Prodded by the Congressional Black Caucus, President Reagan yesterday made good on his promise to help keep predominantly black colleges alive by ordering special aid for Meharry Medical College in Nashville, Tenn. But officials there said the move was just "a first step" to correcting discriminatory policies dating back to the 1940s.

Meharry, founded in 1876, has trained about 40 percent of the nation's black doctors and dentists. It was saved from severe financial trouble and possible suspension of its accreditation by the presidential order, which expands the affiliation between Meharry and the Veterans Administration hospital in Murfreesboro, Tenn., about 33 miles away.

However, college officials and Rep. Harold E. Ford (D-Tenn.), a member of the caucus, said the action was just a short-term solution to the situation.

"There's still a question of fairness when you consider that most black doctors will practice in an urban setting," said Ford. "Meharry is still committed to working out something with Vanderbilt University Medical School and the VA in Nashville."

Meharry President Richard G. Lester said in a formal statement that the action was "a very positive step" although "not completely consistent with our proposal."

The Liaison Committee on Medical Education, which accredits medical schools nationwide, had found that Meharry students had too little access to patients to get enough practice in healing. It ordered the 400-student college to show by next Monday that it had taken steps to achieve the recommended 6-to-1 patient-student ratio or face possible suspension.

Meharry had tried unsuccessfully to gain access for its students to the giant VA hospital in Nashville, which has a $6 million annual contract with Vanderbilt's predominantly white medical school. The college proposed to phase in Meharry faculty, staff and students until the numbers equaled Vanderbilt's participation. It enlisted alumni and the caucus in its aid.

The caucus wrote Reagan in April reminding him of his pledge to support the nation's 111 historically black educational institutions, many of which are struggling for survival in a time of declining enrollment.

"The caucus' point was why does the VA maintain an exclusive contract with one private college and not with another one down the street," said Ford's aide, John Matlock. "Race was always there in the background of this. It was obvious to everybody, but it never came to the table, I'm happy to say."

Relations between the Nashville VA and Vanderbilt University date to the 1940s, an era of firm discrimination in Tennessee, Matlock said. Meharry was never considered for affiliation when the VA hospital opened in 1963, he added.

Reagan appointed a five-agency task force, "and we thought it was a stalling tactic," said Matlock. "But it worked quickly."

Yesterday's order will provide up to 200 additional teaching beds at the Murfreesboro facility for Meharry over the next one to three years. The Department of Health and Human Services will shoulder the balance of a $29 million loan that built Meharry's Hubbard Hospital, and the task force will organize talks between Meharry and Vanderbilt University on interim access to the VA's Nashville hospital for some Meharry staff and students.