On the tdi, the 4 pin sensor controls the temp gauge on the instruments and is a switch for the auxiliary water pump (hence having 4 pins) the 2 pin one you have pictures is the one that gives the ecu a signal.
They are both located on the same 'T' piece which sticks out of the engine. The 4 pin is supposed to be on the bottom and is really hard to see from the top, and the 2 pin is on the top.
I have just changed the 4 pin sensor on my ACV 2.5tdi engine to fix my dash gauge and my constantly running aux pump.

It is really easy to change these sensors, you slide the little plastic clip off and pull it out. The worst part is you will lose coolant. No big deal.

 

so from my symptoms on this thread do you think this is the correct one to be swapping?

 

hi there ive had same problem with no dash temp gauge i have changed the 4 pin sensor but still no dash temp gauge working can anyone help please ...shes a 2003 2.5 tdi

 

This thread (post #10) seems to be the info you need: http://www.vwt4forum.co.uk/showthread.php?t=197001

 

Connect vagcom and then disconnect sensor, whichever one that when disconnected causes jump in vagcom value is the sensor you need.

 

If I remember right, the temp sensor is green.

 

My ACV has a 2-pin (top) and another one (presumably 4-pin) at bottom of the tee.

My temp gauge works OK. Unplugging the 2-pin has no effect on the temp gauge - it still works.

I can't even figure out how to unplug the 4-pin one. (Anyone?)

 

same as the two pin?

when i had issues, i just replaced both and job done!