You did not read correctly. Both are temperature sensors. One reads temperature out of the radiator and one reads temperture at the cylinder head. Both sensors are of the same type according to the WDS. They are not switchs. The DME directly controls the operation of the fan.
Before replacing any sensors I would check to see if the DME is reporting a sensor error and storing a code. I would also check for connector corrosion, loose pins and possible harness issue from the DME to the fan. One thing you have not mentioned. The fan. The fan may have a dead spot in the comutator and may not start if the stator is in a certain posistion but starts in others.
Quote:
Coolant Temperature Sensor at Radiator Outlet
This temperature sensor is fitted in the outlet hose of the radiator and registers the coolant temperature after it flows out of the radiator.
The quantitative signal of the sensor is the most important factor for activation of the electric fan. The engine management system activates the electric fan.
Electric fan, S54 engine
Under certain operating conditions, the engine control activates the electric fan at various speeds.
The electric fan is activated by means of a power output stage directly on the fan motor. The motor control module activates this power output stage by means of a square-wave signal with duty factors (variable pulse width) between 10 and 90 % thus controlling the various speeds of the electric fan. Pulse duty factors less than 5 % and greater than 95 % do not trigger activation but rather they are used for fault detection purposes. The power output stage features its own positive and ground supply.
The fan speed is influenced by the coolant temperature at the radiator outlet and the pressure in the air conditioning system. The fan speed is reduced as the vehicle speed increases.
Coolant temperature sensor, S54 engine
The temperature sensor is screwed into the cylinder head. A varistor (NTC = Negative Temperature Coefficient) serves as the measure for the coolant temperature. A resistor of this kind is also used to sense the radiator outlet temperature.
The coolant temperature sensor, S54 engine, also serves as a measured variable for the following functions:
Start volume calculation
Injection volume calculation
Nominal idle speed
In the event of the sensor failing, a fault code is entered in the DME control module and a substitute value is calculated from the value of the intake air temperature sensor with the aid of a temperature model.
|