I have a similar, but very different, issue with my 450SLC fan. I have only had my car for a short time so I am not too sure of what all the controls do. However, there is not too much to know about a five position switch (off and speeds 1 to 4). When switching on the ignition the fan has the four speeds you refer to. All speeds seem to work okay and the fan does not operate in the off position. However after the car has been running for a few miles the fan suddenly switches itself on to high power or top speed even though the switch is in the off position. I can not switch the fan off. The engine can be turned off, but until the car is cold you can not turn on the ignition without the fan blowing at top speed! Result I drive around with fan on in high power mode all the time. Very frustrating! Anyone got any ideas?