Extruder Controller v2.2: analogReference(INTERNAL) with no effect ?

Hi

I'm trying to build a heated bed. So it wouldn't be bad to have something to control the heater

For the first test I wanted to use my Extruder Controller (because It's already there).
But I don't have another termistor.
The only thing I could find for measuring the temperature was a 1n4148 diode.

So it would be nice to use the internal voltage reference of the atmega168 (which is 1.1V).

The schematic shows that aref is coupled with ground through a capacitor, so it`s safe to use the internal reference - I think.

My next steps were:
-conected the diode to the thermistor screw terminals of the EC
-started Arduino v17
-progamed the EC with the termistor test progam from there
-changed "#define THERMISTOR_PIN" from 0 to 3
-checked the readings in the serial monitor (all fine, raw value 121 at 21°C and changing with temperature)
-added "analogReference(INTERNAL);" in the setup section -> no change, raw value still at 121
-moved "analogReference(INTERNAL);" to the main section -> no change
-added "delay(500);" to add some time to let things settle -> no change

Then I rebuild the circuit on my evaluation board with an atmega8 and an atmega328. Unfortunately i dont have an atmega168 in PDIP to check on my board.

The same program worked as desired - no matter in which section "analogReference(INTERNAL);" was called.
With 1.1V internal reference the raw values are useable (range from 583 at 21°C to 221 at 240°C).

Can somebody please confirm this?
Am I wrong?
Is it the Extruder Controller?
The atmega168?
Arduino v17?

regards

Michael