Contents: Overpressure relief valve ↡ Vacuum valve ↡ Coolant indicator sensor (diesel…↡ Examination ↡ Removal ↡ Installation ↡
Using a standard test device, you can check for leaks in the cooling system and the operation of the relief valve in the threaded cap of the cooling system.
Attach the test device to the filler neck of the expansion tank. Use the hand pump of the device to create excess pressure of about 1.4 bar. If the pressure drops, find the leaky spot where the coolant is leaking and eliminate the leak. If the pressure drops without any signs of coolant leaking or even increases, then it is possible to assume that there is a loss of coolant inside the engine, for example, due to damage to the cylinder head gasket or a crack in the housing.
Overpressure relief valve
The safety valve is installed in the radiator cap. Its task is to open the cooling system starting from a certain excess pressure and thus create conditions for excess coolant to flow into the compensation tank.
Screw the radiator cap (2) onto the tester (1). Create excess pressure. The safety valve should open at a pressure of 1.3–1.4 bar. Otherwise, replace the cap.
Vacuum valve
Material borrowed from an online resource MercedesMan
1. Pull the vacuum valve with your fingers until it opens. Check if the valve closes completely when released. The vacuum valve in the cap opens at 0.1 bar of reduced pressure.
2. Screw the cap onto the radiator. If damaged, replace the rubber sealing ring.
Coolant indicator sensor (diesel engine)
The coolant level indicator system mainly consists of a sensor in the expansion tank and a control lamp on the instrument panel.
The coolant level indicator with the engine running warns of too low a coolant level and, therefore, of possible engine overheating. As soon as the ignition is switched on, the control lamp lights up for 30 seconds. When the engine is running, this lamp goes out immediately.
The coolant level in the expansion tank is measured using a float (551). When the coolant level is low, the contact (550) in the sensor closes and the control lamp lights up. Depending on the driving mode, the control lamp first flashes briefly and then continuously. If the control lamp flashes, add coolant.
Examination
1. Start the engine and let it idle.
2. Pull out the plug on the coolant level sensor.
3. Connect an ohmmeter between both contacts on the sensor. When the coolant level is at the "max" mark, the measuring device should show infinity. When the expansion tank is empty, the resistance should be about 5 ohms.
4. Stop the engine.
5. If the sensor is OK although the indicator lamp does not light, check the electrical lines and indicator lamp according to the circuit diagram.
Removal
1. If necessary, drain some coolant.
2. Use a screwdriver to lift the safety ring (550a).
3. Pull the sensor out of the expansion tank.
Installation
1. Insert the new sensor with a new sealing ring (552) so that the different width protrusions on the sensor are fixed in the hole of the expansion tank.
2. Press the safety ring.
3. Fill with coolant.