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Help! Can someone point me to the wiring diagram for the radiator fan circuit from the SAMs through the relay and to the fan? I cannot find it in the factory manual nor on the internet. Problem is car's fan wont work so I thought the best place to begin troubleshooting was to check the control/power circuits to the fan. I found on alldata/owners manual the fan relay location and power side fuse. Upon checking the relay i see voltage at the coil side which is strange as a/c if off and car engine is cool. Also, i found no voltage from the power side fuse to the common terminal. i checked the fuse 28 and found it to be good. I believe the way this circuit works is the the SAMs unit switches the coil voltage which then trips the relay and applies power the fan. Does anyone else have more info or experience with this circuit. Curious why is the wiring schematic is hard to find?

Note I want understand the control circuit and validate its operation before i go yanking out the fan. Any help on this path is appreciated.

mdm
 

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Mercedes (and smart) are very secretive about most things on their cars. the factory manual only exists on their computer website and is quite expensive unless you have a need for it say like a dealership. Most of their circuits are powered all the time and they control the ground side, which is the opposite of US cars.

the fan circuit should run the fan when it gets above a specific temp outside and the AC is turned to the full on position I usually also turn on the recirculate switch too. the fan should run continuously. I have seen problems with the fan mounts breaking, bushings seizing. and wire breaks on the fans.To replace the fan, the whole front of the car has to be removed to get to the fan assembly, including the radiator and condensor, it sounds worse than it is. but still a good 10 hour job. When my fan mount broke on my '09, I added a second fan on the front of the radiator that pushed air through. Fan was around $70 and a lot less time even with figuring out some sort of mount. I used the current fan wiring and I ran the car for 2 1/2 years with no problems until it had other issues not related to the fan. From the outside , you couldn't tell any changes were made. Just saved a ton of money and time.
 

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The smart service info is worthless for this. The component location index shows the radiator fan relay but the wiring diagram for the power and ground distribution shows the radiator fan powered directly from the SAM on a RED/BLU wire. The BRN wire on the fan connector is ground. Nowhere do the wiring diagrams show a radiator fan control relay. Not in the cooling system, the engine management or the climate control system. Once again, the German Engineering left hand is not talking to the right hand.

If you have 4 wires to the radiator fan relay the heavier gauge wires are the switched power to the fan motor and the smaller gauge are the control wires. Pretty much all modern vehicles have the control relays ground controlled so the relay coil will always be powered with the key on and the ground to the relay coil switched on and off to operate the relay.

Item M4/2 on the attached diagram is the radiator fan motor.

The simple way to test this is with a capable scan tool that will activate the radiator fan. By operating the fan from the scan tool you can verify whether the relay is working when commanded. The poor man's way is with the key on-engine off switch the a/c on and off to see if the fan relay operates at which time you can test for power in and out of the relay. I am assuming the fan will be switched whether the engine is off or on. I have never tried it. You may have to test with the engine running. If you have power out of the relay to the fan motor the next step is to test at the fan itself. In fact, that might be the first step. If power is there and the fan is not running then it is NFG. One can tap on the fan motor with it powered and see if it starts to run.
 

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