Diagnostics

During braking, if the ABS modulator detects that a wheel deceleration indicates imminent wheel lock, the relevant inlet valve is closed, preventing any further increase in hydraulic pressure to the brake caliper (pressure hold). If the wheel speed remains low, the associated outlet valve is opened briefly to allow brake fluid to flow from the caliper to the accumulator (pressure dump), hence reducing brake fluid pressure at the caliper. Brake fluid in the accumulator is pumped back into the pressurised side of the circuit by the return pump, ready for re-application. When the wheel is up to speed, the ABS modulator repeatedly deenergises the inlet valve briefly, allowing re-application of fluid pressure to the brake caliper (pressure build up) until excessive wheel deceleration is again detected. The process of pressure hold, pressure dump and pressure build up is repeated until the vehicle speed is below 5 mph (8 km/h), the risk of wheel lock subsides or the brake is released. During ABS operation, it is normal for pulsing to be felt through the brake pedal.

Diagnostics

The ABS modulator has a self diagnosis function which operates whenever the ignition is switched to position II. During the self test, the ABS modulator earths the UR wire from the ABS warning lamp in the instrument pack (C0234) for approximately 3 seconds to illuminate the lamp for a bulb check. When the vehicle moves off and reaches a speed of approximately 5 mph (8 km/h), the ABS modulator carries out a number of dynamic tests which include running the return pump and energising the inlet and outlet valves. If the brake pedal is being pressed at this time, a pulsing of the pedal may be felt.

The ABS modulator continues to monitor system performance while the ignition is in position II. If a fault is detected, the ABS modulator disables the system and illuminates the ABS warning lamp. With the system disabled, normal, non ABS braking is available. The ABS warning lamp remains illuminated until the ignition is switched OFF. If the fault is still detected when the ignition is next switched to position II, the ABS warning lamp remains illuminated after the bulb check. If the fault is not detected, the ABS warning lamp goes off after the bulb check.

If the ABS modulator detects a fault, a code relating to the fault is stored in the ABS modulator memory. If the fault is a temporary one, the code remains in the ABS modulator memory for 20 cycles of the ignition switch, after which the fault code is erased from ABS modulator memory. The ABS modulator memory can be interrogated for fault code retrieval using TestBook. TestBook communicates with the ABS modulator on the K wire between the diagnostic socket (C0040) and the ABS modulator (C0501).