Here is how it works:

The ECU sends pulses to the Fan Module. (A narrow pulse is just an "Are you there?" message, a wider pulse tells the FCM to run the fan, the wider the pulse, the faster the fan is to run.)
The Fan module responds with an "I'm here" pulse.

If the Fan Module does not get pulses from the ECU, it puts the fan motor in emergency mode, and the motor runs full speed.
If the ECU does not get the "Im here" pulses from the Fan Module, the ECU lights the little "radiator" light that people think is the low coolant light.

It is that simple. If you have a fan motor running fast with a "radiator light" on, you have a breakdown in communication between the two modules. This is usually a sign of a bad Fan Controller.

If you have a fast running fan and NO light on the dash, it means the fan module is telling the ECU that things are OK but still running the fan too fast - change the Fan Module.
Or.....
If you have a fast running fan an NO light on the dash, it COULD mean the ECU is bad but I doubt it.

I'm leaning toward a bad Fan Control Module at this point, but if you dont even know if there is a light on the dash, I can't really help you, as you are not providing enough information. One thing is sure, I'd not change the ECU until I'd exhausted all other avenues. ECUs simply don't fail very often.

 

But if the car runs, it's not the RCM. The reason that the fan runs fast due to a bad RCM is this:

If the RCM's engine control relay output fails, then what you get is a powered up Fan Controller (with the key on) and a NOT powered up ECU - cause the Fan Controller is powered directly from the ignition switch, where the ECU is powered up by the RCM's engine control relay output. If the engine runs, the engine control relay output of the RCM has to be OK, otherwise, the engine would not run.

So if the car runs, the engine control relay output of the RCM is good, in such a case, the fast running fan is not caused by the RCM.

 

Great explanation.

 

I believe that number is for the automatic transmission. The number for a manual transmission is 112 153 57 79 with a Bosch number of 261 208 353
There are several listed on ebay.

 

thanks for all your answers, after the good attempt with the 112 153 53 79 I bought a new one, let's try again!
really hope to fix this issue, for sure I'll keep you up to date.

 

Hi Tom

I've just been reading about your problem. I'm not sure what's going on with your car, but there's no way your mechanic could take the PCM from another car and use it to start your car, it just doesn't work like that and I'd ask him if he really did it? If you buy used PCM and swap it into your car, it still wont work. Even if you can get a "new" PCM, it wont start your car without being mated up to your immobiliser and coded for your car. If it truly is your PCM, then I'm happy to fix it for you (if you cover the postage, as I live in Australia).

BTW. I have seen cases of the fan running flat out before, and it hasn't been the PCM. Fan controllers usually do this just before they fail completely (some of the soldering inside the fan controllers isn't that good). My suggestion would be replace the fan controller first. Did your Mechanic really try and swap this before and if he did, did he use the exact same type (as they're all coded differently in the PCM)?

 

I hope you did not buy a 112 153 53 79, because it is not the correct replacement for your car, as you said your car has a manual transmission. Read CL770 post and understand that a new or used replacement PCM must be programmed for your car with the correct VIN so that it can communicate with the SKREEM anti-theft ( demobilizer) which is located behind the instrument cluster. I had hoped the mechanic you are working with was able to do the programming as you had said your car was running good after he had placed a denoted PCM in your car. The keys/fob, SKREEM, PCM and the anti theft module in the trunk area all must work together as a unit for your car to work properly. The PCM is not a plug and play replaceable part.