System Description - Pre. 1990 Model Year - Page 2
System Description - Pre. 1990 Model YearCAUTION: The hydraulic power system uses CASTROL HYDRAULIC SYSTEMS MINERAL OIL (HSMO). Use of other oil may result in system failure. Store the HYDRAULIC SYSTEM MINERAL OIL ONLY IN ITS ORIGINAL CONTAINER. Keep the container clean and tightly closed as contamination will result in mechanical failure or costly repairs.
The hydraulic power braking and payload compensated levelling system uses a power hydraulic system to provide power assisted brake operation and can also provide self-levelling, which automatically adjusts the vehicle trim to maintain the correct body attitude with respect to the road. Trim adjustments undertaken by the system are limited to those which compensate for laden/unladen (bulk load) changes over the rear axle rather than those experienced when cornering or negotiating undulating road surfaces.
CAUTION: Since the system corrects the vehicle trim to compensate for rear axle load changes, it is possible to grossly overload the vehicle without any perceptible difference in trim height. It is essential that the specified vehicle weight is not exceeded.
The system, which responds to trim deflections maintained over and above a period of 20 seconds, contains the following items:
Accumulator assembly: A vessel within which hydraulic system mineral oil is stored at a pressure of 91-101 bar (nominal), being supplied by a mechanically driven pump.
Charge switch: a pressure actuated switch which responds to oil pressure changes within the accumulator, and transmits a signal to the unloader solenoid valve.
Low pressure switch: a pressure actuated switch which, in addition to providing low pressure warnings on the instrument pack, inhibits suspension-up valve operation at system pressures below 54 bar (800 lb./sq.in.) Non-return valve: Prevents pressurized oil escaping to the reservoir from the accumulator, via the valve block assembly.
Solenoid valve block: contains a solenoid unloader valve, which distributes the oil flow in the system. Suspension-up/down solenoid valves which, under ride height sensor control, increase or decrease the oil pressure applied to the struts. Trapped line pressure-valve (integral-part of down solenoid valve assembly): A non-return valve which ensures that a minimum oil pressure of 27 bar (400 lb./sq.in.) is retained within the levelling system.
Pressure relief valve: permits high pressure oil to return to the reservoir in the event of gross vehicle overloading or the system becoming over-pressurized.
Pump: a three piston radial device which operates at 0.75 times engine speed and delivers 0.7 litre/min (0.153 gal/min) at 600 rev/min.
Fig. 1. Power Hydraulic Circuit - Pre. 1990 MY (schematic only):
Oil reservoir: a 1.5 litre (0.33 gal) vessel which stores a supply of mineral oil for use by the system. A float-switch is incorporated to give low oil level warnings on the instrument pack.
Levelling struts: an integral part of the rear suspension which extend and retract in response to oil pressure changes made by the suspension up/down valves.
Ride height sensor: a device which monitors the body-to-axle distance and controls the suspension up/down valves to maintain the correct trim.
Brake booster: This unit fits between the brake pedal and the master cylinder using the accumulator pressure to boost the pedal effort.
Note: In the event of a system failure, a number of power assisted brake applications are available from the pressure remaining in the accumulator and self-levelling of the vehicle is abandoned. Un-assisted brake operation still remains after the accumulator is exhausted.
A schematic illustration of the power hydraulic circuit for pre. 1990 model year vehicles, is shown in Fig.1.