W221 S320cdi Cooling fan always on

[leicschris]

Hi All,
I have a W221 S320cdi, 2007 which I have not used for a couple of months, the MOT ran out over Xmas and kids Xmas was the priority.

I started it a couple of days ago to take it for its MOT and the radiator fan is running at full speed constantly.

I have googled and read the forums and haven't found a straight answer, some say the engine ecu at fault and some say the fan control unit.

I have an icarsoft mb2 and when the engine is running, the fan operation on live readings is 10%, which means fan off. When the engine is stopped this goes to 90%, and back to 10% as soon as the engine is restarted.

I have checked the voltages at the fan plug and have 12v + and - on the big wires, 12v on the switched thin wire and around 3 to 4 volts on the signal wire so taking that, and the 10% from the ecu, I am thinking the ECU seems to be working as normal.

Icarsoft also shows a P1636-4 fault "check component M4/7 suction fan discontinuity of fan signal" which I haven't found an answer when searching.

Had anyone any ideas, I don't want to change the fan unit if it's not the fault.

Thanks
Chris

 

[Muckypaps]

You need to check your Aircon refrigerant pressure, as a failed sensor can cause the fans on full, without showing a fault code.

 

[ALFAitalia]

My A old SLK did similar with the fan when the MAF went duff....had no fault codes either!!!

 

[leicschris]

Hi,
I did check the aircon system.
Was showing pressure around 3bar raising when I rev the engine.
I had the aircon repaired during last summer as one of the pipes developed a hole and needed to be replaced.
I'm not sure if the pressure sensor was changed at the same time.
What should the pressure read

 

[Muckypaps]

It’s not your aircon pressure sensor causing it then, it would give a stupidly high reading and wouldn’t change pressure if it was at fault.

 

[leicschris]

Hi
No, I didn't think it was to do with the aircon, it's showing no faults and working well.

I am leaning to the fan control unit but not a lot on Google saying that, most in the US steer towards a faulty Bosh ECU, but again, they are all petrol v8's as they don't have the diesel versions over there, or if they do they are very rare