LV29 steering systems (2)

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kap all phase & 6/11/03 11:37 am Page 29 Student Workbook LV29 Steering Systems (2) LV29/SWB Student Workbook for Technical Certificates in Light Vehicle Maintenance and Repair MODULE LV29 STEERING SYSTEMS (2) Contents Page Introduction Overview of Steering Systems: Maintenance requirements for nonpower assisted steering systems Steering wheel installation Steering wheel free play Wear in the steering linkage Wheel bearing play 3 3 Power Assisted Steering Systems: Operating principles of power assisted steering Vehicle speed sensing type Engine rpm sensing type Power steering components The power steering pump Flow control valve and control spool Exercise Maximum load idle-up 9 10 12 13 15 Control Valves: Control valve operation Rotary valve Spool valve Flapper valve Progress check 16 17 17 22 27 30 Page Maintenance: Fluid pressure checks 31 31 Progressive Power Steering 33 Electro-Hydraulic Power Assisted Steering 35 Vane Pump and Motor Construction: Pump motor control Progress check 37 37 41 -1Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue -2Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Introduction During this course of study the maintenance requirements for non-assisted steering systems and the operating principles of power assisted steering will be outlined Overview of Steering Systems Maintenance requirements for non-power assisted steering systems The maintenance checks on non-power assisted steering are very small but some of the checks needed are described below Steering wheel installation By moving the steering wheel in all directions, it is possible to check that the steering wheel has been correctly installed This test will also check that the main shaft bearing is not loose and that the steering wheel is secured correctly to the main shaft Steering wheel free play Whilst sitting in the same position steering wheel free play can be checked Place the steering wheel in the straight-ahead position, and check that the front wheels correspond to the steering wheel Now move the steering wheel slightly to the left or right without the wheels actually moving Check the manufacturers’ specifications for the amount of free play allowed -3Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue If there is excessive free play then one of the following problems may have occurred: • worn steering linkage • steering wheel not secured properly • worn wheel bearings • worn steering rack, or incorrect adjustment of steering gear • loose main shaft joint Wear in the steering linkage Raising the front of the vehicle off the ground and moving the wheels back and forth is the recommended method for checking the wear in the steering linkage If there is excessive movement then either wheel bearings or worn steering linkage are possible causes To raise the front of the vehicle either use a two-poster ramp or a trolley jack; if a trolley jack is used then axle ramps must be used to support the vehicle -4Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Wheel bearing play As with the steering linkage the wheel bearing play can be checked by first raising the front of the vehicle off the ground and then gripping the wheel at the top and bottom and checking if there is any play in the wheel If there is play, then the same task must be carried out with the brakes applied If the amount of play is reduced and not eliminated totally then it is probably not a wheel-bearing fault Rods and arms for damage Gear housing for damage Steering linkage for looseness Gear housing for oil leaks The four diagrams above show other checks that can be carried out on the steering system -5Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Power Assisted Steering Systems Main shaft steering column Control valve Reservoir tank Gear Housing Power housing Vane pump As motor vehicles continue to develop, new systems need to be introduced Modern vehicles are now fitted with wider and lower profile tyres to improve the handling of the vehicle The negative consequence of this is the increase in effort needed to turn the steering wheel, due to the extra friction generated between the tyres and the road Steering load can be lightened by changing the steering ratio within the steering box This has the disadvantage that it also increases the amount of turns needed by the driver, to turn the steering from lock to lock The introduction of power assisted steering has helped to solve this problem by assisting the driver to turn the steering wheel without increasing the amount of steering wheel rotation needed Power assisted steering can be used on various steering system configurations, although in this workbook the rack and pinion steering system will be used to explain the principles -6Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Operating principles of power assisted steering When Turning Neutral position Control valve Power cylinder Cylinder piston Power assisted steering utilises hydraulic pressure to assist the driver to turn the steering wheel Either an engine driven pump or an electrically driven pump generates the hydraulic pressure needed to assist the driver The pressure generated by the pump acts on either the left or right hand side of the power piston and is directed there by the control valve When the steering wheel is in the straight-ahead position, the control valve is in