The RS232 line is likely to be the source of power. The signal at an IR receiver isn't enough energy to supply RS232 which in today's terms is pretty power hungry. Good news is, they can steal a lot of power from a proper RS232.
First thing to check is, does the computer supply a proper RS232 voltage and current? TXD(3) should be a solid +V when idle, preferably 6V or more (spec is 3-12). They probably are stealing -V from RTS(7) or DTE(4). One or both of them should have -6V or better. If you don't get good readings on the open computer port, it's a wimpy RS232 with no power available to steal.
The RD line should be idle at +3 to 12 volts, and active at -3 to 12V. If you place an LED with anode to ground, and a diode and 2.2k resistor in series to RD, it should blink on any activity.