Vacuum system

The vacuum system controls various pneumatic or vacuum-operated functions on MAN and Mercedes trucks, buses and vans. Depending on the engine family, emission standard and vehicle specification, this includes vacuum pumps, vacuum reservoirs, solenoid-controlled control valves, vacuum hoses, non-return valves and diaphragm actuators. Correctly regulated vacuum is essential for the predictable operation of, for example, exhaust brake functions, turbo controls and air valves. In MAN applications, the system may control the EVB (Exhaust Valve Brake) exhaust brake; Mercedes engines use vacuum-controlled throttle valves and associated control components.

The configuration differs significantly between engine series such as MAN D08, D20, D26 and D38, and Mercedes OM 904, OM 906, OM 457, OM 460, OM 471 and OM 473. Older engines may use a mechanically engine-driven vacuum pump, while newer Euro 5 and Euro 6 versions combine vacuum functions with electronically controlled solenoid valves, position sensors and engine control units. Connections, nominal vacuum levels, hose diameters, electrical connector pin assignments and control valve calibration also differ by version.

Vacuum lines must be oil-resistant, temperature-resistant and dimensionally stable. A soft, porous or collapsed vacuum hose causes pressure loss, even if no crack is visibly present. Non-return valves must also be installed in the correct flow direction: an incorrectly fitted valve prevents vacuum from building up or being retained in the reservoir. Therefore, always identify parts using the vehicle chassis number. Visual comparison or selection based solely on the model name is never sufficient, particularly for similar engine variants with different EVB, turbo or throttle valve controls.

Common complaints include a weak or slow-reacting EVB exhaust brake, fault codes for control deviations, insufficient turbo boost pressure, a throttle valve that does not close fully, increased fuel consumption or reduced engine power. In Mercedes applications, a faulty vacuum control system may prevent the throttle valve from reaching the requested position. This affects air mass, EGR control and potentially the regeneration conditions of the exhaust aftertreatment system. In MAN vehicles, a leak or defective control valve can result in insufficient EVB operation, with noticeably reduced engine braking power.

First read fault codes and live values using diagnostic equipment. Compare the requested and actual positions of the controlled valve or actuator. Then measure with a vacuum gauge directly at the vacuum pump and subsequently downstream of the reservoir, non-return valve and control valve. A circuit must retain sufficient vacuum after the pump is switched off; a rapid pressure increase indicates a leaking hose, diaphragm actuator, connection or non-return valve. Test solenoid valves electrically for coil resistance and power supply, but also check their mechanical flow when actuated. A valve may be electrically sound but internally contaminated or unable to seal properly.

When a leak has been identified, do not replace only the visibly defective hose. Check all connections, T-pieces, seals, the vacuum reservoir and the actuator diaphragm chamber. Oil or soot deposits around control valves may indicate external contamination or an internally defective actuator. After installation, hoses must be routed without tension, protected against abrasion and heat from exhaust components, and connected according to the original hose routing.

At Braem, technical support can assist with chassis-number-based identification of vacuum components. This helps you avoid an incorrect valve characteristic, wrong hose connection or incompatible actuator and reduces unplanned downtime.