Suspension Control ( Automatic - Electronic ): Description and Operation

Hydraulic System 1988-89:

XJ6 sedans for 1988-89 have an engine driven power hydraulic system which supplies hydraulic pressure to rear suspension for ride leveling, and to brake booster for power brake assist. The power hydraulic system is completely separate from the brake hydraulic system. Each system uses a different hydraulic fluid. The power hydraulic system uses Hydraulic System Mineral Oil (HSMO). The brake system uses DOT 4 brake fluid.

Hydraulic Pressure

Hydraulic pressure is not dependent on engine speed, load or temperature. A reservoir supplies HSMO to an engine-driven pump which supplies high pressure HSMO to the valve block. The valve block uses solenoid valves for three functions; brake boost charge, rear suspension up, and rear suspension down.

Brake Power Boost

An accumulator is used in the brake boost portion of the system. To be sure that a reserve capacity of pressurized HSMO always exists, charging the accumulator takes priority over ride leveling. The accumulator is equipped with a charge switch and a low pressure switch. The valve block charge valve is activated by the charge switch. A low pressure warning signal to vehicle condition monitor (VCM) is supplied by the low pressure switch and disables the operation of the up solenoid until a minimum pressure is established in the accumulator.

Ride Leveling (Rear Suspension Only)

The ride leveling part of of the system operates automatically to maintain minimum ride height. Trim adjustments are made by the ride leveling circuit to compensate for extra weight changes over the rear suspension. The system will not compensate for small load changes, loads imposed by rough roads, or cornering loads.

Valve Block

Valve Block Up To VIN 653345:

The operation of the three valve block solenoid valves is controlled by the charge switch and the ride height sensor. The charge valve is normally open and the up and down valves are normally closed. The charge valve is activated by both the charge switch and the ride height sensor. The up valve is activated by the ride height sensor via the suspension relay. The down valve is activated directly by the ride height sensor.

No Ride Leveling Operation / No Accumulator Charging:

When there is no call for accumulator charge pressure or ride leveling pressure, the charge valve is open, directing oil flow back to the reservoir. The up and down valves are closed to the ride leveling circuit.

Accumulator Charging Operation:

Accumulator charging occurs when the charge solenoid is activated and it closes the charge valve. Pressurized oil is applied to brake boost circuit to recharge the accumulator. The pressure relief valve in the valve block protects the system from over pressurization.

Ride Level Up Operation:

When ride leveling up pressure is required, the charge and up valve solenoids are activated to close the charge valve and open the up valve in vehicles up to VIN 653345. In vehicles from VIN 653345, the charge valve solenoid is activated to close the charge valve. Pressurized oil is directed to the ride leveling circuit to raise the rear of the vehicle. Pressurized oil is also applied to the brake boost circuit in 1988-89 vehicles. When the ride leveling compensation is achieved, the up valve closes and the charge valve opens directing oil from the pump to the reservoir.

Ride Level Down:

When ride leveling down is required, the down valve solenoid is activated to open the down valve allowing oil return to the reservoir to lower the rear of the vehicle. The trapped line pressure valve located within the down valve, maintains a minimum pressure in the rear suspension.