XJ40 Brake Briefs

This month's tip discusses the brake systems used on the 1988-1994 XJ40 Jaguar sedans.

With the introduction of this new rectangular-taillight car in 1988 (1987 in the UK), the braking system was changed substantially from the prior Series III models. In some ways, the brake system was simplified and in others it became more complicated.

The simplification was that the inboard rear brakes which Jaguar had used for many years on the XJ6 were discontinued, with the new XJ40 having the outboard rear brake design used by virtually all other manufacturers (although the inboard rear brakes continued to be used on the XJ-S for a few more years). Outboard rear brakes mean that the brake rotors reside out by the wheels, rather than in by the differential as on the earlier cars. This makes it much easier to replace the rear brake rotors and pads on an XJ40 than on the Series III and prior Jaguars.

The complications were introduced by a new brake power assist system which used hydraulic pressure to provide boost, rather than using engine vacuum as is much more commonly seen and as was used in the earlier Jags. In this system, a hydraulic pump mounted on the upper part of the engine's timing cover is used to pressurize mineral oil. This pressurized mineral oil is used in conjunction with a hydraulic boost cylinder to provide power assist to the brakes (as well as to provide fluid pressure to operate the self-leveling rear shocks used on many of the XJ40 cars.) There are several other components to this fairly complex system, i.e. a pressure accumulator vessel and several pressure switches. This system can be difficult to diagnose when problems occur and a description of its operation is beyond the scope of this essay. Point your browser to http://www.jag-lovers.org/modern/xj40book/book_susp-brakes.html for a troubleshooting sequence for this system.

One important point: if the mineral oil level is allowed to get too low, then there will be no power assist to the brakes! It is a good idea to carry an extra bottle or two of the proper Jaguar Castrol HSMO (Hydraulic System Mineral Oil) in the car as this fluid is very difficult to buy outside of the Jaguar dealer system or independent Jaguar specialists. The 1988-89 XJ40s use a mineral oil reservoir which requires a special adapter when adding oil. The Jaguar mineral oil bottle comes with this adapter, although it is not required on 1990 and later XJ40s.

In 1990, Jaguar changed the brake boost system again. The engine-driven pump was eliminated and the Teves system was installed. This system uses a motor-driven pump on the firewall to provide the mineral oil pressure to boost the brakes. This Teves system also differs in that it does not use a traditional master cylinder but rather has an "actuation assembly." The mineral oil reservoir was changed to allow fluid addition without the previously-required adapter. The ABS function was also incorporated as an integral part of the brake system. This system has a couple of variants, depending on whether the car was produced with the self-leveling rear suspension or not. (For the new XJ6 introduced in 1995, the so-called X300 model, Jaguar reverted to vacuum-boosted brakes).

With the Teves system, it is very important during brake pad installation that the flexible brake hose be clamped, and the bleed screw loosened, when pushing the caliper piston deeper into the caliper to allow for the insertion of thicker new brake pads. This procedure expels the dirty fluid from behind the caliper piston and prevents it from being forced back toward the ABS unit. Not bleeding a Teves-equipped car in this manner can cause a problem with violent pulling to one side upon brake application. If you have a car which exhibits this symptom, there is an ABS valve-cleaning procedure detailed at the URL shown above which may stop this problem; there is also a repair kit available from Jaguar.

All of the XJ40 cars use a brake caliper design which utilizes sliding pins. When doing pad replacement or any other work at the calipers, it is wise to check for free movement of these sliding pins. It is fairly common for the sliding pins to bind or to seize, resulting in uneven pad wear and pulling under brake application. If seized pins are found, they must be freed up and lubricated with a high-temp grease to ensure proper caliper function. Replacement sliding pin kits are also available if needed. This is something that needs to be checked at every pad replacement.

One further issue involves the 1990-on XJ40s. On these cars, if the front brake pads are allowed to wear very low, and if the front brake rotors are also allowed to wear substantially under the specified minimum thickness (or are turned below the minimum), it is possible for the caliper piston to fall out of its bore and to lock up the wheel. On the 1990 and later cars, it is highly recommended not to let the front brake rotors go below the minimum thickness specification due to the possibility of this problem occurring.

As with all Jaguars, replacement of the brake fluid is specified every two years and this is a very wise thing to do, as Jaguars do not use sealed brake systems as with most American cars. The prevalence of internal brake caliper corrosion (which we see in our caliper rebuilding operation) will be reduced if this normally-forgotten service is performed as listed in the Jaguar maintenance schedule. Always use DOT4 spec brake fluid (not DOT3) when performing this service. Additionally, it is not recommended to use DOT5 silicone brake fluid in any ABS-equipped Jaguar.