Cape Canaveral - Nasa is scrambling to figure out what's wrong with a key cooling unit on the International Space Station.On Wednesday, one of two identical cooling loops shut down. The system uses ammonia to dissipate heat from on-board equipment. One line was getting too cold. Engineers suspect a bad valve.Mission Control ordered the six-man crew to turn off some science experiments and other non-critical equipment. Those instruments stayed off on Thursday, while the astronauts remained safe and comfortable.The suspect valve is in a pump that was replaced by space walking astronauts three years ago. Flight controllers are looking at ways to fix the valve. A software repair would be the easiest option, a spacewalk the most complicated.