Help!

I have been reading posts on the diesel discussions for some time but finally decided to join in, in desperation. I just changed the transmission fluid filter and pan gasket. My fluid was low but could not ever detect a leak. Clean as a whistle. I put 5 L of new fluid in and crawled back underneath only to find the little devil. Near the rear of the unit is a plastic probe with a cable leading up over the top of the transmission. It looks like it bolts i right at the base of the transportation pan. A rubber boot covers a coupling in the plastic. It looks like it may be cracked. Can someone help a novice, what is this?