7.4 - ABS Warning System ( ,
7.4.1 - Overview - 1987 thru 1989 models ( ,
The default condition of the ABS indicator is ON. The ABS computer pulls a line down to turn the light off if all is well. Thus, if the ABS computer doesn't have any power, the light is on, indeed, if you remove the computer, the light is on.
The "over-voltage-protection" relay, when working properly, supplies power to the ABS computer only when the line voltage is between 13 and 15 (rough numbers) volts. So, the ABS light will be on until the alternator starts charging, and the line voltage comes up... at which point
The computer runs a static system check on power-up. If all checks well, it requests the light to be turned off.
And finally, the system runs a dynamic check when it senses a vehicle speed greater than 3 mph. If this fails, it turns the light back on, and keeps it on.
Whilst in operation, the system runs checks, and turns the light on if anything is found wrong.
One note here is when the system detects a fault in the ABS system rather than an electrical fault, it shuts down and turns the light on for the duration... the light will not go out until you shut the key off.
If the light comes on and goes out randomly during a single driving session, the system is losing power. This could be caused by electrical connection faults or charging system faults, but is far and away most likely to be a flaked out relay.
Wheel sensors are the single biggest real failure in the ABS (biggest is dodgy over-voltage relay which is easily fixed). And, just for the sport of it, a few diagnostic tips:
1. The sensors get weak before they die totally. Causes intermittent "failures".
2. Proper testing requires a lab scope, and a reference pattern/ amplitude. however, you can do a rough-n-tumble by hooking a AC volt meter to the unplugged sensor. Compare side to side. At the sort of speed you can spin the front wheels by hand, you should see about 1 volt. At about 0.7v, the system gets upset.
3. Ever notice how badly XJ40’s chew up the brake rotors? Well as those filings from the rotors seem to gravitate (magnetate?) to the ABS sensors. Before replacing a suspect sensor, try cleaning all of the swarf from it and the reluctor wheel.
4. The most common cause of the warning is due to a failed solder connection in the overvoltage relay. This relay is mounted in the left hand side of the boot behind the carpet. There is a line of relays just under the fuel filler pipe. Figure out which one it is, take it out, slip off the black plastic cover, where the two boards are joined, the solder fails due to fatigue. Resolder and replace.
The ABS does a two pass self test. First test is done directly at startup. When this passes, it will do the second part as soon as the car exceeds 5km/h. This second test is dynamic, firing off the pump and cycling all of the dump valves. It is audible but not too loud and creates a strange feeling if you happen to brake at the same time it does it. If the failure light comes on at any time this second test will occur again as the failure light goes out.
Usually (But of course not always) ABS Failure is caused by a faulty Overcharge relay. If your car claims ABS failure from startup, then you probably have this problem!
This relay is located on the left hand side of the boot. It's the tallest of the 3 or 4 relays back there. Open the relay up, and you should find some broken contacts on the circuit board. Since this relay costs $48+++ it's worth fixing
7.4.2 - Overview - 1990 thru 1994 models ( John Ping,
January 16, 2001
The Teves Anti-Lock Braking System as installed on Model Year 1990 through 1994 Jaguar XJ40s has demonstrated both high reliability and excellent durability. The system is not exclusive to Jaguar as it was used on other European manufactured vehicles (Alfa Romeo, Saab, Mercedes-Benz and certain Ford applications). It is considerably more than just an anti-lock braking system; it also provides the power-assisted boost for the master cylinder. It is however, quite non-conventional in comparison to other contemporary ABS designs. Instead of the ubiquitous vacuum assist booster for the master cylinder, it uses a hydraulic boost provided by a small electric motor driven pump. To prevent continuous pump operation, high-pressure hydraulic fluid is stored within an inert gas filled accumulator. Whenever the driver applies the brakes, this stored hydraulic energy is used to boost input pressure to the master cylinder. Normally (non-ABS operation), the front wheel brake hydraulic circuitry is controlled in a conventional manner by the master cylinder in what is termed "static" operation. However, the rear wheel brake hydraulic circuitry is controlled by the complex pressure / position interactions of the hydraulic control unit in what is termed "dynamic" operation.
The heart of the Teves ABS is the electronic control unit (ECU), which is located in the trunk (boot) behind the molded carpeting on the fuel filler side. The Teves system performs an initial ABS "self-check" routine upon startup and subsequently, in a periodic manner, during vehicle operation to guard against hidden system malfunctions. If any system malfunction is detected by the ECU, the ABS function will be inhibited (conventional braking remains) and the "ABS Warning Light" will be illuminated. With the vehicle in operation, the ECU continuously monitors signal inputs from speed sensors located at each wheel. Additionally, it monitors system pressures by function of the "combined pressure warning switch" and brake fluid levels by operation of the "twin reed switches" mounted within the reservoir.
During normal vehicle operation, the ABS ECU constantly checks for wheel speed differentials. Normally, all wheel speed inputs should be relatively equal. But if the inputs are different, the ECU interprets this data as imminent wheel lockup and modulates the circuit control valves in a manner as to prevent wheel lockup (decreases hydraulic circuit pressure). Twin hydraulic circuit control solenoid valves are provided for each of the three hydraulic circuits (right front wheel, left front wheel and rear wheels). Each of these valves will open or close as necessary to eliminate wheel lockup. Wheel speed data manipulation and solenoid valve control is performed by the ECU. A main solenoid valve is energized during ABS operation to provide "dynamic" operation of the front wheels.
