Replacing the MFTS

[michael]

Gentlemen,

there has been discussion of the MFTS on this list.  It appears  
replacement is not as straightforward as it would seem.

Coolant leak discovered at MFTS.
Audi replacement MFTS has 3 terminals instead of the original 4.  The  
+Battery terminal has been eliminated.
After installing the 3 terminal MFTS, the temperature gauge does not  
work.

I installed the same 3 terminal MFTS in a 5000 and the temp gauge  
works there.

The voltages on the wires coming from the mating connector (to the 3- 
terminal MFTS) are as follows.

5000:
10V      12.5V
+Bat      10V

200tq20v:
10V      9.4V
0V        10V

What's causing the difference here?

The wiring diagram shows one of the wires going to the instrument  
cluster.
Could this be an artifact of my flaky instrument cluster (which I  
have resoldered, etc. but which still acts up)?

- Michael