There was a time when diagnosing hydraulic power steering issues was easy: a time when the most challenging issue was diagnosing the so-called “morning sickness syndrome” that affected the operation of the power steering on some applications until they were fully warmed up. Today though, speed-sensitive hydraulic power steering is a bit more complicated and diagnosing issues have become correspondingly more difficult. In this article, we will briefly discuss the operating principles of modern hydraulic power steering systems, as well as some of the failure modes that occur most frequently, starting with this question-
In the main, only two types of hydraulic power steering are used on passenger and other light vehicles. The first type consists of a hydraulic actuator that is incorporated into the steering box, and is connected to the recirculating ball nut that is in turn, connected to the steering shaft. In these designs, the output shaft is known as a “Pitman arm”, which is connected to a draglink that connects the front wheels both to each other, and to the steering mechanism through one or more idler arms.
The other type uses a modified rack-and-pinion mechanism that encloses a double-acting hydraulic actuator that acts on the front wheels through conventional tie rods. This is the type of hydraulic power steering that most of us are most familiar with, and which is fitted to almost all passenger vehicles.
As a practical matter though, the required pressure to operate the system derives from engine-driven, positive displacement pumps that deliver pressurised fluid to the actuator through a control valve. On older systems, the control valve regulated the flow of pressurised fluid based on rotation of the steering shaft, but through a reduction to produce more assistance at slower vehicle speeds. However, the biggest disadvantage of this system was that since vehicle speed was not one of the primary control parameters to calculate the required level of steering assistance, the steering became extremely sensitive at high speeds.
NOTE: In recent years, it has become more common for manufacturers to drive the power steering pump with an electric motor, since an electric motor does not add to the parasitic drag factors that reduce fuel efficiency.
To reduce steering sensitivity at higher road speeds, car manufacturers started introducing a variable pressure/flow control mechanism in the 1980’s. Known as an EVO (Electronically Variable Orifice), this device is mounted at the power steering pump’s outlet, and it consists of a simple solenoid that can control the flow of the pressurised fluid with a moveable pintle that varies the effective diameter of an orifice.
In practice, the solenoid is controlled by a dedicated microcontroller that uses the vehicle’s speed as the primary input data; at low-speed manoeuvres such as during parking, when a lot of steering assistance is required, the controller employs a pulse width modulated current to retract the pintle to allow more pressure to reach the hydraulic actuator. Conversely, when little or no steering assistance is required at higher speeds (typically, highway speeds), the controller extends the pintle to close off the orifice, which has the practical effect of removing all steering assistance in order to allow full steering feedback to the driver.
Under these conditions, the pressurised fluid merely circulates between the pump and the fluid reservoir through a pressure relief valve, regardless of whether a rack-and-pinion or recirculating ball mechanism is used. It should be noted however, that the process of increasing or decreasing steering assistance is linear, in the sense that the amount whereby steering assistance is increased or decreased is directly related to the vehicle’s road speed.
However, modern hydraulic power steering systems also use an additional control mechanism, which is commonly known as the-
Located behind the steering wheel, the HWSS measures the rate of rotation (as opposed to the angle of rotation) of the steering wheel, and based on the rate of rotation, it generates a variable analogue signal that is used by the steering controller to control the solenoid that ultimately controls the level of steering assistance.
In practice however, the signal from the HWSS is not constant, since the HWSS consists of four separate potentiometers arranged in two pairs, with one pair producing a low voltage, and one pair that produces a higher voltage, relative to the position of the steering wheel. The two pairs of potentiometers are arranged so that if the steering wheel is turned from the dead-ahead position, the low voltage at “dead ahead” will change to a progressively higher voltage until the steering wheel reaches a rotation at 90 degrees. Between 90 degrees of rotation and 180 degrees of rotation in the same direction, the HWSS will generate a progressively lower voltage.
For instance, from the “dead-ahead” position the signal voltage generated by the HWSS will change from 0.2 volts to 4.8 volts as the steering wheel is turned to the 90-degree position. From 90 degrees to 180 degrees, the voltage will drop from 4.8 volts, back to 0.2 volts in a pattern that repeats itself through every 360-degree rotation of the steering wheel.
