What year car? If you are talking about the BLUE switch(2 pin connector) on the front of head, then that is probably the problem. On 1988 & later cars that sensor is a varible resistor & must send the correct info to the A/C pushbutton or the fan will run! Unplugging or jumping this switch makes the fan run!

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MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. TECH
190E 2.3 ITS RACECAR