Recurring troubles in Columbia's cockpit electronics forced postponement of the second flight of the space shuttle for a fourth time, though shuttle officials were hopeful that the newest delay would be no longer than several hours.

The scheduled launch time for Columbia and astronauts Joe Henry Engle and Richard H. Truly was tentatively set back from 7:30 a.m. to 10 a.m. Thursday. The countdown appeared likely to be a cliffhanger with the strong possibility of a second scrubbed launch in two weeks.

"They're working on a fairly tight schedule but they think it's do-able," said space agency spokesman Hugh Harris.

The latest troubles were in an electronic instrument in the shuttle cockpit that collects information about the shuttle's condition from all over the spacecraft, sorts out the information, displays it on video terminals for the astronauts and beams it to the ground for analysis by engineers.

The faulty instrument was replaced today with a substitute that did not work as well as the one it was replacing.

In haste, shuttle officials ordered two of the 15-pound instruments, a "multiplexer-demultiplexer," removed from the space shuttle Challenger, the still unfinished second shuttle that is being built at the Rockwell International factory in Palmdale, Calif., and flown to Kennedy Space Center.

The Challenger instruments arrived in Florida about 9 p.m. today. One went into the Columbia cockpit to replace the faulty instrument and the second may be carried as a spare.

A midnight check of the instrument showed that it was working, but it still meant that the countdown to launch, originally scheduled for 10:10 p.m., was delayed 2 1/2 hours. Starting early this afternoon, the countdown went into a built-in "hold" where nothing took place and which is designed for emergencies like the one today.

Shuttle officials planned to resume the countdown early Thursday and begin loading 3 million gallons of supercold liquid hydrogen and oxygen fuel aboard Columbia's huge external fuel tank Thursday, which would delay the launch from 7:30 a.m. to no earlier than 10 a.m. EST. Columbia can be launched as late as 12:10 p.m. Thursday. The constraint on launch other than weather on the launch pad is that there must be daylight on at least two of the emergency landing fields in Spain and California the shuttle might have to use if it aborts its mission early in flight.

The faulty instrument is one of seven in the cockpit that collect and sort out temperature and pressure measurements throughout the spacecraft. The instrument flashes the data on a screen for the astronauts to check whenever they need to. It also feeds them to another device called a pulse-code modulator, which beams the information to the ground for analysis by engineers checking progress of the flight.

Even if the astronauts take off by 10 a.m., the latest delays in the second shuttle flight will now cost the National Aeronautics and Space Administration more than $26 million. Every day of delay, including the eight in this last postponement, costs NASA $3 million.

The latest delay cost an extra $2 million in propellants lost from handling and replenishment, overtime to mission contractors, extra travel costs for personnel from outside Kennedy Space Center and money to the Pentagon for launch and landing tracking support.

Engle, 49, and Truly, who will be 44 Thursday, will rocket Columbia away on a five-day flight that will carry their DC9-sized spaceliner around the world 84 times. When they lift off, they will become the first astronauts to reuse a spacecraft that has already been flown in space.

They will leave earth at a higher speed and a steeper angle than the first flight to put more aerodynamic force on the spaceliner as a test of its space fitness. The second day in orbit, Engle and Truly will exercise a 50-foot-long robot arm that sits in the shuttle's cargo bay to deploy satellites and retrieve them from orbit.

Engle and Truly will spend more than 90 percent of the time in orbit upside down. Once they are in orbit and have opened the cargo bay doors, they will roll the spacecraft upside down so the open bay faces the earth, a position Engle and Truly will maintain until they are ready to return.

Flying upside down, Engle and Truly will aim five instruments at earth to study and map global terrain, photograph differences in ocean color to pinpoint the whereabouts of fish and measure pollutants like carbon monoxide in the atmosphere.