Workers unwrap a new Enhanced Main Events Controller (EMEC) to be installed in Endeavour. Photo: NASA

A faulty avionics control unit that scuttled this week's shuttle launch has been replaced aboard Endeavour.

Workers swapped out the failed 65-pound Enhanced Master Events Controller No. 2 with a spare box on Thursday. Follow-up testing of the new EMEC was conducted on Friday, verifying the unit was operating properly.

Endeavour has two EMECs, located in the aft engine compartment, that relay commands from the shuttle's main computers to ignite the twin solid rocket boosters at liftoff, and later separate the spent boosters and external tank during flight.

Built by Rockwell's Satellite Space Electronics Division, Anaheim, Calif., the suspect EMEC No. 2 failed a standard health check at T-minus 29 minutes during Monday's countdown.

NASA managers decided to replace the unit fearing a hardware problem inside the box, which measures about 20 inches long, 13 inches wide and 8 inches tall.

Currently Endeavour and Columbia are the only two orbiters with the advanced EMECs. Discovery and Atlantis are outfitted with older models, NASA says.

At pad 39A, workers move the replacement EMEC into Endeavour's aft compartment via an access door. Photo: NASA

The weekend will be spent reconnecting ordnance lines and closing out the aft compartment.

Endeavour's six astronauts, led by commander Kevin Kregel, are slated to arrive at Kennedy Space Center at 12 noon EST (1700 GMT) on Monday. The three-day launch countdown should begin at 5:30 p.m. EST (2230 GMT) on Tuesday.

Liftoff is now scheduled for 12:30 p.m. EST (1730 GMT) on Friday, the opening of a two-hour, 10-minute window.

The 11-day STS-99 mission will use a radar system to map a vast majority of Earth to create the highest resolution 3-D surface elevation map of the planet.