Can you drive a truck with a failing power steering pump?

Only to reach a safe place, slowly. In classic hydraulic power steering the wheel can still be turned through the mechanical link when the pump drops out, and European type-approval rules (UN R79) require steering within a set force limit even with failed assist. But the force required rises a lot: for heavy trucks (N3) the limit is more than double that of an intact system. If necessary, have it recovered to a workshop by a tow truck.

What causes a power steering pump to whine?

A constant whine or squeal usually means the pump is not getting enough fluid, that is, cavitation. Check the fluid level first; then inspect the suction hose, clamps, reservoir filter and the cap's vent hole. A short noise on a cold first start, or a brief noise from the pressure relief valve at full lock, may not be a fault in itself. If the noise persists, remove the cause before cavitation damages the pump's internal surfaces.

Why is my power steering fluid foaming?

Foam shows that air has mixed into the fluid. Air mostly enters through a loose clamp, a cracked hose or a leaking fitting on the pump's suction side; at a low level, the pump also draws in air directly. A wrongly fitted reservoir filter element can cause foaming too. If the reservoir level rises when the engine stops, air is trapped in the steering gear. Once the leak is fixed, bleed the air by the method in the service documentation.

Should a power steering pump be repaired or replaced?

The decision depends on the type of fault. If an external leak comes from the shaft seal or O-rings, the pump can be repaired with the manufacturer's seal kit, then flow and pressure are checked. If the pump cannot produce enough flow and pressure, or internal parts show clear wear marks, replace rather than repair. Either way, find the root cause first; if low fluid, contamination or an air leak persists, a new or repaired pump soon fails again.

Truck Power Steering Pump Failure Symptoms

Power steering pump failure symptoms in a heavy truck are usually heavy steering, pump whine, fluid that foams or drops in the reservoir, and a leak around the pump. First check the fluid level, as the service documentation specifies, and the fluid's color, smell and foam. If the fluid is fine, a flow and pressure test shows whether the fault is in the pump, steering gear or lines.

In short:

Whine and foam usually point to a problem on the suction side: air enters the system or the pump cannot get enough fluid.
If the heaviness is felt in only one direction, look at the steering gear or at air in the system rather than at the pump.
The pump-versus-gear decision is made by flow and pressure measurement, not by guesswork.
After a pump is replaced, the system is flushed according to the manufacturer's procedure, the filter and fluid are renewed, the air is bled and the root cause that started the failure is removed.

What does a power steering pump do and how does it work on a heavy truck?

A power steering pump is the hydraulic unit that converts the engine's rotary motion into a flow of pressurized fluid to feed the hydraulic steering gear. The pump's flow determines how fast the steering wheel can be turned, and the pressure it can reach determines the force the steering gear can produce to turn the wheels.

On a heavy truck, the pump is driven either directly by the engine or by a V-ribbed belt; on some vehicles it is driven from a through-drive on the back of the air compressor. The common design is a vane pump: vanes in the rotor slots slide along a cam ring and push the fluid. On some engines the power steering pump is built as a tandem pump that runs on the same shaft as the fuel feed pump.

The flow control valve in the pump limits flow to the gear by returning surplus fluid as engine speed rises; the pressure relief valve, in the pump or the steering gear, limits the maximum system pressure. Some steering gears also have relief valves (often called poppets) that lower pressure as the wheels approach the end of their travel. The fluid is usually drawn from a separate reservoir that often has a replaceable filter element.

On some vehicles with a heavy front axle load or more than one steered axle, dual-circuit steering is used to meet legal safety requirements: besides the engine-driven pump there is an emergency pump driven from the driveline (usually the transmission) that works while the vehicle is moving. An auxiliary steering cylinder or a second steered axle also raises the flow demanded from the pump.

Power steering pump failure symptoms

Power steering pump failure symptoms generally come from three mechanisms: reduced fluid flow to the gear, air in the fluid, or loss of sealing. The table below links each symptom to a first check; the final decision is made by measurement.

Symptom	Likely cause	First check
Steering gets heavy, especially at idle and when maneuvering	Low fluid level, air on the suction side, internal wear in the pump, flow or pressure valve fault, drive problem	Fluid level and condition; flow and pressure test against the manufacturer's values
Assist cuts out intermittently	Intermittent binding in the steering intermediate shaft, sticking flow valve, overheated (burnt) fluid, restricted line	Whether the intermediate shaft moves freely, smell and color of the fluid, condition of the hoses, response of the flow valve on a test unit
Constant whine or squeal from the pump	Low level, loose or cracked suction hose, clogged reservoir filter or cap vent, internal wear	Level, foam, suction hose and clamps, filter element
Foam or bubbles in the reservoir	Air entering on the suction side, wrongly fitted filter element, low level	Suction line connections; whether the level in the reservoir rises when the engine is stopped
Fluid loss, pump area wet	Pump shaft seal or gaskets, fittings, hoses and pipes	Leak traces on the pump body and along the lines
Heaviness in one direction only, or poor return to center	More likely the steering gear, air in the system, front-end alignment or a seized kingpin	The diagnostic order below

Brief heaviness or noise on the first start in cold weather can come from the vanes of some vane pumps extending late, and it clears when the revs are raised briefly. If the symptom becomes constant, the suction side and the fluid level should be checked.

Why do power steering pumps fail on trucks?

The main causes of power steering pump failure are low fluid and air on the suction side, dirty or out-of-specification fluid, overheating, drive problems and a pump that does not suit the application. On a heavy truck, the pump delivers continuous flow while the engine runs and builds high pressure when maneuvering, so any problem in the fluid shows up directly at the pump.

