The fan used on the PS2 is a DC brushless fan, controlled by an internal integrated circuit. Instead of spinning in reverse (which is what would happen if it was a regular brush based motor with fixed magnet) it will fry the controller IC and hal effect diode (spin detection/speed sensor device) making the fan useless.
Originally Posted by SP193
No matter what DO NOT INVERT the polarity.
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)