What Part Controls Power Steering?

In hydraulic systems, the control of power steering assist is handled by the rotary control valve inside the steering gear (rack or box), while in electric systems it’s governed by the Power Steering Control Module (PSCM) using inputs from a torque sensor and other vehicle signals; the pump (hydraulic or electro-hydraulic) supplies pressure but does not make the control decisions. This article explains how each system decides when and how much assist to deliver, the parts involved, and how to diagnose issues.

How Power Steering Works

Modern vehicles use one of three architectures: hydraulic power steering (HPS), electro-hydraulic power steering (EHPS), or fully electric power steering (EPS). All aim to reduce steering effort, but the “brains” that control assist differ by design.

Hydraulic Power Steering (HPS)

Traditional HPS uses an engine-driven pump to create hydraulic pressure. A torsion bar and a rotary (spool) control valve built into the steering gear sense driver input: as you turn the wheel, the torsion bar twists and the valve meters fluid to one side of the rack piston (or steering box), generating assist in proportion to effort. Some systems add a speed-sensitive solenoid or variable orifice to trim assist at higher speeds.

Electro-Hydraulic Power Steering (EHPS)

EHPS replaces the belt-driven pump with an electric pump controlled by an electronic module. The steering gear still uses a hydraulic control valve to direct fluid, but pump output is modulated by the module based on vehicle speed, steering input, and temperature for efficiency and variable assist.

Electric Power Steering (EPS)

EPS eliminates hydraulic fluid. An electric motor on the column or rack provides assist, commanded by the Power Steering Control Module (often integrated with or networked to the vehicle’s main control systems). The PSCM uses a torque sensor, steering angle sensor, vehicle speed, yaw rate, and other signals to calculate assist in real time, often integrating with stability control and lane-keeping features.

The Controlling Parts by System

The specific component that “controls” assist depends on the steering architecture. Below are the key control elements you’re most likely to encounter by system type.

• Hydraulic (HPS): Rotary/spool control valve and torsion bar inside the steering rack or steering box (primary controller of assist flow and direction)
• Hydraulic (HPS) with variable assist: Electronic variable orifice (EVO) or flow-control solenoid commanded by a control module to reduce assist at speed
• Electro-Hydraulic (EHPS): EHPS control module regulating the electric pump output; steering gear’s control valve still meters flow to the rack piston
• Electric (EPS): Power Steering Control Module (PSCM) commanding a column- or rack-mounted assist motor using inputs from torque and angle sensors and vehicle dynamics

While pumps, hoses, and motors provide the “muscle,” the steering gear’s control valve (hydraulic) or the PSCM with sensors (electric) are the parts that actually decide when and how much assist to deliver.

Symptoms When the Controlling Components Fail

Issues with the control valve, sensors, or control module often feel different from simple fluid loss or belt problems. Watch for the following signs that point specifically to control-related faults.

• Assist is uneven or pulls to one side during turns (sticky or worn rotary control valve)
• Assist is fine when parking but abruptly too light or too heavy at highway speeds (faulty EVO/solenoid or PSCM calibration)
• Intermittent assist with warning lights such as EPS or steering wheel icon (PSCM or torque sensor fault; DTCs stored)
• Notchy or self-steering feel near center (torsion bar or valve wear; column torque sensor drift)
• Assist changes with temperature or electrical load (EHPS pump control or voltage supply issues)

Consistent, speed-sensitive changes typically implicate electronic control, while asymmetrical or mechanical-feeling issues often indicate a hydraulic valve or torsion bar problem.

How to Diagnose Which Part Is at Fault

A structured approach can separate supply issues (pump/motor) from control issues (valve/module/sensors).

1. Identify the system: Confirm HPS, EHPS, or EPS via service info or a quick visual (fluid reservoir and pump vs. electric motor on column/rack).
2. Check basics: Fluid level and condition (HPS/EHPS), belt condition/tension, battery and charging system voltage (EPS/EHPS need stable 13.5–14.5 V).
3. Scan for codes: Use a scan tool to read PSCM or related modules; look for torque sensor, steering angle, vehicle speed, CAN communication, or pump control faults.
4. Road test: Note when symptoms occur—parking speeds vs. highway; left vs. right turns; temperature dependence.
5. Steering angle/torque calibration: Perform zero-point and angle calibrations after battery work, alignment, or column service.
6. Hydraulic checks: For HPS/EHPS, measure pump pressure and inspect for internal leakage; if pressure is normal but assist is erratic, suspect the control valve in the rack/box.
7. Electrical checks: For EPS/EHPS, verify power/ground, inspect connectors, and load-test circuits to the motor and module.

These steps help isolate whether the issue is mechanical/hydraulic (valve or torsion bar), electrical/electronic (PSCM, sensors), or simply a supply problem like low fluid or weak voltage.

Common Misconceptions

Because multiple parts interact, it’s easy to misattribute the cause of steering problems. Keep these clarifications in mind.

• The pump doesn’t “control” assist in HPS—it supplies pressure; the control valve meters it.
• Replacing fluid won’t fix a worn torsion bar or sticky control valve, though it can reduce noise and extend life.
• In EPS, there is no fluid or pump; the PSCM and torque sensor govern assist via the electric motor.
• Wheel alignment issues can mimic steering pull but won’t trigger EPS warnings or change assist effort with speed.
• A battery or alternator problem can cause intermittent EPS/EHPS assist loss without any mechanical fault in the steering gear.

