Electrically powered hydraulic steering (EPHS) is an intermediate technology on the path to full electrically powered steering (EPS). The system is used in many high-volume models, with several million vehicles on the UK roads equipped with this technology. In this article, ZF Aftermarket outlines a structured approach to diagnosing and curing the problem of unwanted noise in the system.
EPHS comprises an electric motor, a gear pump, a fluid reservoir and a control unit within a single housing. Supplied by the vehicle electrical system, it runs completely independently of the engine. The pump runs continuously at low speed, ready to provide immediate steering assistance, while the control unit uses steering wheel angle and speed inputs plus vehicle speed to determine the correct rate and magnitude of EPHS response. As EPHS does not require a belt-driven steering pump like a conventional hydraulic system, fuel savings are possible due to reduced vehicle weight and up to 75% lower energy usage. Reduced maintenance is a further advantage due to the absence of the drive belt.
NOISE EMISSION AND DIAGNOSIS
If loud noises occur during steering manoeuvres, it’s important to adopt a structured approach to fault finding. While the temptation may be to simply replace the motor and pump assembly, to do this prematurely can be cost prohibitive, without any guarantee of success.
The diagnosis should start with a detailed evaluation of the vehicle. In ZF Aftermarket’s experience, increased noise emission often occurs after inexpert repairs on the steering or suspension, so if possible, check with the vehicle owner whether any recent work has been carried out.
A certain level of noise is normal, especially at the limit of right and left steering lock when the pump note can be heard to change. However, if a high-pitched howl is emitted when the servo load changes, the problem could be in the close vicinity of the motor/pump unit.
VEHICLE ELECTRICAL SYSTEM
Check the power output of the vehicle electrical system. If the voltage is low, the power consumption (current draw) of the steering automatically increases, causing higher noise emission that can be heard in the passenger compartment.
FIGURE 1 Oil temperature, condition and fill level should be checked. Leaks in the system or insufficient bleeding after repairs tend to generate noise; if the oil foams in the reservoir when the system is running, this suggests that air is trapped in the system and more thorough bleeding is required.
FIGURE 2 Whenever the steering wheel is turned, the speed of the motor/pump unit increases to generate pressure to provide steering assistance. This causes high pressure pulsation in the hydraulic pipework connecting the motor/pump unit and the steering gearbox, which manifests itself as vibration. To absorb these pulsations and vibrations in the steering system, rubber- insulated connecting lines are fitted, with vibration-absorbing hose clamps used to attach the lines to the car body.
Any part of the connecting pipework contacting the car body, or a motor/pump unit installed in such a way that it creates tension in the pipework, is enough to increase noise emission. Ensure all pipework is undamaged, correctly routed and only suitably insulated clamps are used to attach it to the bodywork or chassis.
FIGURE 3 The motor/pump unit is installed in a bracket incorporating rubber buffers for insulation against vibration. Inexpert handling or incorrect installation here can also result in excessive noise emission. When removing or replacing the pump unit, any ancillary items such as rubber buffers and/ or insulated brackets should always be reinstalled. Similarly, when installing a new replacement unit, any noise insulating components supplied with it must also be fitted.
Following the advice above will help to ensure a successful repair of the electrically powered hydraulic steering system. For other repair and routine servicing operations on passenger cars, ZF Aftermarket also supplies a comprehensive range of replacement braking, chassis, steering and suspension parts from its Lemförder, Sachs, TRW and Boge brand portfolio.