If you're TCAS is on then you are visible to other aircraft.
I'm afraid I don't know enough about TCAS to know what will happen if the a/c using it has no working transponder. Certainly, it won't co-ordinate with nearby TCAS systems as a collision advisory. But does the pilot get to see some kind of display of a/c in the vicinity? Does it still give data, but with no alarms going off?
And I don't think TCAS on /transponder off makes you visible to other a/c? Does it?