Anti-lock Braking System

Models with higher performance level came fitted with ABS.

The principle of operation is the same as on cars with conventional braking systems but the layout is much simpler as all we need to control the operating pressure of the brakes are a few electro-valves.


During breaking, the ABS computer monitors the changes in the rotational speed of each roadwheel, communicated by inductive magnetic sensors reading the individual cogs of a toothed wheel fitted inside the cavity of the brake discs. The computer does not interfere with the braking if the vehicle speed (as measured with the same sensors) is below 5 km/h.

If any of the wheels begins to slow at a faster rate than the others, the ABS reduces the hydraulic pressure fed to the brake caliper of the wheel in question to avoid the wheel being locked. Although every wheel has its own sensor, the rear brake calipers receive the same pressure, only the front ones are fed separately. As soon as road grip is regained, the hydraulic pressure to the brake will be restored. The computer is capable of cycling the pressure with a frequency of several times a second.

To actually control the pressure, the system uses a three-unit hydraulic block (one block each for the front brakes, one for both rear brakes). All three units comprise two electro-valves, an inlet 1 and a return 2 valve.

During the rising period of normal braking, without the need for the intervention of the ABS computer, the brakes operate in phase 1: the inlet valve 1 is open but the return valve 2 is closed. The braking functions as in a system without ABS: the incoming hydraulic pressure is directly routed to the brake caliper.

Under constant breaking (phase 2) both valves close to maintain a steady hydraulic pressure in the brake calipers.

When the ECU senses the need for intervention, the electro-valves proceed to phase 3: the inlet valve 1 closes while the return valve 2 opens. Hydraulic pressure will be released from the brake caliper, reducing the braking force. To restore the braking effort, the ECU will return to phase 1 in a short while.

The ABS computer has a built-in diagnostic feature, checking the components both when the ignition is turned on and during braking. Any failure will be reported by a warning lamp or a warning message of the board computer. As you can see from the illustration, the springs inside the valves are located in such a way that the mechanical default mode is phase 1—the normal braking—for all three hydraulic blocks. Any failure in the ABS system will therefore revert it to the usual, non-assisted braking.

Early CXs have a slightly different ABS system. The general layout is the same, but the hydraulic block only has three valves, one for each brake circuit, however, they have three positions. Without energizing current, they route the fluid coming from the brake accumulator to the brakes. In phase 2, a medium current switches them to isolate the brake calipers, while a larger current opens it completely to let the pressure escape from the brakes into the return lines.

On the XM the hydraulic block only has five electro-valves. Two valves serve the front left and right wheels each, and the remaining fifth valve controls the rear brake circuit.

The front valve pairs start working in phase 1. Both valves are open, not energized by the ABS computer, letting the brake pressure build up in the brake calipers just like in the simpler, non-ABS braking system.

In the next phase, the valve 1 will close the inlet from the brake valve and open up the return to the reservoir but the behavior of valve 2 will depend on the speed of the pressure dropping in the circuit. If the ABS ECU senses a swift drop of pressure, it will leave this valve in its original position, opening up the way for the pressure to exhaust into the return line quickly—this can be called phase 2.

If, however, th pressure is dropping slower, the ECU will close the valve 2, only allowing the pressure to escape via the damper 3...