Topic

I know there are different sensors for the MEMS 1.9, MEMS 2.0 and MEMS 3 .What does the MEMS stand for and how do I know which one to buy. My TF is from December 2004 135

Modular engine management system.

All TFs are MEMS 3

As per David - MEMS is a serious of ECUs designed by Motorola - Wikipedia tells the story - https://en.wikipedia.org/wiki/Modular_Engine_Management_System

MEMS 1.9 and MEMS 2.0 are the same except the 1.9 is for the non VVC and 2.0 is for VVC.

MEMS 3 is slightly different but all three use the same coolant temp sensor for the dash gauge.

However this isn't connected to the operation of the cooling fan which is actually controlled by the ECU.
And the ECU triggers the fan when it reads that the coolant temp is high based on the reading from it's coolant temp sensor which is black on MEMS 3 (brown on earlier cars), all three sensors are different are not interchangeable.