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 XJ40ís 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