System Operation - Pre. 1990 Model Year - Page 4

System Operation - Pre. 1990 Model Year

The operation of the power hydraulic system can be divided into the following functions:
Charging the system. Trim up compensation. Trim down compensation. Power assisted brake operation.

Charging the System

When the ignition is switched on and the engine has remained stationary for a period sufficient for the pressure in the accumulator to drop below 54 bar (800 lb./sq.in.), the action of the low pressure switch causes a low pressure warning to be given on the instrument pack. This is a normal situation, the warning being initiated by the removal of a logic earth from the power brake relay which in turn inhibits suspension-up valve operation. Charging the system occurs when the engine is started. The unloader valve closes and thus, forces the oil flow from the pump into the accumulator. The accumulator contains a diaphragm which divides the unit into two chambers; an upper chamber within which nitrogen gas is contained, and a lower chamber into which mineral oil is forced under the influence of the pump. As mineral oil enters the lower chamber, the diaphragm is moved upwards against the nitrogen which in turn is compressed until the system operating pressure of 100 bar (1500 lb./sq.in.(nominal)) is reached. At this pressure, contacts of the charge switch open and the unloader valve de-energizes thus allowing oil to flow back to the reservoir. Pressure loss within the accumulator occurs during normal braking applications. Eventually the contacts of the charge switch will close and result in the charging cycle being repeated.

Trim Up Compensation

Provided that the engine is running and the pressure within the accumulator is greater than 54 bar (800 lb./sq.in.) levelling up can take place. The ride height sensor energizes a relay after a 20 second period of it detecting a car low condition. The relay in turn energises the solenoids of both the unloader and suspension-up valves which link the pump line to the suspension line and accumulator. The accumulator charge is maintained by its non-return valve. Accumulator charging will then occur when the levelling system pressure reaches the accumulator stored pressure. Should a low pressure condition exist, i.e. low pressure switch contacts open, actuation of the suspension-up solenoid is inhibited and the unloader solenoid valve assembly is actuated by the charge switch.
Trim Down Compensation

Irrespective of whether the ignition is on or off, the ride height sensor actuates the solenoid of the suspension-down valve after a 20 second period of it detecting a car high condition. With the suspension-down valve open, mineral oil within the rear damper/struts is vented back to the reservoir as the struts retract to the correct trim height. A minimum pressure of 27 bar (400 lb./sq.in.) is maintained by the trapped line pressure valve.

Power Braking Operation

Accumulator oil pressure is applied to the brake booster at all times and produces a return force of approximately 9 kg (20 lb.) to push the piston to the fully off position. When brake application is made oil pressure is fed to the servo chamber, giving assistance to the application. The degree of assistance is governed by a rubber reaction disc located between the booster and the output push rod. The force from this reaction disc causes the high pressure feed to the servo chamber to be closed, creating a "balance" condition. If additional braking is required then further pressure on the pedal will re-open the high pressure oil port to the servo until "balance" is again achieved. This "balance" point may be achieved at any given pedal effort up to the limit of boost. When the brakes are released the servo pressure is vented to the reservoir, the piston assembly will return to its fully off position under the influence of the inherent 9 kg (20 lb.) return force.