The Pentagon, in a turnaround, announced yesterday that it will keep open the famed Army training center at Ft. Dix, N.J.

The decision, backed by Army Secretary Clifford Alexander and endorsed by the deputy defense secretary. W. Graham Claytor, reversed the conclusion of former deputy secretary Charles Duncan last March that the base should be closed.

It came after a vigorous campaign by a coalition of members of Congress from New Jersey, Pennsylvania, Delaware and New York.

Freshman Sen. Bill Bradley (D-N.J.), who led the drive, said yesterday, "i'm really quite pleased, because keeping it open is in line with my contention all along that the Army's training needs require the base to stay open."

Bradley said that 20 percent to 25 percent of Army recruits come from the Northeast, and that it would not be desirable to leave only Ft. Jackson, S.C., in operation because recruits should be trained under a variety of climactic conditions. Bradley also said that since Army recruitment has been lagging and will have to rise next year, the added training capacity at Dix is needed.

He said studies by the General Accounting Office and the Arthur D. Little Co. showed there wouldn't be any saving by the Army in closing Dix. l

The famous base, first opened in 1917, has trained millions of recruits.

The shutdown plan didn't involve the entire base, but only the recruit-training operations, which required 2,367 military personnel and 865 civilians. A much smaller operation for training cooks and other specialists would have been left open, with the basic-training facilities kept in mothball status for possible future use. The estimated saving was about $15 million a year, a Bradley aide said.

According to estimates by the members of Congress, New Jersey and nearby states would have lost at least $157 million a year in purchases made by the Army, and the equivalent of 10,000 to 15,000 local jobs in businesses serving the base and its personnel.

Bradley aides said it is their understanding that the base will continue operations at current levels.

Secretary Alexander, in announcing the decision yesterday, said that because of recruiting shortfalls this year, the Army might need 'to recruit and train more personnel nexy year than originally anticipated" and therefore the shutdown decision had been "placed in abeyance pending a further review of Army recruit training needs over the next 12 to 18 months."