[fireant911]

I have been reading both the Charles Probst's Bosch Fuel Injection & Engine Management & the Bentley Porsche 911 SC Service Manual to hopefully find an answer to my problem with no luck thus far. The 15° C temperature switch on the right front timing chain cover controls the CIS system from going from open loop to closed loop. Since I can measure a constant duty cycle at the three pin test connection, I realize that I am always in an open loop mode (the duty cycle is always at 50% when the engine is fully up to its normal operating temperature). I had removed the 15° C temperature switch and tested its operation in hot water and the switch closes as designed. When the switch closes it is supposed to provide a ground input to the Lambda control unit (lead number 7 on the harness) which should allow the system to go into closed loop operation. My mixture is indeed being controlled by the computer because I can both feel this control as well as see it on my Air / Fuel Ratio meter but the system (at idle) always remains at 50% duty cycle (open loop). What am I missing here? Why will the duty cycle not deviate from 50% - and yes, I realize that 50% is the target value but it appears that I am always there regarless of the position of 3mm basic mixture adjustment screw.
My goal is to address, identify, and correct this issue so simply unplugging the Oxygen Sensor is NOT the answer I am hoping to obtain.