Faulty Control Unit or Sensor?

Hello, fellow diesel enthusiasts!

Our Mercedes W203 CR-diesel is malfunctioning - after three days of rain, the engine will no longer start. The engine check light is on, and after about 3 seconds, the engine fan starts running. The information display shows "Waiting period required" during the oil level test.

The BOSCH CR2.11 control unit shows signs of oxidation on the lower edge of the circuit board (on the serial EEPROM) when opened.

Given that the control unit appears to be in good condition both visually and electrically – with reasonable signal levels and access to memory, etc. – I had hoped that the fault might be due to one of the sensors.
Is it normal for the ECM (Engine Control Module) to turn on the fan after 3 seconds, even when it's not connected to any sensors? According to the wiring diagram, pin 3/46 of the control unit is connected to the SRB (Steering Rack Booster) and is labeled "Voltage Supply Relay Control" - however, this pin is high-impedance.

Should I connect voltage dividers to the temperature sensor inputs to simulate "normal" ambient conditions?

I am very grateful for any help.


Best regards from Vienna.
Thomas.

Due to water ingress in the W203 (a known issue), the pins of the 8-pin serial EEPROM were oxidized.

Symptom: Check engine light is illuminated - Engine fan runs after 5 seconds, and the information display shows "Please wait" - Diagnostic tool cannot establish a connection.

Type of control unit: BOSCH EDC-15C6.

So, in similar cases, drying and visually inspecting the printed circuit board might save you around 1500 euros.

Best regards from Vienna.

Thomas.