Better Beaters - Building Cars and Daily Drivers

Sometimes you can't help but learn something new.

Anti Lock Braking Systems (ABS) are now standard equipment on most new cars.
At one time they were seen as an exotic offering on high end cars.

Learning more about potential problems with thirty year old anti lock braking system.

This black ball is a gas pressurized accumulator and an indication of the existence
 of a Teves Mark II  ABS system.

I didn't delve into this subject because I was intrigued. I did it because I became aware of certain problems that could affect my XJS's ABS system.

Like total brake failure.

Or the steering pulling hard to the right or left upon brake activation.

I was well acquainted with possible brake failure with older single circuit systems.

 A leak in a line or in a wheel cylinder could result in a loss of all braking power.

The switch to dual circuit systems isolated the front and rear brakes

A leaking master cylinder could still cause problems with both systems.

Leaking wheel cylinders would result in the inside tire wall being covered with brake fluid.

On American cars a leaking master cylinder would often leak into the area in front or behind the firewall.

Of course unless you regularly kept an eye on fluid levels you might never notice it until it was too late.

I once had a front brake line rupture on my '77 Datsun 280Z as I backed out of a parking space. I felt a sudden decrease in pedal resistance but the car still came to a stop.

I stopped, got out and looked at the parking slot. It looked to me as if oil had been sprayed onto the pavement.

Luckily, I was only a short distance from my house so I carefully and slowly drove the car home.

I replaced both front lines, bled the brakes and was good to go.

My '89 XJS is equipped with the Teves Mark III ABS system.

It does not have the familiar vacuum booster on the firewall, instead it utilizes an electric pump to provide a source of high pressure brake fluid. A spherical pressure reservoir (accumulator) is mounted on a manifold on the opposite side of the firewall. These black balls are quite distinctive.

The accumulator stores enough pressure to provide boosted stopping for several applications even if the pump fails. Much like the more familiar vacuum booster which does much the same thing.

The accumulator/ pump manifold is energized by turning the ignition on. The pump runs until it pressurizes the fluid in the accumulator ball. Then the  pump shuts off and awaits the need to pressurize the system during a brake application. Warning lights on the instrument panel advise of this process.

The master cylinder is directly activated by the brake pedal. The hydraulic fluid is directed to the front brakes as well as the accumulator manifold. The pressure opens a valve which directs pressurized  fluid from the accumulator ball back to the master cylinder, where it helps provide boost to the master cylinder piston. This pressurized fluid is also routed to activate the rear brakes.

The wheels are equipped with speed indicating rings with pick ups. This info is fed to the ABS computer which will determine if a wheel has locked up and is skidding.

The computer activates electrical solenoids that control the amount of fluid flowing to the brake caliper at each wheel. If a wheel is determined to be skidding the solenoid can stop the flow until the computer can determine that the wheel has started rolling again. This takes place very quickly and it is the basis for the "pulsing" felt by the driver at the brake pedal.

The actual braking action is supplied by conventional discs and calipers.

The XJS and other high end cars were the first to be equipped with these systems. However this same system was used on a surprising variety of cars both foreign and domestic. It was high tech and functioned well back in the day.

In my case that day was thirty years ago. What can go wrong? Plenty, of course. We can leave the regular hydraulic components and fittings out of the discussion for now.

I visit the Jaguar forums on a daily basis and I look through all of the new posts as well as search for answers to my specific questions.

I stumbled across several threads with frightening titles such as Total brake failure, Sudden brake failure. Occasional problems stopping, and, The car darts suddenly upon braking. Not too reassuring.

Pressurized brake fluid is supplied and stored by the ABS pump and accumulator ball. If the pump fails than there will be no assist. When the accumulator fails than there is no safety reservoir of pressurized fluid and again assist is lost. The front brakes should still remain operable but require much higher foot pedal effort to stop. Pretty much like any conventional vacuum boosted system.

It seems that an equal concern is the failure of the solenoids that control the distribution of brake fluid. If they get stuck in the closed position than that wheel will lock up and stay locked up. If a single front wheel locks up it will cause the car to pull severely to that side. It seems that some of the forum members have ended up in a roadside ditch.

photo source: jaguarforums com
These are the six solenoids and you can see the ribbon wire connections.

Those solenoids are connected to the connector plug with ribbon wires. There are reports of these ribbons breaking due to old age.

What can be done to prevent these problems? Keep in mind that these cars can be thirty years old. Components can and will wear out and require replacement.

First of all is performing the proper maintenance, which includes using the proper brake fluid

This actually seems to be the most important factor. The specified fluid is DOT 4. DOT 5 silicone fluid is not compatible with these ABS systems. Since it doesn't absorb water, the water sits in different parts of the braking system causing corrosion and it can boil under high heat conditions. It was also noted that it can foam when pressurized by the pump.

Also very important is to change the fluid regularly to keep it clean. The fluid in this system doesn't just sit in the reservoir. It circulates throughout the system constantly. Dirt and contaminants will be spread throughout the system instead of just settling to the bottom. Change the fluid and flush the system by bleeding the brakes. This is important to keep the distribution solenoids clean and working freely.

Of great importance is that when changing brake pads the caliper pistons should never be forced back without opening the bleeder valves. This could force dirty fluid and contaminants back into the distribution block. This could result in the solenoids sticking resulting in seizure or erratic behavior. This is what causes the car to pull to the side.

The accumulator ball is a very important safety component. Not only does it provide a storage area for pressurized brake fluid but it also controls the length of time that the ABS pump has to run. These balls are gas charged and on average lose 10% of their charge every year. As they degrade the ABS pump must work longer to bring them up and maintain the required pressure. These pumps were never designed to work 100% of the time, they were designed to bring the pressure up when the vehicle is started up and they will kick in to keep the system at proper pressure. Think of it like your home's heater thermostat, it's there to maintain a certain temperature. It shouldn't keep the furnace running all the time! If the pump is activated after every brake application due to a faulty accumulator, it will eventually overheat when repeatedly used, and will shut down, with predictable results.

Obviously it's extremely important to pay attention to how long the ABS pump runs upon initial start up.

First things first. I'm going to replace the brake fluid in the master cylinder using a turkey baster. I know there's one in the kitchen somewhere! Then I'll see about bleeding the brakes. That's probably going to have to wait until the Holidays are over.

I'd like to thank the Jaguar Forums, and Jaguar Lovers websites as well as Kirby Palm and his book; Experience in a book, Help for the Jaguar XJS owner.  They have made an enormous amount of valuable information available to the enthusiast.