Electric motor driven hydraulic pump operation is controlled via the combined pressure warning switch. When stored system hydraulic pressure decreases below 140 bars (˜ 2050 psig), the pump-motor is activated. System hydraulic pressure builds and is stored within the accumulator until a threshold of 180 bars (˜ 2650 psig). Normal system operating pressure will be contained within these limits based on braking demands. The entire hydraulic system is protected from over-pressure conditions by a pressure control valve which releases pressure at 210 bars (˜ 3100 psig). The hydraulic accumulator is a spherical device internally separated with a flexible membrane to create two independent chambers. The upper chamber is filled with inert nitrogen to approximately 84 bars (˜ 1235 psig) and acts as a continuously applied force upon the lower chamber, which is filled with brake fluid. The action of the hydraulic accumulator provides a working reservoir of high pressure brake fluid.
The combined pressure warning switch also provides other ABS pressure related alarms. In the event of an electrical or mechanical malfunction (i.e. faulty pump or hydraulic accumulator) is experienced within the ABS, the combined pressure warning switch will illuminate (via the ECU and instrument panel logic) the "ABS Warning Light" when system pressure decreases below 130 bars (˜ 1900 psig). Additionally, the ECU will partially inhibit ABS operation by defeating the front wheel ABS control. At 105 bars (˜ 1550 psig) decreasing, the "Brake System Warning Light" will be illuminated. This latter action represents a very serious braking system abnormal condition and its cause should be immediately investigated and rectified. It is dangerous to drive the vehicle in this condition! Keep in mind, that if only the "Brake System Warning Light" is illuminated, the problem could be simply the parking brake is applied or the brake pads are excessively worn.
Upon normal vehicle startup (after several hours of shutdown), its normal for both the "Brake System Warning Light" and "ABS Warning Light" to be illuminated since system hydraulic pressure has partially bleed off. After approximately thirty (30) seconds or less, the system hydraulic pressure will increase (via pump operation) extinguishing first the "Brake System Warning Light" and immediately followed by the "ABS Warning Light". It is also normal for certain ABS relays to momentarily "cycle" during non-braking operations as this signifies performance of the ECU "self-check" routine. Increased operation of the hydraulic pump in conjunction with numerous illuminations of the "ABS Warning Light" is likely to represent a problem with the hydraulic accumulator.
The internal operating aspects of the hydraulic brake booster, valve block, master cylinder, pump/motor and the ECU are complex. Due to their complicated nature (and lack of refurbishment parts), repair of these circuits is usually far beyond the capabilities of even the most diligent and knowledgeable owner. Replacement of the hydraulic accumulator, combined pressure warning switch, motor-pump and wheel sensors is within the grasp of the technically competent owner.
The actual Teves Anti-Lock Braking System requires minimal "special" maintenance, but indifference or negligence in regard to system operation can be very expensive for the owner. As with any braking system, the longevity and reliability can be greatly enhanced by periodic flushing and replacement of the brake fluid. A dedicated flushing routine will eliminate oxygenated brake fluid and corrosion products from the brake system. Unlike many brake systems that are completely sealed via an expandable membrane, the Teves system uses a vented reservoir. Oxygen can be dissolved into the fluid, which can result in oxidized working surfaces and decreased seal life. Unless vehicle operation conditions are very severe, it is prudent to replace the system fluid every two years or approximately 30k miles (48k kilometers).
Fluid replacement can be performed using numerous traditional methods. However, the use of a manual (hand operated) vacuum pump with fluid reservoir makes it a simple one-person operation. These tools are very common and readily available at any quality tool shop or automotive supply store at a reasonable cost. The Teves ABS as installed on XJ40s requires the use of DOT 4 rated brake fluid and other types (silicone based fluids) should be avoided. As with any braking system work, always be vigilant for system leakage. Never push back a brake caliper piston without first opening the caliper vent and installing a drain tube. Contaminated fluid with particulate matter can have a notable damaging impact on valve block and hydraulic booster operation if allowed to re-enter the system.
During brake pad replacement or fluid change intervals, the wheel sensors should be cleaned and electrical connections checked for integrity. It is not necessary to remove the sensors, simply brush them off or clean them, as needed using a quality electrical contact cleaner. A buildup of ferrous compounds (brake rotor wear) on the sensors can alter their operating characteristics. Do not utilize strong petroleum solvents as this could adversely impact the sensor materials.
After approximately 100k miles (160k kilometers), it is prudent, though not mandatory, to replace the three (3) ABS electrical relays. The ECU and Main relays are contemporary five-conductor automotive type with 30 amp rated contacts. With these two relays, both the normally open and closed contacts are wired and perform a function. The Pump relay is of heavier design. A 40 amp rated, normally open contacts relay should be used for this application. A conventional five-conductor relay will fit this application as well. All three of the ABS relays are located under the driver’s side (LHD) footwell electrical component panel.
The Teves ABS has an ECU "interrogate" feature that can be accessed by the owner. The fault code is provided to the owner by a series of "blinks" from the "ABS Warning Light". The process of retrieving the codes is conventional after the appropriate "conductor jumpering" sequence has been initiated at the ECU diagnostics connector located near the ECU in the truck and ignition switch manipulated. A full description of ABS fault codes and recovery sequence is beyond the scope of this article.