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Lotus Service Notes Section JL
Pressure Increase (Re-apply)
As soon as imminent wheel lock is detected by the ECM, and the ABS control system is activated, the
pump motor is energised. When the dump valve is opened, and fluid is allowed to bleed off from the wheel
brake circuit into the low pressure accumulator, this fluid is scavenged by the pump and returned back into the
input circuit through an attenuator orifice. This action is the origin of the ‘pedal pushing back’ sensation felt by
the driver, with the pressure pulsations from the pump damped and quietened by the restriction of the orifice.
When wheel speed has been restored and the brake pressure is required to increase, the isolation valve is
momentarily opened, to allow master cylinder/pump pressure to raise the pressure in the wheel brake circuit
in increments. If imminent locking is again sensed, the isolation valve closes, the dump valve opens and the
cycle repeats, with the whole process occuring several times a second.
Isolation valve
pulsed open
Increasing
pressure
to LHF
brake
Inlet from
master
cylinder
Attenuator
Pump orice
operating
Accumulator
discharges into
pump
j149d
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