What Controls Power Steering Pump Pressure and Volume

In hydraulic power steering, flow (volume per unit time) is governed primarily by pump speed and displacement and moderated by an internal flow-control valve, while pressure is created by steering load at the gear’s rotary control valve and limited by a relief valve; in electro-hydraulic systems an ECU modulates pump motor speed to control flow, with a relief valve still capping maximum pressure. Here’s how the system decides how much assist you feel at the wheel.

How Flow (Volume) Is Controlled in Hydraulic Systems

Most belt-driven pumps are fixed-displacement vane pumps. They move a set amount of fluid each revolution, so raw flow rises with engine speed. To keep assist consistent and prevent overboost at high rpm, the pump’s flow-control (bypass) valve diverts excess fluid back to the inlet or reservoir once a target flow is reached. The result is near-constant assist over a wide speed range, even though the pump spins faster.

Key hardware that governs flow

These components and conditions determine how much fluid the pump actually delivers to the steering gear at any moment.

• Pump displacement and type: Fixed-displacement vane pumps move a fixed volume per revolution; some systems use variable-displacement designs that reduce delivery at high rpm to save energy.
• Pump speed (engine rpm): Higher rpm increases potential flow; the flow-control valve curbs the surplus above its setpoint.
• Flow-control/priority valve: A spool-and-orifice assembly that maintains a target flow by bypassing excess back to the inlet/reservoir.
• Internal leakage and clearances: Wear increases internal leakage, reducing delivered flow, especially when hot.
• Fluid viscosity and temperature: Thin, overheated fluid leaks more internally and can reduce effective flow; cold, thick fluid can restrict inlet flow and cause noise or cavitation.

Together, these factors determine whether the system can deliver sufficient flow for smooth, quick assist at idle and steady, not overboosted assist at higher engine speeds.

How Pressure Is Controlled

Pressure in a hydraulic steering system is demand-driven: it rises only when the steering gear imposes resistance to flow. The rotary control valve in the steering rack or box meters fluid to either side of a power piston. When you turn the wheel and load the tires, the valve creates a restriction, pressure builds to move the piston, and you feel assist. When you’re straight ahead, the valve is nearly neutral, restriction is low, and pressure is minimal. A pressure relief valve inside the pump sets the absolute ceiling (often roughly 800–1,500 psi depending on vehicle). If steering resistance exceeds what the system can supply, the relief opens to protect components.

Key hardware that governs pressure

The following elements set, limit, or influence hydraulic pressure during steering.

• Rotary control valve (in the steering gear): Meters flow based on torsion-bar twist; more twist equals more restriction and higher pressure to the assist piston.
• Steering load at the rack/piston: Tire scrub, vehicle weight, road surface, and maneuver (e.g., parking vs. cruising) dictate how much pressure is needed for movement.
• Pressure relief valve (in the pump): A spring-loaded valve that opens at a calibrated pressure to prevent overpressure; it does not create pressure, it only limits the maximum.
• Speed- or assist-control features: Some systems add an electronic/solenoid valve or variable orifice to reduce assist (i.e., limit pressure rise) at higher vehicle speeds.
• System condition: Aeration, contaminated or incorrect fluid, kinked/soft hoses, and collapsed reservoir strainers can cause erratic pressure or noise.

In normal use, actual working pressure constantly varies with steering demand, while the relief valve intervenes only at extremes (for example, when holding the wheel against the stop at full lock—something best avoided).

Variable-Displacement and Speed-Sensitive Designs

To cut parasitic losses and stabilize feel, many modern hydraulic pumps are variable-displacement: a movable cam ring reduces eccentricity as discharge pressure rises, trimming flow at higher rpm. Others keep a fixed displacement but tune the flow-control valve for a lower setpoint at speed. Some systems add an electronically controlled valve that bleeds or shapes control pressures to modulate assist versus vehicle speed, improving road feel.

Electro-Hydraulic Power Steering (EHPS)

EHPS replaces the belt drive with an electric motor powering a conventional pump. An ECU varies motor speed based on inputs such as vehicle speed, steering angle/torque, engine state, and temperature. That motor-speed command controls flow, while a mechanical relief valve still caps maximum pressure. EHPS blends the familiar hydraulic feel with efficiency gains and is common on many 2000s–2010s vehicles. Note that fully electric power steering (EPS) eliminates hydraulic pumps and fluid entirely, so pressure/flow control does not apply there.

What It Means for Symptoms and Diagnosis

When pressure or flow is off, steering feel quickly gives it away. These common symptoms help pinpoint whether the issue is flow-, pressure-, or control-related.

