sorry mate... I have a W140 which is on a M104 engine. I think yours is a M103 so I cannot help you too much.

The fans come on when:

1) CTS gives the right signal (in ohms); and
2) when the A/C system pressure is above a certain bars.

To have the fan turn on, all we have to do is lower the ohm on the CTS sensor or alternatively, play with the pressure sensor on the A/C system.

To give you more details, have a look at Jimf's web site whom tells you all about the aux.fan and a mod. that he sales.

http://pages.prodigy.net/jforgione/MB_CTS.html

So in your case, either you have unpluged your CTS sensor or you have stuffed the refrig pressure sensor.