Lexus SC300 / Lexus SC400. Manual - part 741

OPERATION
NEUTRAL (STRAIGHT–AHEAD) POSITION

SR0NU–01

Fluid from the pump is sent to the control valve. If the control valve is in the neutral position, all the fluid
will flow through the control valve into the relief port and back to the pump.  At this time, hardly any pressure
is created and because the pressure on the cylinder piston is equal on both sides, the piston will not move
in either direction.

WHEN TURNING

When the steering main shaft is turned in either direction, the control valve also moves, closing one of the
fluid passages. The other passage then opens wider, causing a change in fluid flow volume and, at the
same time, a differential pressure is created between both sides of the piston. Consequently, the piston
moves in the direction of the lower pressure so that the fluid in the cylinder is forced back to the pump
through the control valve.

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STEERING

POWER STEERING

SR–103

SERVICE HINT

Troubles with the power steering system are usually concerned with hard steering due to the fact that there
is no assistance. In such cases, before attempting to make repairs, it is necessary to determine whether
the trouble lies with the pump or with the gear housing. To do this, an on–vehicle inspection, can be made
by using a pressure gauge.

ON–VEHICLE INSPECTION

Power steering is a hydraulic device and problems are normally due to insufficient fluid pressure acting on
the piston. This could be caused by either the pump not producing the specified fluid pressure or the control
valve in the gear housing not functioning properly so that the proper fluid pressure can not be obtained.
If the fault lies with the pump, the same symptoms will generally occur whether the steering wheel is turned
fully to the right or left. On the other hand, if the fault lies with the control valve, there will generally be a
difference between the amount of assistance when the steering wheel is turned to the left and right, causing
harder steering. However, if the piston seal of the power cylinder is worn, there will be a loss of fluid pressure
whether the steering wheel is turned to the right or left and the symptoms will be the same for both.
Before performing an on–vehicle inspection, a check must first be made to confirm that the power steering
system is completely free of any air. If there is any air in the system, the volume of this air will change when
the fluid pressure is raised, causing a fluctuation in the fluid pressure so that the power steering will not
function properly. To determine if there is any air in the system, check to see if there is a change of fluid
level in the reservoir tank when the steering wheel is turned fully to the right or left.
For example, if there is air in the system, it will be compressed to a smaller volume when the steering wheel
is turned, causing a considerable drop in the fluid level. If the system is free of air, there will be very little
change in the level even when the fluid pressure is raised. This is because the fluid, being a liquid, does
not change volume when compressed. The little change in the fluid level is due to expansion of the hoses
between the pump and gear housing when pressure rises. Also, air in the system will sometimes result in
an abnormal noise occurring from the pump or gear housing when the steering wheel is fully turned in either
direction.
This on–vehicle inspection must be performed every time to ensure that the power steering system is work-
ing properly after overhauling or repairing the pump or gear housing.

SR–104

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STEERING

POWER STEERING

VANE PUMP

The main component parts of the vane pump, such as the cam ring, rotor, vanes and flow control valve
are high precision parts and must be handled carefully. Also, because this pump produces a very high fluid
pressure, O–rings are used for sealing each part. When reassembling the pump, always use new O–rings.
In the flow control valve, there is a relief valve which controls the maximum pressure of the pump. The
amount of this maximum pressure is very important; if it is too low, there will be insufficient power steering
assistance and if too high, it will have an adverse effect on the pressure hoses, oil seals, etc. If the maximum
pressure is either too high or too low due to a faulty relief valve, do not disassemble or adjust the relief valve,
but replace the flow control valve as an assembly.
The clearance between the flow control valve and pump body installation hole is very important. After
manufacture, the factory measures the size of the installation hole and outer circumference of the flow con-
trol valve, and punches a mark accordingly. Therefore, when replacing the flow control valve, be sure to
do so with one having the same mark in order to ensure the proper clearance.

The functional parts of the pump which produce fluid pressure are the cam ring, rotor and vanes, and these
should be checked for wear. If the clearance between each is not within standard when reassembling, any
worn parts should be replaced.
In this case, the replaced cam ring and rotor should be of the same length (have the same mark), and the
vanes should be replaced with those having a length corresponding to that mark, otherwise the proper
thrust clearance cannot be obtained. If there is too much thrust clearance, there will be insufficient fluid
pressure at low speeds. If there is too little thrust clearance, it may result in seizure of the vanes.

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STEERING

POWER STEERING

SR–105

GEAR HOUSING
If the gear housing is secured directly in a vise during overhaul, there is danger of deforming it, so always
first secure it in the SST provided (rack & pinion steering rack housing stand) before placing it in the vise.

The oil seals on both sides of the power cylinder are for the prevention of leakage of the high pressure fluid
which acts on the piston. Always use new oil seals when reassembling and be very careful not to scratch
or damage them.
Because of the high pressure, even the slightest scratch will cause fluid leakage, resulting in an inoperative
power steering system.
Also, be very careful not to scratch the sliding portion of the rack which makes contact with the oil seals.
When removing the rack ends from the rack, it is very easy to cause a burr when holding the tip of the rack
with a wrench. Therefore, before assembling the rack, first check the tip for burrs and remove any with an
oil stone.
Teflon rings are used for the piston and control valve. These teflon rings are highly durable against wear,
but if it is necessary to replace them, be careful not to stretch the new ones.
After installing a teflon ring into its groove, snug it down into the groove before assembly of the cylinder
or housing to prevent possible damage.

SR–106

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STEERING

POWER STEERING