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boblane,
I can only give you info that pertains to my '97 E420. There the W/S switch simply supplies 12 volts to pin 3 on the Transmission Control Module when it is in W and 0 volts when it is in S mode. If your car is similar, the only way that I could imagine how this switch could cause limp mode is if instead of the full 12 v you are getting too low a voltage , either because the contacts are dirty and have built up resistance, or, because there is some current leakage through the coffee left on the switch. If the switch is not working alltogether the car would just be in S mode instead or W or vice versa not cause limp mode.At any rate, it should be easy to measure the funtionality of this switch at the TCM connector.
The main gear selector switch is a little more complicated, it is essentially a 4 bit binary switch which means it can give 2x2x2x2=16 different responses. Examples: 0001 ; 0010 ; 0011 ; 0100 ; 0101 ; 0110 ; 0111 etc. Out of these possible 16, only 8 are used and needed ( 1,2,3,4,D,N,R,P) So MB skips every other response which means that any single failing contact in the switch can not yield a "wrong" but nonetheless "valid" response from the switch. The only way it can go from one valid to the next valid reading is by two of the 4 bits changing state ( going from 1 to 0 or vice versa) . One faulty contact ( regardless of whether it is stuck "open" or stuck "closed")in the switch will cause a single bit change which will cause an invalid reading. This off-course the TCM can detect and it will most likely trigger the limp mode.
Attached is the combination of voltages that are valid for my car, notice that instead of the 0 and 1 in my example, they use 0 and 10 volts.
No idea if yours is identical so beware.
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