Exhaust throttle valve

Depending on the design, engine brake valves regulate the compressed air supply to the exhaust brake actuator cylinder or control another engine braking mechanism. A control valve that switches correctly ensures predictable braking performance and prompt release when accelerating. Always distinguish between the pneumatic control valve and the exhaust brake flap in the exhaust gas stream: both can cause similar symptoms, but require different testing methods and replacement parts.

Always identify parts using the vehicle's chassis number. Visual comparison or selection based solely on the model name is never sufficient. For example, on MAN TGL and TGM models with D08 engines or Mercedes Atego models with OM 906 engines, the engine brake equipment, connections and control system vary according to year of manufacture and specification. The engine brake configuration fitted to buses must also be checked. Vans do not necessarily have a separate engine brake valve; check the factory-fitted equipment first.

There are mechanically operated pneumatic valves and electropneumatic solenoid valves. A 3/2-way valve has three ports and two switching positions, allowing the actuator cylinder to be pressurised or vented. Check the default position, nominal orifice size, permissible operating pressure, venting capacity, thread and air line diameter. For electrically operated versions, the connector pinout, rated coil voltage, for example 24 volts where specified, and any integrated suppression diodes are also determining factors. Aluminium or brass valve bodies and elastomer seals must be suitable for the mounting location, temperature exposure and compressed air quality. Matching external dimensions do not guarantee the same switching function.

Internal leakage caused by worn seals results in insufficient cylinder pressure and weak engine braking. Condensation, corrosion and contamination can cause the valve spool to stick. A blocked vent causes delayed release: the exhaust brake remains partially engaged, resulting in power loss and excessively high exhaust back pressure. Open-circuit coils, contact resistance in connectors and broken wires cause failure or temperature-dependent faults.

1. Secure the vehicle against rolling and allow hot exhaust components to cool. On electronically controlled systems, read the fault codes and enabling conditions via the ECU (Electronic Control Unit). Check that an engine brake command is actually being issued.
2. Using suitable pressure gauges, measure the supply pressure upstream of the control valve and the outlet pressure to the actuator cylinder. Record pressure build-up and release during a specified actuator test. Compare these with the vehicle-specific values; reservoir pressure alone does not prove correct operation.
3. Check for external leaks using leak detection fluid. If outlet pressure remains after switching off, inspect the vent port and connected lines for blockages. Keep hands clear of the actuator's range of movement.
4. Measure the supply voltage and voltage drop under load using a multimeter. Only check coil resistance with the power disconnected, and compare it with the manufacturer's data. For pulse-controlled versions, use an oscilloscope to assess the control signal.
5. If the control pressure is correct, check the cylinder stroke, return spring and free movement of the exhaust brake flap in accordance with the workshop instructions. If necessary, measure exhaust back pressure at the specified test point to distinguish between mechanical jamming and an exhaust restriction.

Depressurise the relevant compressed air circuit before removal. Mark the supply, working port and vent; incorrect connections can cause continuous brake activation. Fit suitable seals and observe the specified tightening torques. Prevent sealing material from entering the air passages, and do not alter any factory-set stops or actuator settings.

If water or compressor oil residue is found in the control valve, also address the air dryer and compressed air supply. Otherwise, the new component may seize again. After installation, check for leaks, full actuator travel and rapid pressure release. Then carry out a controlled functional test in accordance with the manufacturer's procedure. Braem can provide technical support in selecting the correct version using the chassis number.