Line

The pipes and hoses between the power steering pump, steering box and hydraulic fluid reservoir help ensure the reliability of the hydraulic power steering system. They must accommodate the required flow rate, absorb pressure pulses and withstand engine movement without kinking or chafing. On MAN and Mercedes trucks, buses and vans, leaks, air ingress or internal restrictions can cause pump noise, uneven steering assistance and heavy steering. Do not automatically replace the power steering pump: a damaged suction hose can cause very similar symptoms.

The circuit includes high-pressure lines, return lines and suction hoses. High-pressure versions typically consist of formed steel pipes with corrosion protection, reinforced elastomer hoses or combined pipe-and-hose assemblies with crimped fittings. Return lines carry hydraulic power steering fluid back to the power steering fluid reservoir, sometimes via a cooler. Suction hoses must be vacuum-resistant: a hose that appears intact on the outside can collapse during operation.

Always identify parts using the vehicle's chassis number. Visual comparison or selection based solely on the model name is never sufficient. On MAN TGS models with D26 engines and Mercedes Actros models with OM 471 engines, for example, the pump version, connection position and chassis configuration affect the choice of lines. Line lengths and connections also vary between bus and van versions; fully electric power steering does not use hydraulic steering lines.

Other examples include MAN TGL and MAN TGM with D08 engines, MAN TGX with D26 or D38 engines, Mercedes Atego with OM 934 or OM 936 engines and Mercedes Arocs with OM 470, OM 471 or OM 473 engines. Bus examples include MAN Lion's City with D20 engines and Mercedes Citaro with OM 936 or OM 470 engines. Relevant van applications include Mercedes Sprinter versions with OM 651 or OM 642 engines and hydraulic power steering. Year of manufacture, engine type and steering system configuration remain decisive; these examples do not replace a chassis number check.

Check the internal diameter, wall thickness, permissible working pressure, pressure impulse resistance, temperature range and minimum bend radius against the vehicle specification. Threads, sealing cones, banjo connections and O-ring seats must match exactly. Do not compensate for fitting alignment or positioning discrepancies by forcibly bending the lines. Integrated pulsation dampers or calibrated restrictors must be retained; a universal hose that appears to fit can cause troublesome steering vibration.

Typical signs of damage include pitting corrosion beneath line clamps, chafing against chassis components, hardened hose covers and leaking crimp sleeves. Internal hose layers that separate can restrict flow without any visible external damage. A leaking suction connection can draw in air without obvious fluid loss, resulting in foaming and cavitation noise.

1. Start with the system depressurised: secure the vehicle, allow the system to cool and depressurise it according to the manufacturer's instructions. Clean the connections and check the fluid level, fluid condition, clamps and the entire line routing. Never use your hands to search for high-pressure leaks.
2. Check the suction side: use a suitable vacuum gauge at the pump inlet to measure the vacuum at the specified engine speed and fluid temperature. An abnormal reading calls for an inspection of the suction hose, reservoir outlet and any suction filters fitted.
3. Measure pressure and flow: use a calibrated hydraulic test kit with suitable adapters. Compare pump pressure and flow rate at the specified engine speed and steering load with the manufacturer's values. Do not hold the steering at full lock for extended periods.
4. Locate restrictions: where specified, measure return line back pressure or the pressure differential across suspect sections of line. Also check the cooler and reservoir filter to avoid incorrectly attributing a blockage elsewhere to the power steering pump.

Replace damaged line assemblies in full; emergency repairs using hose clips are unsuitable for high-pressure connections. Cap open connections immediately to keep out dirt. Use the specified new seals and tightening torques. Fit hoses without twisting, observe the bend radius and replace any missing clamps and heat protection. Check that there is sufficient clearance for engine movement and suspension travel.

Only use hydraulic power steering fluid with the specified manufacturer approval. If there is internal hose damage or contamination, flush the circuit according to the repair instructions and replace the power steering fluid filter where specified. Bleed the system using the vehicle-specific procedure without allowing the pump to run dry. Afterwards, check the fluid level, foaming, leaks and steering operation at operating temperature. Braem can provide technical support in selecting the correct line version.