the neutral position This means that the pressure generated by the pump is allowed to travel directly down the relief port, so very little pressure is sent to either side of the power piston The pressure that is sent to the power piston acts equally on both sides, so no steering is achieved When the steering wheel is turned either to the left or right, the control valve is also moved This is because the steering wheel is connected to the control valve via a torsion bar As the control valve moves it opens one port and closes the other This allows hydraulic pressure generated by the pump to be directed to either one side or the other of the power piston The side of the power piston that does not have hydraulic pressure applied has its fluid directed to the intake side of the pump through the relief port This causes a pressure difference between one side and the other and thus aids the driver to turn the steering wheel As described earlier, power steering is required to aid the driver to turn the steering wheel due to increased friction generated between the tyre and the road The amount of assistance needed by the driver is dependent on vehicle speed When the vehicle is stationary or travelling very slowly (when being parked) the driver needs maximum assistance, but as the vehicle speeds up the amount of assistance needed reduces -7Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue When the vehicle is travelling at high speeds very little or no assistance is needed, this is due to a reduction in the friction between the tyre and the road If maximum assistance is given to the driver at high speeds, then the driver will tend to over steer due to the small effort needed to turn the steering wheel To compensate for this different devices are fitted to the steering system These enable the steering system to increase or decrease the amount of assistance given to the driver under different driving conditions Two types of system are explained below, but will be explained in full later in the workbook These two systems are often used together to gain the desired result Vehicle speed sensing type Vane pump Gear housing Solenoid valve Power steering ECU Speed sensor The vehicle speed sensing type detects the speed of the vehicle via a speed sensor normally fitted at the rear of the combination meter From this signal the power steering ECU is able to determine the speed of the vehicle and adjust the amount of assistance given to the driver -8Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Engine rpm sensing type Flow control valve Most power steering systems are designed to send the same amount of fluid flow to the control valve regardless of engine rpm With the engine rpm sensing type, as the speed of the engine increases and reaches a given point, the fluid sent to the control valve is reduced This in turn reduces the pressure applied to the power piston Power steering components Torsion bar Sleeve valve Rotary valve Spool valve Pinion Rack Spool valve type Rotary valve type The diagrams above show two different types of valve and their operation will be outlined during this course of study -9Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Flapper valve The flapper type control valve is used with the recirculating ball type steering system and is integral with the torsion bar The high-pressure oil generated by the power steering pump is first directed through valves V1 and V2 These valves control the direction in which the fluid flows Fluid is allowed to flow from P to A to T or from P to B to T depending on which direction the driver turns the steering wheel These first two valves form flapper No.1 The second two valves act as pressure control valves, controlling the pressure at points A and B, depending on the amount of force generated by the drive turning the steering wheel These two valves form flapper No.2 If the steering wheel is in the straight-ahead position so the control valve is in neutral, all four of these valves are open, so there is no pressure difference at points A and B If the driver now turns the steering wheel to the left, V1 off flapper No.1 is opened and V2 is closed Flapper No will also be operating at this point, partially opening valve V3 and fully opening V4 By opening and closing these valves, pressure will rise at point A forcing the piston to the right thus giving assistance to the driver -27Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Neutral position V1 Open V2 Open V3 Open V4 Open When the steering wheel is in the straight-ahead position, the control valve is in the neutral position Neither flapper No.1 nor flapper No.2 are moved so all four valves are open This allows fluid to travel from the power steering pump directly back to the reservoir through all the passages As all the valves are open, there will be no pressure difference between left and right hand sides of the cylinder, so no assistance is given Turning right V1 Closed V2 Open V3 Open V4 Partially Open As the driver turns the steering wheel to the right, pressurised fluid will travel through valve V2 to the right hand side of the power piston If the driver continues to turn the steering wheel then the pressure acting on valve V4 caused by the increased force generated by the driver, will close V4 tightly This will cause the fluid pressure to rise giving maximum assistance to the driver As the amount of turning force exerted by the driver