The Cooling Fan is Running at Maximum Speed

[BStar]

I have a problem with my Mercedes C350E (model S205).
The cooling fan does not stop when I turn on the ignition.
I have already done some diagnostic measurements.
I measure that the PTCU is sending the correct lock signal, however that lock signal is not getting to the fan motor, low temp cooling pumps, low temp cooling valves correctly.
So I suspect the problem is in the LINBUS cable.
Has anyone encountered this problem and/or have any suggestions?

 

[xeoninside]

It is possible that one of the two fans is faulty, so the other fan should run at full speed. First, check if the problem is hardware related.

 

[BStar]

Thanks for your reaction.

I only have one large fan and it is marked as M4/7 on the diagram.
As mentioned, the LINBUS (BKYE) wire sends the correct lock signal from the PTCU, but after the X25/7(3) connector the lock signal is no longer a lock signal but a more or less constant 9V, which I did not measure on the mentioned actuators.
It's strange that I measure 5VDC instead of 9V on M43/6.
I have already replaced the low temperature cooling pump M43/7, but the problem remains the same.

I appreciate every answer that will help solve the LINBUS wiring problem.

 

[eng.aws]

The easiest way to diagnose is to take the wire from the back of the PTCu, connect it to the fan and see if the fault goes away and the fan runs normally, while disconnecting other units such as the auxiliary water pump. Then connect them back one by one and test.

 

[vecinga]

I have seen this a few times. Once it was on the Z-connector Z142z1 located behind the bulkhead near the high pressure fuel pump on LHD vehicles or on the right headlight on RHD vehicles. Another time it was similar to what you are describing and after the X25/7X3 was disconnected I still had voltage on the fan LIN line. It ended up being a frayed wire where the harness goes through the firewall from the footwell to the engine bay.
There were also broken insulated wires.
Hope this helps.

 

[B0bby92]

Check the fuse box first, if it is ok, consider replacing the whole fan unit. They have control units that go bad. A used fan will work fine for less money.

 

[KG7222]

Check the actual values of the HVAC pressure sensor, I think this is the problem.

 

[kwikautoservice]

Common problem W205. Replace cooling fan.

 

[Kubis_09]

LIN BUS cable in the wiring harness under the windshield wiper motor is broken (oxidation).

 

[thxg]

I hope this helps you.