Understanding which parts command assist helps avoid unnecessary replacements and speeds up accurate repairs.

Typical Repair Options and Costs (2024–2025)

Costs vary by vehicle and access, but these ballpark figures can guide expectations for control-related fixes.

• HPS steering rack/box (worn control valve/torsion bar): $500–$1,500 parts, $250–$700 labor
• EPS torque sensor or column assembly: $150–$400 sensor; $700–$2,000+ for full column, $200–$600 labor
• PSCM/module programming or replacement: $200–$800 parts, $150–$300 labor; programming often required
• EHPS electric pump assembly: $400–$1,200 parts, $200–$500 labor
• EVO/assist solenoid (where equipped): $100–$300 parts, $100–$250 labor
• Fluid service (HPS/EHPS): $80–$150; may help noise but won’t fix a failing control valve

Always confirm with a scan and system-specific tests; many electronic components require calibration or programming after installation.

Maintenance and Prevention

Good maintenance can prolong the life of both hydraulic and electronic control components.

• Maintain clean power and grounds; weak batteries and poor grounds stress EPS/EHPS modules and motors.
• Use the correct fluid and change it on schedule for HPS/EHPS to minimize valve varnish and pump wear.
• Perform steering angle and torque sensor calibrations after alignments, suspension work, or battery disconnects.
• Address leaks early; running an HPS/EHPS pump dry can damage both supply and control components.
• Keep tires properly inflated and aligned to reduce assist load and heat.

These habits help the system’s controlling parts operate within design limits and avoid premature failure.

FAQ

This section addresses quick clarifications that often come up when identifying the control component in power steering.

• Which single part “controls” power steering? In HPS, the steering gear’s rotary control valve; in EPS, the PSCM with the torque sensor.
• Is the pump the controller? No—hydraulic pumps supply pressure; control valves and/or modules decide assist.
• Can a software update change steering feel? Yes—EPS/EHPS updates can alter assist curves and fix glitches.
• Why does assist change with speed? Variable-assist systems reduce help at higher speeds via an EVO (HPS) or PSCM logic (EPS/EHPS).

Matching the symptom to the system type is the fastest way to pinpoint the true controlling component at fault.

Summary

The part that controls power steering depends on the system: a rotary control valve inside the steering gear governs assist in hydraulic setups, while a Power Steering Control Module—using torque and angle sensors—commands the electric motor in EPS. Pumps and motors provide force, but valves and modules make the decisions. Accurate diagnosis hinges on identifying the system, scanning for codes, checking fundamentals, and testing the control elements before replacing major components.

What controls my power steering?

Hydraulic Power Steering System
The power steering pump is turned by a serpentine belt, or accessory drive, providing pressurized power steering fluid to the power steering hose and ultimately delivering it to the power steering control valve at the steering gear.

Is EPS expensive to repair?

Just so you know, that $3000 estimate is excessively high for this problem. Even if the motor and the entire EPS assembly are faulty, the parts shouldn’t cost more than $800, and labor would take around $800-900!

What part is the power steering connected to?

Hydraulic power steering systems may use either a steering rack and pinion assembly or steering box, depending on the vehicle design. From there, the steering linkage connects to the steering knuckles, which, in turn, attach to the wheels and tires.

What is the most common failure in a power steering system?

The most common causes of power steering failure are a low fluid level due to a leak and a worn-out or slipping drive belt that powers the pump. Other causes include a failing power steering pump, a damaged or failing power steering rack, or issues with the system’s hoses, couplings, or, in the case of electric power steering, a faulty control module or motor.
 
For Hydraulic Power Steering (Fluid-Based)

• Fluid Leaks: This is the most frequent cause of power steering issues, leading to low fluid levels and a loss of hydraulic pressure. Leaks can originate from cracked hoses, loose couplings, or a failing power steering rack. 
• Worn-Out Drive Belt: A slipping or damaged belt that drives the power steering pump will prevent the pump from operating, resulting in heavy steering. 
• Failing Power Steering Pump: The pump, which pressurizes the fluid, can wear out over time, leading to reduced pressure, whining noises, and eventual failure. 
• Worn Hoses: Over time, hoses can become cracked, brittle, or lose their seals, leading to fluid leaks. 
• Faulty Power Steering Rack: The steering rack can develop wear and tear, including faulty seals, that can cause fluid leaks and power steering failure. 

For Electric Power Steering (EPS)

• Faulty Control Module: Modern vehicles often use electric power steering, and the onboard computer module is a common point of failure. 
• Failing Electric Motor: The electric motors that assist steering can also fail, leading to loss of power assist. 

How to Troubleshoot

1. Check Fluid Level: Inspect the power steering fluid reservoir and ensure the fluid is at the proper level. 
2. Look for Leaks: Check for any visible fluid leaks under the car, especially around the hoses and steering rack. 
3. Inspect the Belt: Examine the power steering belt for cracks, fraying, or signs of slipping. 
4. Listen for Noises: Pay attention to any whining or grinding noises when turning the steering wheel, which can signal a pump problem. 

If you notice any signs of power steering failure, have the system inspected by a professional to prevent more significant damage and ensure safe driving conditions.