Caba,
Further to Hotdogs response, TCAS is dependant on having a functioning mode S transponder attached.
2 main reasons.
Firstly, the data linking process between the aircraft uses the transponders. I.E. your TCAS talks to his transponder, his TCAS talks to your transponder. Hence without the transponder their can be no TCAS to TCAS coordination.
Secondly, your TCAS actually gets its altitude data from your transponder.
Hence, if transponder turned off, TCAS stops working.
Incidentally, this is why it is important not to turn transponder to Standby when changing code, an arguemnt that pops up here every now and again.
CPB