the system while the other person opens one of the bleed screws to
allow the fluid to squirt out. A small piece of hose routed into a container helps reduce the mess. When the bleed
screw is opened, the pedal will go to the floor, and the person in the car must keep it there until the bleed screw is
closed once again; if the pedal is allowed to rise, it will draw air back in through the bleed screw. After the bleed screw
is closed, the pedal is pumped back up and the cycle is repeated. If a clear hose is used on the bleed screw, it is easy to
see when the crud is gone and clear fluid is coming through. You must pause every now and then to top up the
reservoir with fresh fluid.

If you want to make things easier or require only one person, there are numerous gadgets available to help. Dan
Welchman suggests the “Eazi-bleed” kit: “I think they’re made by Gunsons but I’m not sure. This device uses your
spare tyre as a compressed air reservoir which pressurizes a large plastic brake fluid reservoir via a hose and footpump-
style valve connector. A tube dips into the bottom of this reservoir and feeds fluid up to an air-tight cap that screws
onto your brake fluid reservoir on the car (the kit comes with a wide range of different caps and rubber gaskets to fit
most cars).

“Once you’ve fitted this and got it air-tight you can bleed the brakes by just opening each of the nipples and letting the
fluid flow out until it’s bubble free (or longer if you’re changing the fluid). No pumping, no assistant needed, and no
worrying about having to top up the reservoir for fear of running out of fluid and pumping air through the system.
They’re extremely cheap and work pretty well.”

Brian Schreurs reports: “I shall quote directly: “One of the hottest new items at the SEMA [Specialty Equipment
Manufacturers Association] show (winning the ‘Best Engineered New Product’ category) was Russell Performance
Products’ Speed Bleeder brake bleeder screw. This unique product replaces the stock bleeder screw on your car, truck,
or motorcycle. It has a patented, internal, spring-loaded check ball that permits air and fluid to escape when the pedal is
depressed, but automatically seals when the pedal is released.” They are supposedly available nationwide, but if all else
fails the company can be contacted directly at

225 Fentress Blvd

Daytona Beach, FL 32114

+1 (904) 253-8980