Lotus 2004 Eleven Service Notes page 269

Lotus Service Notes
JJ.12 - ABS THEORY OF OPERATION
The Kelsey-Hayes antilock brake system is an 'add on' type used to supplement the dual circuit, tandem
master cylinder, vacuum servo assisted brakes fitted to the Elise. A single electro-hydraulic unit comprising a
hydraulic modulator, hydrauliic pump, microprocessor and solenoid valve bank, is mounted in the front serv-
ices compartment and plumbed into the front and rear brake circuit lines from the tandem master cylinder.
The microprocessor (ECM) receives signals from magnetic wheel speed sensors integrated into each of
the four road wheel hubs, and interprets the individual wheel acceleration, deceleration, and comparative
wheel speeds. From this data, the ECM is able to determine if any wheel is tending to lock up, and if imminent
lock up is sensed, the microprocessor commands the relevant solenoid valves firstly to reduce pressure in that
particular brake circuit in order to restore wheel speed, and then to modulate pressure to that providing the
maximum braking force consistent with continued wheel rotation. The system is able to monitor and independ-
ently control each of the four wheel brakes, and is referred to a 4-channel system.
In order to achieve the required pressure modulation, three basic modes are used:
- Pressure hold;
- Pressure reduction;
- Pressure increase;
In order to maintain the safety provision of two entirely independent hydraulic circuits, one for the front
brakes, and one for the rear, the hydraulic elements of the control unit are doubled up, with no part of the
system shared between the two circuits. For the pressure hold function, four isolation solenoid valves are
used, one in the hydraulic circuit for each wheel brake. The pressure reduction function is achieved by a
separate dump solenoid valve in each of the four wheel brake circuits, and the pressure increase provided for
by a single electric motor operating two hydraulic pumps, one serving the front, and on the rear brake circuit.
Separate low pressure accumulators are used for the front and rear circuits.
Electro-Hydraulic Control Unit
The electro-hydaulic control unit comprises an alloy valve block containing the four isolation valves, four
dump valves, two hydraulic pumps and two accumulators, with the single pump motor screwed to the housing,
and with a solenoid block and ECM unit attached to the topside. The complete assembly is flexibly mounted
via three rubber isolator bushes to a steel cradle, which is itself mounted on three rubber bobbins to the
passenger side front chassis. The eight solenoid valves are grouped in two rows, with the valve plungers
protruding in sealed canisters from the topside of the unit where each one is surrounded by a solenoid coil. The
isolation and dump valves share a similar construction, but the spring loaded isolation valves are normally
open, and the dump valves normally closed.
Anti-lock ECM
Valve block
ECM isolator grommet
Cradle mounting bobbin
Passenger side damper bracket
Page 24
Section JJ
Control unit
mounting cradle
j190