is reduced, so is the amount of twisting of the torsion bar This causes valve V4 to open slightly, thus reducing the amount of assistance given -28Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Turning left V1 Open V2 Closed V3 Partially open V4 Open When the driver turns the steering wheel to the left, the flapper valves move in the opposite direction to when the driver is turning to the right The operating principles are the same as when carrying out a right hand turn, except the pressurised fluid is directed to the left hand side of the power piston -29Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Progress check Answer the following questions: Why is power steering required? What is the purpose of the flow control valve? Name three types of control valve: Why are idle-up devices fitted? -30Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Maintenance Items to be checked • fluid level • drive belt tension • hydraulic pipes and gear housing for leakage • steering wheel free play • steering linkages • steering joints and bushes • idler gears • steering column • replacement of power steering fluid The mechanical items like steering linkages are checked in the same manner as non-power assisted steering, with the explanations given at the start of the workbook In this section explanations will be given on how to carry out fluid pressure checks, which will assist in the diagnosis of the power steering system Fluid pressure checks Disconnect the pressure line from the gear housing and connect in the pressure gauge Bleed the power steering system as described by the manufacturer and recheck the fluid level Bring the fluid level to the correct temperature; this may vary depending on the manufacturer -31Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Close the valve on the pressure gauge and check the reading displayed Compare the reading obtained against the manufacturers’ specifications If the pressure is low replace the power steering pump Re-open the valve on the pressure gauge and take a reading at 1000 rpm and at 3000 rpm Check the manufacturers’ specification, and if the difference between the two readings is too great, then replace the flow control valve within the power steering pump Check that the valve on the pressure gauge is still fully open Then turn the steering wheel to full lock and take a reading If the pressure is lower than the manufacturers’ specification, then there is an internal leak in the gear housing The gear housing must be repaired or replaced -32Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Progressive Power Steering Vane pump Gear housing Solenoid valve Speed sensor Power steering ECU At the beginning of the workbook an overview was given with relation to progressive power steering As a vehicle speeds up the amount of friction, generated between the road surface and the vehicle tyres, is reduced This means that the amount of assistance needed by the driver is also reduced Different manufacturers will use different systems, each controlled by an electronic control unit to achieve progressive power steering Two examples of systems used today are: • types that use a bypass circuit to reduce the amount of pressure delivered to the power piston • types that control the twisting of the torsion bar within the control valve An overview will now be given of a type that reduces oil pressure by the use of a bypass circuit -33Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Low speeds Medium and high speeds The operation of the control valve is controlled by the amount of twisting force exerted by the driver on the torsion bar As the driver turns the steering wheel the pressurised fluid is directed to either one side of the power piston or the other by the control valve When vehicle speed increases the amount of assistance needed decreases, so there needs to be a way to reduce the amount of pressure being directed to the power piston This is achieved by the introduction of a bypass circuit that links the right and left hand chambers Within this bypass circuit a solenoid valve is fitted, this allows varying amounts of fluid to travel through the circuit The solenoid valve is controlled by an electronic control unit, which is fed information on vehicle speed through the use of a speed sensor When the vehicle is travelling at low speeds the solenoid restricts the size of the passage thus giving the driver greater steering assistance As the vehicle speed begins to increase, then the amount that the solenoid restricts the passage is reduced, thus reducing the amount of assistance given to the driver Duty ratio signals are sent by the electronic control unit to control the position of the solenoid If the amount of time the voltage is turned on is greater than the amount of time it is turned off, the solenoid will see this as a high average voltage and adjust the opening of the bypass circuit accordingly If the amount of time that the voltage is turned on is less than the time it is turned off then the average voltage detected by the solenoid will be low -34Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Electro-Hydraulic Power Assisted Steering In conventional power assisted steering systems, an engine driven power steering pump generates the hydraulic pressure needed to assist the driver With electro-hydraulic power steering the engine driven pump is replaced by an electric motor There are two main