300SDL temp sensor question

The auxiliary cooling fan on my 1986 300SDL was not coming on so my
mechanic replaced the temperature sensor. It lasted about 2 weeks and
then was not coming on again. I took it back and he said the sensor
was defective and still under warranty so he replaced it again. I
noticed afterward that as soon as the key was in the "ON" position the
auxiliary fan would come on and run all the time. A few weeks more and
the fan stopped running agin. I took it back to the same mechanic and
found he has closed. I started loojing things over to see if it was
something simple, like a loose plug on the temp sensor when i found
the sensor was not even plugged up. It has a grey base/ 3 prong
sensor. The plug for the 2 top prongs had been hot-wired (a jumper
wire completing the circuit and taped in place). I unplugged the fan
from the wiring harness behind the drivers side headlamp and attached
a 12V power supply to it and the fan comes on, so i know it is good. I
took out the jumper wire from the temp sensor plug and plugged it up
and with the car at about 90C the fan still did not come on. I found
the aux. fan fuse had blown and when i replced it the fan came on
right away. 
   A few more days and sitting in traffic the temp was coming close to
100C and when i checked the fan is not coming on again but the fuse is
still good. That has been going on for about 2 weeks now. Today when i
went to get in the car after work, my wife had been in the car for
about 2 minutes with the engine running and as i walked by i heard the
aux fan running but the cars temp gauge was only reading approx 60C.
When i got home, i had been using the A/C and the fn was still
running, but as soon as i cut the A/C off, the aux fan cut off, even
though the temp gauge was reading approx 85C. When the car had only
been running for 1 minute and the fan was running, the A/C was not on.
   So why is it on when the car is cold, later on only when the A/C is
being run and not on when the car is the same temp (apprx 85-90 or
higher at times)  and the A/C is off?   
TIA

Shannon
86 300SDL and 28mpg hwy ;-)
80 450SL