Low fluid and air on the suction side: When the pump cannot get enough fluid, cavitation forms; continuous cavitation damages the internal surfaces of the pump.
Restricted suction: A clogged or wrongly fitted reservoir filter, a blocked cap vent, or a crushed suction hose or one with a separated inner lining starves the pump.
Dirty, mixed or out-of-specification fluid: Dirt and foreign matter clog the filter and the suction line; mixing different fluids or using unapproved fluid can damage seals and cause leaks; the vehicle's service documentation governs the fluid type.
Overheating: Holding the steering at full lock for a long time forces fluid through the valves under high pressure and heats it quickly; burnt fluid wears hoses and seals.
Use and load: Frequently turning the steering while the truck is stationary, overloading and front-end alignment faults strain the steering parts continuously.
Drive problems: A slipping belt reduces pump speed; if the drive gear or coupling wears, the drive can be lost completely.
A pump that does not suit the application: Too high a flow leads to heating and steering that reacts abruptly; too low a flow gives heavy, slow steering.

Heavy steering: pump, steering gear or lines?

The source of heavy steering is found by working from simple checks to measurement; this order keeps a good pump from being replaced needlessly.

1. Check the fluid level, color, smell and foam. Burnt-smelling, darkened or foamy fluid is a problem to fix before measuring.
2. Inspect the pump body, shaft seal, hoses, steering hydraulic pipes and fittings for leaks and damage; the suction hose must not be crushed or loose.
3. Rule out mechanical factors: low front tire pressure, overloading, a dry fifth wheel on a tractor and binding in the steering intermediate shaft also make steering heavy. Kingpin and drag link binding is tested in the workshop by trained personnel like this: with the engine off, the front wheels are put on turntables and the drag link is disconnected from the steering gear; a wheel pulled by hand to the stop and released should return close to straight ahead. If it does not, the kingpins or the linkage joints are binding. Do not turn the steering wheel while the drag link is disconnected; when the link is reconnected, the joint is tightened and secured according to the manufacturer's instructions.
4. Note the condition of the symptom: if steering is heavy only at idle and when turned quickly, think of insufficient flow (pump wear, suction restriction); if it is heavy in one direction only, look for internal leakage in the steering gear or air in the system.
5. Connect a test unit made up of a flow meter, a pressure gauge and a load valve to the pressure line between the pump and the steering gear. Compare the flow measured at idle and at high revs, and the maximum pressure, with the vehicle manufacturer's values; if the values are low, the pump is repaired or replaced.
6. If the pump values hold, the opening pressure of the gear's relief valves is checked, then its internal leakage is measured; these tests are done according to the manufacturer's procedure and with the front axle secured. After that, the intermediate shaft and front-end alignment (tie rod, kingpin, caster) are addressed.

Safety: Most of these tests are done with the engine running. Secure the vehicle with the parking brake, chock the wheels and keep away from rotating belts and fans; support the front axle on stands if it is raised; if the cab is tilted, make sure it is secured in the fully open position.

Close the load valve of the test unit slowly, do not take the pressure above the limit of the unit and hoses, and do not keep the valve closed for more than a few seconds. Do not hold the steering at full lock either; the fluid heats quickly. Tests are done with the fluid at operating temperature; wipe up spilled fluid because of the burn and fire risk.

In the internal leakage test, the spacer block placed at the axle stop can fly out; do this test only by the manufacturer's procedure. Before opening any lines, stop the engine and wait for the pressure to fall, and never feel for a pressurized leak with your hand. Steering tests must be done by trained personnel.

Repair kit or complete pump?

Whether a power steering pump needs a repair kit or a complete pump depends on whether it has lost only its sealing or also its pumping ability.

Repair kit (seal kit): If the problem is limited to an external leak from the shaft seal or gaskets and the pump passes the flow and pressure test, renewing the seals may be enough.
Complete pump: Replace the pump if flow or pressure is low, if there are grooves you can feel with a fingernail on the cam ring, rotor, vanes or pressure plate, or if the housing is cracked or the shaft is damaged.
Repeat failure: A new pump fitted without removing low fluid, dirty fluid, a suction-side leak or a pump that does not suit the application will fail for the same reason.

When a pump or steering gear is replaced, whether the part is new or remanufactured, flush the system, replace the reservoir filter, fill it with approved fluid and bleed the air. When filling, the engine is run in short bursts and stopped and the level topped up; the detailed sequence is in the vehicle's service documentation.

What should you look for when choosing a power steering pump?

Pumps that look the same from outside can differ in flow, pressure limit, rotation direction, shaft and connection dimensions; so selection is made by part number and vehicle data.

OEM/part number: The number stamped on the pump body or on the label, and the vehicle manufacturer's part number.
Chassis number (VIN), engine type and model year: Different pumps can exist within the same model depending on engine and equipment.
Pump design and drive: Vane, variable-flow or tandem type combined with the fuel pump; gear, coupling or pulley drive; shaft profile and rotation direction.
Flow and pressure class: Must match the needs of the steering gear and, if fitted, an auxiliary cylinder or a second steered axle.
Connections: Flange type, inlet and outlet port threads, gasket or O-ring.

In the MPT catalog you can search power steering pump models by OEM number; what to watch for when matching numbers is covered in how to find the right spare part using an OEM number. If the steering gear or connecting parts are also being renewed, match steering system parts with the same chassis data.

If the steering suddenly gets heavy, if fluid is dropping fast or if a leak is reaching hot engine parts, do not drive the vehicle and go to a workshop.

Power Steering Pump Failure Symptoms and Causes in Trucks