• Heavy steering especially at idle: Often insufficient flow (worn pump, slipping belt, clogged inlet screen, thick/cold fluid, or failing flow-control valve).
• Assist fades when hot: Increased internal leakage from wear or thin fluid reduces both pressure under load and available flow.
• Whine, shudder, or foaming fluid: Aeration from low fluid, inlet air leaks, or cavitation starves the pump and destabilizes pressure.
• Intermittent hard spots: Contamination or a sticking rotary control valve alters restriction unpredictably.
• Loud groan at full lock: Relief valve held open—avoid holding the wheel against the stop to prevent overheating and pump wear.

Addressing these issues usually involves correcting fluid level and type, bleeding air, inspecting belts and hoses, cleaning or replacing the reservoir screen, and testing pump output pressure/flow to confirm pump wear.

Maintenance and Best Practices

Preventive steps help the pump maintain proper pressure and flow across temperatures and operating conditions.

• Use the manufacturer-specified fluid; the wrong viscosity can impair both flow and pressure stability.
• Bleed the system after any service that opens hydraulic lines to remove air and prevent cavitation.
• Check belt condition and tension (for belt-driven pumps) to ensure the pump reaches commanded speed.
• Inspect hoses and the reservoir strainer; restrictions at the inlet cause starvation and noise.
• Avoid holding the steering at full lock; it forces the relief valve to dump at maximum pressure, heating and stressing the system.

Following these practices preserves the internal clearances and valve behavior that underpin accurate flow and pressure control.

Summary

Flow (volume) from a hydraulic power steering pump is primarily a function of pump displacement and speed, moderated by an internal flow-control valve and influenced by fluid condition and leakage. Working pressure is created by the steering gear’s rotary valve in response to driver demand and road load, and it is capped by a pump-mounted relief valve. Variable-displacement and speed-sensitive systems refine this behavior, while EHPS uses an ECU to vary pump motor speed for precise flow with a mechanical relief still limiting pressure. In EPS, there is no hydraulic pressure at all.

What sensor is connected to the power steering pump?

The PSP sensor like the pressure switch (PSS) also monitors the power steering load. The sensor transforms the varying power steering load into a varying output voltage that is utilised by the PCM to assist in activating the throttle control system for a more stable RPM/load result.

How do I know if my power steering pump or hydroboost is bad?

To determine if your power steering pump or hydroboost is bad, look for external fluid leaks, especially from the hydroboost unit or its hoses, and listen for a whining, groaning, or squealing noise from the pump when turning the wheel or starting the engine. You should also perform a basic brake assist test by holding the pedal down while starting the engine; the pedal falling slightly indicates a functioning power section, while it remaining stiff suggests an issue. If you have steering but not power steering, a leaking hydroboost is possible, but if the steering works after bypassing the hydroboost, then the unit itself is faulty.
 
Symptoms of a Bad Power Steering Pump

• Noises: A whining or groaning sound, especially when turning the steering wheel or at idle, indicates the pump is struggling. 
• Stiff Steering: Difficulty turning the steering wheel, particularly when the vehicle is stopped or moving slowly. 
• Fluid Leaks: Look for signs of power steering fluid leaking from the pump or its hoses. 
• Squealing on Startup: A squealing noise when the engine first starts can also point to a problem with the power steering pump. 

Symptoms of a Bad Hydroboost Unit

• Fluid Leaks: Opens in new tabLook for power steering fluid leaking from the hydroboost unit itself, particularly from around its mounting surface on the firewall. 
• Brake Pedal Feedback: Opens in new tabThe brake pedal may feel stiff, or you might experience pedal feedback, like a slight jiggle, as you initially apply the brakes. 
• Sinking Pedal: Opens in new tabA sinking or long brake pedal when you first press it can be a sign of a failing hydroboost. 
• No Power Steering: Opens in new tabIf your power steering works but the brakes don’t have the proper assist, the hydroboost unit could be the issue. 

Diagnostic Steps

1. Check Fluid and Belt: Ensure the power steering fluid is at the correct level and the serpentine belt is in good condition and properly tensioned. 
2. Listen for Noises: Start the engine and turn the steering wheel lock-to-lock to listen for any whining or groaning sounds from the power steering pump. 
3. Check for Leaks: Inspect the hydroboost unit, power steering pump, and all hose connections for any signs of fluid leaks. 
4. Perform the Brake Assist Test: With the engine off, pump the brake pedal several times to discharge the accumulator. Then, hold the pedal down with moderate pressure and start the engine. The pedal should fall slightly if the hydroboost is working correctly. 
5. Bypass the Hydroboost (If Necessary): If the steering is still difficult after confirming the power steering pump is okay, you can temporarily disconnect and join the input and output lines to the hydroboost unit. If the steering then works correctly, the hydroboost unit is faulty. 

What controls fluid pressure in a power steering pump?

Now when your pump spins up at higher engine RPM. It’s going to create more flow. And you don’t necessarily need all that pressure at higher RPM.

Which component limits the power steering pump pressure?

flow control valve spool
The pump has a pilot operated relief valve built into the flow control valve spool which will limit the maximum pressure the pump can produce.