advantages to this type of system One is that the speed of the pump can be controlled via the engine’s electronic control unit, and by varying the speed of the pump progressive steering which can be achieved easily The second is that there is no power loss due to the driving of the vane pump as with conventional power assisted steering The system diagram is shown above The layout of the system makes it particularly suited to mid-engine front wheel drive vehicles as all the components can be grouped in the front of the vehicle -35Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue The combination meter sends a vehicle speed signal to the power steering’s electronic control unit A warning light is also fitted to the combination meter to alert the driver if there is a malfunction within the system Pump Pump Power Power rack rack EHPS EHPS relay relay EHPS EHPS ECU ECU The vane pump assembly incorporates the vane pump, reservoir, pump motor and the power steering electronic control unit The vane pump and reservoir supplies the steering gear box with the appropriate amount of fluid, depending on vehicle speed and steering conditions The pump motor drives the vane pump, and also transmits the steering conditions back to the electronic control unit, as the amperage needed by the motor changes A power steering electronic control unit then controls the speed of the electric motor based on signals received from the speed sensor and the pump motor Idle up functions are also controlled via the power steering electronic control unit When there is an increase in electrical load, rpm is increased to reduce poor engine performance If there is an actuator malfunction the power steering electronic control unit is able to switch off the relay and stop motor operation It is also able to output diagnostic codes to help find any malfunctions in the system The engine’s electronic control unit sends a signal to the power steering electronics control unit when the vehicle is started, if the engine is cold or if the engine speed is low to enable it to make the appropriate adjustments Finally the power steering relay supplies power to the pump motor -36Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Vane Pump and Motor Construction As described earlier the vane pump assembly consists of the reservoir, vane pump, pump motor and power steering electronic control unit Within the assembly a relief valve is fitted which relieves pressure to the inlet side when discharge pressure is too high, thus preventing abnormal pressure build up Pump motor control Signals from the pump motor and the vehicle speed sensor are sent to the power steering electronic control unit The electronic control unit judges the condition of the vehicle and adjusts the fluid flow accordingly The fluid flow is adjusted by either speeding up or slowing down the motor The pump motor control consists of three maps: normal, non-steering and km/h vehicle speed As the vehicle speed increases the pump motor speed decreases as a reduction in voltage is applied -37Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Normal condition The fluid flow of the pump is controlled based on the assist map, vehicle speed and steering conditions are taken into account Non-steering state If the steering is not operated for a long period of time, the control of the fluid flow transfers to a standby map to save energy Non-steering state If the vehicle is not steered for a long period of time when the vehicle is stationary the fluid flow is reduced even further -38Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Electro-hydraulic power steering during acceleration • wheel speed increases • wheel speed sensors inform ABS ECU • ABS ECU informs EHPS ECU • HPS ECU reduces motor pump speed • reduction in pressure/assistance level During deceleration • wheel speed decreases • wheel speed sensors inform ABS ECU • EHPS ECU increases pump motor speed ã increase in pressure/assistance level -39- Copyright â Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Standby • steering state not altered for extended period • current feedback to EHPS ECU from pump motor remains low and constant • pump speed reduced to save money Prolonged non operation of steering • wheel speed mph • wheel speed sensor informs ABS ECU • ABS ECU informs EHPS ECU • EHPS ECU reduces pump speed further ã further reduction in pressure -40- Copyright â Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Progress check Answer the following questions: Why are progressive power steering systems used? If a duty ratio of 50% is sent to a component of a 12 volt system, what voltage will the component receive? How is the power steering pump driven on an EHPS power steering system? Is the pump motor speed increased or decreased during acceleration on the EHPS power steering system? -41Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue ... Repair MODULE LV29 STEERING SYSTEMS (2) Contents Page Introduction Overview of Steering Systems: Maintenance requirements for nonpower assisted steering systems Steering wheel installation Steering. .. Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue -2Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Introduction During this... Rights Reserved LV29: Steering Systems (2) Issue Pump speed over 2500 rpm Relief valve -14Copyright © Automotive Skills Limited 2003 All Rights Reserved LV29: Steering Systems (2) Issue Maximum