Making his third spacewalk in just 10 days, Douglas Wheelock slid the bathtub-size pump into place then bolted it down as Tracy Caldwell Dyson hooked up power cables.
An initial test proved successful. "Sweet," Wheelock exclaimed.
The astronauts then connected all the ammonia fluid lines. To everyone's relief, the work went smoothly, with no apparent leakage of the toxic substance. Back on the first spacewalk, ammonia streamed out and forced NASA to add an extra outing to get the job done.
"We did not see any ammonia come out today, amazingly," Mission Control radioed. Wheelock said a couple frozen flakes may have drifted out, but nothing more.
Wheelock proudly showed off the checklist on his arm cuff to the TV cameras. Printed in black ink were the words, "Game over!!"
Flight controllers still have more tests to conduct. If everything goes well, NASA expects to have the space station's disabled cooling loop back in action by Thursday.
The orbiting lab has been operating on only half its normal cooling capability ever since a crucial ammonia coolant pump failed July 31. Science research was halted and unnecessary equipment turned off to avoid overtaxing the single functioning cooling line.
It took two spacewalks, but Wheelock and Caldwell Dyson finally removed the broken pump last week.
NASA said a fourth spacewalk eventually will be needed to move the failed pump into a better storage location, but managers are uncertain whether this crew or another will carry out the work.
The pumps - weighing 780 pounds apiece - are needed to drive ammonia through cooling loops and keep electronics equipment from overheating. Four spare pumps were on board; the one installed Monday was the oldest of the bunch. It flew up in 2006.
Engineers are uncertain how and why an electrical short knocked out one of the two original ammonia pumps.
NASA said the repair effort was one of the most challenging ever undertaken at the 12-year-old space station. Indeed, the astronauts' work was hampered by the large ammonia leak that erupted during the first spacewalk on Aug. 7.
A special team of engineers has been working practically nonstop ever since the trouble struck.
The space station is home to three Americans and three Russians. It's supposed to continue working until 2020, but that will become increasingly difficult to accomplish once NASA's shuttles stop flying next year. Two shuttle missions remain, with a third possible if the White House and Congress sign off on it.
Once the three remaining shuttles are retired, the Russian, European and Japanese space agencies will take over all crew and cargo shipments until NASA has a new rocket ready to go.