While this pattern of ever-changing voltages might be confusing at first glance, there is a very good reason for it to be the way it is, and here is why-
Since vehicle speed as determined by the ECU is only one parameter used to calculate an appropriate steering assist strategy, the vehicle’s speed is generally not available during parking manoeuvres or other low-speed changes in direction. Therefore, there has to be another way to calculate how much steering assistance to provide under these conditions, and linking a variable signal from the HWSS to the EVO (Electronically Variable Orifice) solenoid provides a way to do just that. Thus, at road speeds that cannot be measured, or be measured accurately, the changing position of the steering wheel is an effective control mechanism, since it is only at low speeds that large steering inputs are usually made, and it is at these times that maximum steering assistance is usually required.
Therefore, and as a practical matter, the position of the steering wheel largely replaces vehicle speed as the operative input when the steering controller calculates steering assistance strategies at very low speeds. Therefore, but provided that the power steering system is fully functional, steering assistance will increase as the steering wheel is turned during a parking manoeuvre, and decrease as the steering wheel returns to the “dead-ahead” position, in which position steering assistance is not required anyway. The practical advantage of controlling the level of steering assistance with a variable signal voltage is that since the steering wheel is usually rotated through large angles during a parking manoeuvre, maximum steering assistance is available when large steering inputs are made in a short time.
From the above, it should be obvious that for the most part, hydraulic power steering systems are relatively straightforward, and diagnosing the most common issues should be well within the capabilities of any trained technician. Below are some details of the most common failure modes-
Noisy pumps
The most common cause of a noisy pump is a low fluid level, although dirty, contaminated, degraded, or unsuitable fluid comes in at a close second place. In the first case the solution is obvious – top off the fluid level - , but in many cases where degraded or unsuitable fluid is the cause, the pump may have suffered irreparable damage by the time it became noisy.
If mechanical damage to the pump is suspected, the entire steering system needs to be purged of the degraded or unsuitable oil before refilling the system with the correct fluid. Note though that on applications that require proper hydraulic fluid, such as many VAG-group applications, even trace amounts of any other type of fluid can cause all or most of the oil seals in the system to fail.
Fluid leaks
Replacement of the fluid in the power steering system is probably one of the most overlooked (or ignored) aspects of routine servicing and since power steering systems do not have the benefit of proper filtration, accumulated dirt and solid particulate matter can cause excessive mechanical wear of moving parts. One common site of excessive wear is on the rack where it passes through the seals in the casing.
Note that if the rubber boots on the ends of the casing are not damaged or perforated, all of the fluid in the reservoir can leak into the boots. Therefore, if the power steering system mysteriously “loses” fluid and there are no obvious signs of a leak, remove the boots to check for a leak between the seals and the rack.
Other common sites for fluid leaks include fluid escaping through the high-pressure hose(s) due to degradation of the hose(s) caused by long use, and almost any threaded connection or crimped fitting. Make repairs or replace hoses as required.
EVO failure
While failure of the solenoid is not common, it can fail in one of two conditions. If the EVO fails in the closed position, there will be no steering assistance at any road speed, since fluid is prevented from reaching the hydraulic actuator. If however, the solenoid fails in the open position, maximum steering assistance will be available at all road speeds, which means that the steering will be extremely sensitive at high speeds.
HWSS failure
As a rule of thumb, a failure of the HWSS will only affect the power steering system at low speeds, and it will generally result in no steering assistance being available at low speeds. However, it must be borne in mind that since the HWSS communicates with both the ECU and the steering control module via the CAN bus system, the failure in communication may have unpredictable results that depend on both the application, and the exact nature of the failure.
However, the good news is that a failure of the HWSS will almost always set one or more generic fault codes, with the most common being-
• C0472 – Steering Handwheel Speed Sensor Signal V Low
• C0473 – Steering Handwheel Speed Sensor Signal V High
• C0495 – EVO Tracking Error
• C0498 – Steering Assist Control Actuator Feed Circuit Low
• C0499 – Steering Assist Control Solenoid Feed Circuit High
• C0503 – Steering Assist Control Solenoid Return Circuit Low
• C0504 – Steering Assist Control Solenoid Return Circuit High
Note though that depending on the application, one or more manufacturer specific codes may also be present, and particularly codes that involve the CAN system. In these cases, it is important to use a high-end scan tool that can access the serial communications system in order to isolate the fault by following the manufacturer’s prescribed procedures. Nonetheless, by resolving codes in the order in which they were set and stored, it is often possible to resolve the root cause of the issue without having to delve into the CAN bus system.
