Commercial vehicles use different types of power steering to reduce the force required at the steering wheel. Hydraulic steering has traditionally been common in trucks and buses, while electrohydraulic systems use an electrically driven pump to provide hydraulic assistance.
Both systems ultimately use hydraulic pressure to assist steering, but they differ in how this pressure is generated and controlled.
For workshops and distributors, understanding this distinction is important because similar symptoms can have different causes depending on the steering architecture.
How does hydraulic power steering work?
In a conventional hydraulic steering system, a mechanically driven pump generates hydraulic flow while the engine is running.
Hydraulic fluid is supplied to the steering gear, where pressure assists the driver’s steering input. The amount of assistance and the exact system design vary between vehicles, but the pump, fluid reservoir, lines and steering gear form the basic hydraulic circuit.
Because the pump is mechanically driven by the engine, it generally operates whenever the engine is running, not only when steering assistance is required.
This relatively direct architecture has been used extensively in commercial vehicles and remains common in many applications.
How does electrohydraulic steering work?
Electrohydraulic power steering retains hydraulic assistance but changes the way hydraulic pressure is generated.
Instead of using a pump driven mechanically by the engine, an electric motor drives the hydraulic pump. The pump can therefore operate independently of engine speed and can be controlled according to steering demand and system requirements.
The steering gear itself can still operate hydraulically, so electrohydraulic steering should not be confused with fully electric power steering, where assistance is generated directly by an electric motor without a conventional hydraulic circuit.
This distinction is particularly important when discussing newer commercial vehicle steering systems.
What are the main differences?
The fundamental difference lies in the pump drive.
A conventional hydraulic system takes mechanical power from the engine. An electrohydraulic system uses electrical energy to drive the hydraulic pump, allowing pump operation to be controlled more independently.
This can reduce the continuous parasitic load associated with a mechanically driven pump and allows steering assistance to remain less dependent on engine speed.
However, the additional electrical components also introduce another possible source of faults. A conventional hydraulic steering problem is primarily investigated through the hydraulic and mechanical system, while an electrohydraulic fault may also involve the electrical supply, pump motor or electronic control.
Common hydraulic steering problems
Increased steering effort is one of the most noticeable symptoms of reduced hydraulic assistance. The cause can lie in the pump, but it can also result from low fluid level, leakage, restricted flow or problems within the steering gear.
Unusual pump noise can indicate insufficient fluid, aeration or other problems affecting hydraulic operation. Visible leakage around hoses, connections, seals or the steering gear provides another important diagnostic clue.
Fluid condition also matters. Contamination can affect hydraulic components and should be investigated rather than simply corrected by repeatedly topping up the reservoir.
A steering complaint should therefore be assessed as a system problem before the pump itself is assumed to be defective.
Common electrohydraulic steering problems
Electrohydraulic systems can produce many of the same hydraulic symptoms, but their electrical side must also be considered.
Intermittent assistance, complete loss of assistance or warning messages can be associated with the electric pump or its control, but voltage supply, wiring and electrical connections can produce similar behaviour.
A pump that does not operate is therefore not automatically mechanically defective. Before replacement, the conditions required for pump operation should be considered according to the particular system.
Likewise, a functioning electric motor does not rule out a hydraulic problem. The pump may operate while the system still fails to generate the expected hydraulic assistance.
Why does steering effort sometimes change at idle?
Changes in steering assistance at low engine speed can provide useful diagnostic information, particularly with conventional hydraulic systems.
Because a mechanically driven pump follows engine operation, problems with pump output can become more noticeable during low-speed manoeuvring or when the engine is idling. Steering may feel heavier when parking even though assistance appears more acceptable at higher engine speed.
With an electrohydraulic system, assistance is less directly linked to engine speed. Similar symptoms may therefore point toward a different combination of hydraulic and electrical causes.
The symptom alone is not sufficient for diagnosis, but understanding the system architecture helps determine where to investigate.
Steering pump or steering gear?
Heavy or irregular steering does not automatically mean that the steering pump is responsible.
The steering gear itself can develop internal leakage, wear or mechanical problems that affect assistance and steering behaviour. Hydraulic lines, fluid condition and other mechanical steering components can also influence the symptoms perceived by the driver.
Diagnosis should therefore distinguish between insufficient hydraulic supply and a problem within the steering mechanism receiving that supply.
This is particularly important when a pump has already been replaced without resolving the original steering complaint.
Replacing steering components
Steering pumps and electrohydraulic pump units can differ in pressure and flow characteristics, connections, mounting arrangements and drive configuration.
Electrohydraulic units also introduce electrical requirements that are absent from mechanically driven pumps. Similar dimensions or connections therefore do not necessarily make two units interchangeable.
The correct replacement must correspond to the steering system in which it operates rather than simply fitting the available mounting position.
Conclusion
Hydraulic and electrohydraulic steering systems both use hydraulic pressure to reduce steering effort, but they generate that pressure differently.
Conventional hydraulic steering uses an engine-driven pump, while electrohydraulic steering uses an electrically driven hydraulic pump. This changes both system behaviour and the diagnostic approach required when assistance is reduced or lost.
For workshops, identifying the steering architecture first helps separate hydraulic, mechanical and electrical causes and reduces unnecessary replacement of steering components.
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