Camshaft Hall effect switch works in other Audis except mine!!!???

[Cwalln at aol.com]

We now know where the problem with the '94 100S that will not start...when 
the camshaft Hall effect switch is taken from a running Audi and placed in my 
100S, the switch will not work as checked at the switch itself (wiring to the 
ECU is good).  

Place the Hall effect switch back in the running Audi, the switch works fine 
as evidenced by the generation of variable voltage...and a running car.  
Disable the switch in the running car, and the same symptoms show up, the car 
will not start/no error codes are generated (the word is that the car must be 
running for the error codes to be generated.)  

The Hall effect switch is triggered by a piece of metal on the camshaft...the 
trigger is still there and visible.  

Anyone hear of such a problem?  

I am told that nearly every Audi expert within 100 miles is stumped by this 
one.

Chris W.