1.3 Hydraulic Power Steering Systems
Key Takeaways
- The vane-type pump is a positive-displacement pump where fluid flow is regulated by a flow control valve and maximum pressure by a relief valve.
- The rotary control valve uses a twisting torsion bar to create a relative displacement between the inner spool and outer sleeve, directing pressure.
- A power steering analyzer evaluates pump flow, relief pressure, and gear seal integrity; the shut-off valve must never be closed for more than 5 seconds to prevent component damage.
- Pump whining or foaming in the reservoir is typically caused by air entering the suction line, leading to aeration and cavitation.
- Never mix mineral-based Central Hydraulic Fluid (CHF) with standard petroleum ATF, as it will destroy rubber seals and cause system failure.
Hydraulic Power Steering Systems
Hydraulic power steering systems use pressurized hydraulic fluid to assist the driver in turning the wheels. A belt-driven pump generates fluid flow and pressure, which is routed through a control valve to a power cylinder. Diagnosing these systems requires an understanding of pump hydraulics, control valve mechanics, and pressure testing.
Power Steering Pump Construction and Operation (Vane Type)
The vane-type pump is the most common design used in hydraulic power steering. It is a positive-displacement pump driven by the engine's accessory belt.
- Rotor and Vanes: The pump shaft rotates a slotted rotor. Rectangular metal vanes slide freely in and out of the rotor slots.
- Cam Ring: The rotor and vanes are housed inside an oval-shaped cam ring.
- Suction and Discharge: As the rotor turns, centrifugal force and fluid pressure force the vanes outward against the cam ring. Because of the oval shape, the space between the vanes increases and decreases twice per revolution.
- As the space increases, a vacuum is created, drawing fluid from the reservoir.
- As the space decreases, the fluid is compressed and forced out of the discharge port under pressure.
- Flow Control Valve: At high engine speeds, the pump produces more flow than needed. The flow control valve contains a spring-loaded spool that opens at a preset flow rate, routing excess fluid back to the pump inlet to prevent excessive steering assist and heat.
- Pressure Relief Valve: Located inside the flow control valve, this spring-loaded check ball opens if pressure exceeds maximum limits (typically 1,100–1,500 PSI or 76–103 bar), venting fluid back to the inlet to prevent hose or seal failure.
Rotary Spool Control Valve and Torsion Bar Operation
The control valve determines when and how much hydraulic assist is applied. It is located inside the steering gear input housing and operates based on the driver's steering input.
- Input Shaft and Pinion: The input shaft connects to the steering column, and the pinion gear (or worm shaft) connects to the steering linkage.
- Torsion Bar: The input shaft is connected to the pinion gear by a thin, spring-steel torsion bar. All steering torque passes through this torsion bar.
- Inner Spool and Outer Sleeve: The control valve consists of an inner spool valve (attached to the input shaft) and an outer valve sleeve (attached to the pinion gear).
- Operation:
- Neutral (Straight-Ahead): With no steering force, the torsion bar is straight. The inner spool and outer sleeve are aligned, directing pressurized fluid equally to both sides of the steering gear power cylinder and back to the reservoir. System pressure remains low.
- Turning: When the driver turns the wheel, road resistance prevents the wheels from turning immediately, causing the torsion bar to twist. This twist rotates the inner spool valve slightly inside the outer sleeve. The misaligned ports direct high-pressure fluid to one side of the power cylinder and vent fluid from the other side back to the reservoir. The pressure difference assists in moving the wheels. When steering torque stops, the torsion bar untwists, returning the valve to the neutral position.
Diagnostic Pressure and Flow Testing (Power Steering Analyzer)
A power steering analyzer (consisting of a pressure gauge, flow meter, and a shut-off valve) is used to locate faults in the hydraulic system.
- Analyzer Connections: Connect the analyzer in series with the high-pressure line between the pump discharge port and the steering gear inlet.
- Preparation: Fill the reservoir, open the analyzer valve fully, start the engine, and cycle the steering to warm the fluid to operating temperature (150°F–170°F / 65°C–77°C).
