7.3 - Pulsing ( ,  )

If you can see the pressure lines pulsing, and hear a knocking sound which varies with engine speed, you might have a sticking charge switch. That's the switch on the accumulator with the yellow-black and black-pink wires.

When low pressure is detected the switch closes, supplying a ground to the charge valve (unloader), which directs high pressure oil to the accumulator.

If the switch sticks closed the pressure builds up until the pressure relief valve opens.

You can check it by measuring the resistance to ground of the yellow-black wire when the pulsing occurs. If it is grounded then the switch is closed and that's where your problem is.

It is worth trying unscrewing them and cleaning.

7.30 - Wheels and Tires ( ,  )

7.30.1 - Wheel sizes ( Lasse Schauman,  December 3, 2001 )

According to dealers, you can fit Jaguar's 15" or 16" wheels as you wish which means that newer wheels can be fitted on older cars. This will, however, affect the tyre size. Some wheels might not only be larger in diameter but also wider requiring a wider tyre, as well.

What you should be cautious about is the total diameter of the tyre. If there are too great variations in this, your speedo will display incorrect speeds. Also, be aware of not fitting tyres too large in diameter (an added problem with this is that too large ones won't fit in the spare wheel space in the boot).

You can calculate the diameter of the tyre as follows:

The hight of the tyre wall (=how much tyre you can see from the side) is a percentage of the tyre width. This is stated e.g. as 205/70. This means that the tyre is 205mm wide and the hight is 70% of 205=143,5mm.

If you have a 15" wheel the diameter of the wheel is 381mm. Thus, the total diameter of the tyre is :

143,5 + 143,5 + 381=668mm.

If you go for the 16" wheels the diameter is 406,4mm. If you wish to keep the original diameter your tyres should be :

668 - 406,4=261,6mm/2=130,6mm.

If the tyre width is still 205mm the profile is 130,6/205x100=63% or about 65. So you should find tyres with the size mark 205/65R16.

I don't think there is such a size so you'll have to find wider tyres, e.g. 225's. The size would then be 225/60R16 (total diameter 676,4mm).

However, the difference in comparison with the original diameter should not be greater than 5% and please be aware that there could be some differences in exact measurement from one manufacturer to the next... just like shoes !

For more on tire size, the Tire Rack ( one of the largest tire retailer in the US ) has a lot of info along with comparison and test.

7.30.2 - Wire Wheels ( ,  December 3, 2001 )

The question about fitting wire wheels to modern Jaguar often comes back as a number of people like the way they look even on the late model cars.

Over the years, Wire Wheels have improved greatly and are now stronger, available in larger sizes and even in tubeless configuration where a special rubber seal is fitted over the end of spoke making the rim airtight. The leading manufacturer of wire wheels is Dayton who offer a variety of size and even finishes... like those gold things you see on low riders...

Jaguar has stopped fitting wire wheels to its saloons when the XJ40 was introduced for reason which now go beyond aesthetics. The factory official stance is that the new Jaguar are too heavy and that wire wheels can not safely cope with the weight, suspension design and performance of an XJ40 or X300.

While you can purchase the appropriate size wire wheel from Dayton, it is not recommended from a safety point of view.

7.30.3 - Tire speed rating ( ,  December 3, 2001 )

That's another common question on the Modern List where many people wonder if they could replace their higher speed rating tires with less expensive tires having lower speed ratings. The argument often given is that they will never drive their cars over a certain speed so they don't see why they need to pay for the extra performance.
P=93 MPH / 150km/h H=130 MPH / 210km/h
Q=99 MPH / 160km/h V=149 MPH / 240km/h
R=106 MPH / 170km/h Z=over 149 MPH / 240km/h and over
S=112 MPH / 180km/h W=168 MPH / 270km/h Cars
T=118 MPH / 190km/h Y=186 MPH / 300km/h
U=124 MPH / 200km/h

87 thru 92 models came with V rated tires and all others came with Z rated tires; indeed most of us will never get to enjoy pushing our cars near their design limits.

However, while saving a couple of hundred dollars is tempting, other factors play a role in tire selection such as weight and heat. Modern Jaguars are heavy cars and even at medium speed this will contribute to the amount of heat generated. The higher speed rating provides an extra margin of safety lower rated tires might not.

In addition to this, modern manufacturers often tune the car's suspension for a specific type of tires and unless you absolutely need to save money on tires, sticking to the original model, size and rating is a safe way to go.

Below is of tires fitted by Jaguar for various models, along with alternatives list members have had good experience with. Obviously, their needs could be different from yours and as often said... your mileage may vary...

Year - Model Size Model
87 - 92 XJ6 205-70VR15 Pirelli P5 (discontinued )
93 XJ6 225-65ZR15 Pirelli P4000E
94 XJ6 225-65ZR16 Pirelli P4000E
94 XJ12 225-60ZR16 Pirelli P4000E
95+ XJ6 225-60ZR16 Pirelli P4000E
95+ XJR 255-45ZR17 Pirelli PZeros

Note that there is difference between the cheaper P4000 and the P4000 E which was original equipment and much better than the first one.