Cooling system fault table (Mercedes-Benz S-Class W126)

Malfunction: Coolant temperature is too high, instrument readings are in the red area.

Cause	Method of elimination
Low coolant in the system	Fill the radiator to the marking, check the system for tightness
Thermostat does not open	Check if the upper coolant hose gets warm. If not, replace the thermostat
Coolant pump defective	Remove and check the pump
Coolant temperature sensor defective	Check the sensor
Temperature gauge defective	Check the index
Radiator cap defect	Conduct a pressure test (in a workshop environment)
The coolant pump belt is loose	Check and adjust belt tension
The radiator fins are dirty	Blow out the radiator with compressed air from the engine side
The radiator is clogged with scale or rust	Replace the radiator
The air bleed tube between the coolant pump and the cylinder head is clogged	Remove the tube and check its free flow
Fan viscous clutch defect	Check the clutch: let the engine speed be 4000-4500 rpm. As soon as the coolant temperature reaches 90-95°C, the fan speed will increase by 1000 rpm, which will be audible