Power Steering Analyzer Test Procedures
| Analyzer Test Step | Valve Position | Engine Speed | Expected Reading (Normal) | Diagnostic Indication (Abnormal) |
|---|---|---|---|---|
| System Flow & Pressure | Fully Open | Idle | Pressure < 150 PSI, Flow 1.5–2.5 GPM | High pressure indicates restriction; low flow indicates worn pump or stuck flow valve |
| Pump Max Pressure | Closed (Max 5 sec) | Idle | Pressure 1,100–1,500 PSI, Flow drops to zero | Low pressure indicates worn pump rotor/vanes or weak pressure relief valve spring |
| Steering Gear Seals | Fully Open | Idle (Wheel held at lock) | Pressure spikes to pump max, Flow drops near zero | Low pressure and high flow (>0.5 GPM) indicates internal leakage past steering gear seals |
- Test Procedures and Interpretation:
- System Pressure and Flow Test (Open Valve): With the engine idling, pressure should be low (under 150 PSI) and flow should meet specification (typically 1.5–2.5 GPM / 5.7–9.5 LPM). Low flow indicates a worn pump or a stuck flow control valve.
- Pump Maximum Pressure (Deadhead Test): Slowly close the analyzer shut-off valve for no more than 5 seconds and read the pressure.
[!IMPORTANT] Closing the valve for more than 5 seconds can overheat the fluid and damage the pump and hoses.
- If the pressure rises to specification (typically 1,100–1,500 PSI) and flow drops to zero, the pump's pressure relief valve and internal components are functioning correctly.
- If pressure is below specification, the pump has internal wear or a leaking relief valve and must be replaced.
- Steering Gear Internal Seal Test: Open the analyzer valve. Turn the steering wheel to the left lock and hold it, then to the right lock and hold it.
- The pressure should rise to the pump's maximum relief pressure, and flow should drop to near zero.
- If pressure does not reach specification and flow remains high (above 0.5 GPM), fluid is leaking past the internal piston seals or control valve rings in the steering gear. The steering gear must be replaced or rebuilt.
Pump Noise Diagnosis and Fluid Service
- Pump Whining/Growling: A high-pitched whine that increases with engine RPM is commonly caused by aeration (air in the fluid). Air enters through a loose fitting or clamp on the pump’s suction/return line. Aerated fluid looks foamy or bubbly in the reservoir. Cavitation (fluid starvation) caused by a clogged reservoir screen or restricted inlet hose also causes a growling noise.
- Belt Squeal: A screeching noise when steering to the locks indicates a loose, worn, or oil-soaked drive belt, or a weak belt tensioner.
- Bleeding Procedures: Air must be purged after replacing components.
- Manual Bleeding: Raise the front wheels. With the engine off, turn the steering wheel lock-to-lock 20 to 30 times. Check the fluid level. Start the engine and repeat.
- Vacuum Bleeding: Apply 15–20 in-Hg of vacuum to the sealed reservoir using a hand vacuum pump. Run the engine at idle and turn the steering wheel lock-to-lock. The vacuum pulls air bubbles out of the fluid.
- Fluid Specifications:
- Dexron/ATF: Used in older domestic and Asian vehicles.
- Mineral-based CHF: Used in European vehicles (such as Pentosin CHF 11S). These fluids have a stable viscosity index across a wide temperature range.
- Universal Fluids: Multi-purpose fluids that may not meet the specific seal compatibility requirements of all vehicles.
[!WARNING] Never mix mineral-based CHF with standard petroleum-based ATF. Mixing these fluids will cause the rubber seals to swell, soften, or disintegrate, leading to system failure and fluid leaks.
A technician is performing a power steering pressure and flow test using an analyzer. When the analyzer's shut-off valve is closed for 3 seconds, the pressure reads 750 PSI, which is below the pump specification of 1,300 PSI. When the steering wheel is held at the left lock with the valve open, the pressure also reads 750 PSI. What is the most likely cause?
A hydraulic power steering pump makes a loud whining noise that increases with engine speed. The technician notices that the fluid in the reservoir is foamy. Which of the following is the most likely cause?
What is the consequence of adding standard Automatic Transmission Fluid (ATF) to a power steering system that specifically requires a mineral-based Central Hydraulic Fluid (such as CHF 11S)?