The Expedition 32 crew finalized preparations Tuesday for a second spacewalk in less than a week in the wake of an unsuccessful attempt to install a replacement power-switching unit on the International Space Station's truss Thursday.

Flight Engineers Suni Williams and Aki Hoshide, who spent the weekend studying revised procedures and fabricating an array of new tools, finished configuring the Quest airlock for a spacewalk beginning at 7:15 a.m. EDT Wednesday to complete the installation of a spare Main Bus Switching Unit (MBSU). The MBSU is a boxy, 220-pound component that relays power from the station's solar arrays to its systems.

A marathon 8-hour, 17-minute spacewalk Thursday - the third longest in history - ended with the spacewalkers unable to attach the switching unit into its housing on the truss.

Engineering teams at the Johnson Space Center in Houston have worked around the clock since Thursday to troubleshoot the problem that prevented the spare MBSU from being installed. The most probable cause is likely a combination of a slight misalignment in the positioning of the spare unit for its installation prior to bolting and possible damage to the threads of the receptacle posts on the S-zero truss to which the MBSU must be bolted in place.

Wednesday's spacewalk is scheduled to last 6.5 hours. If by the 4-hour mark the spacewalkers are still unable to install the MBSU, they will begin the process of cleaning it up and bringing it back inside the Quest airlock for further analysis and troubleshooting within the pressurized, shirt-sleeve environment of the station.

Flight Engineer Joe Acaba, who will lend a hand to the spacewalkers Wednesday from within the station via the Canadarm2 robotic arm, teamed up with Hoshide to review the robotics choreography. Later Acaba, Hoshide and Williams ran through the spacewalk procedures again and afterward participated in a conference call with spacewalk specialists on the ground for one final review.

Meanwhile, Commander Gennady Padalka and Flight Engineer Sergei Revin spent some time preparing for their departure from the station. Padalka, Revin and Acaba are scheduled to return to Earth Sept. 16 aboard their Soyuz TMA-04M spacecraft after four months in space, marking the end of Expedition 32 and the start of Expedition 33 under the command of Williams.

The crew was awakened before the normal scheduled wake time Tuesday when a cooling system valve for hardware in the Tranquility node failed. This has resulted in some equipment in Tranquility being powered down, but is unrelated to the MBSU issues.

In a coincidental but unrelated occurrence, a direct current power switching unit component that enables power to be routed at the proper amperage and voltage tripped late Saturday, causing one of the station array's power channels to default to a parallel channel.

Although the power trip is not linked to the station operating on only three of its four MBSUs at the moment, the station is currently operating on only five of its eight power channels. Even with three such channels unavailable, flight controllers have been able to reroute power to critical station systems and payloads with only a minimal impact on operations.